Saturday, November 21, 2009
Saturday, November 14, 2009
Troubleshooting a Non-Condensing Gas Furnace Pressure Switch
On occasion, the non-condensing furnace control board (called various names like integrated ignition control, furnace control board, DSI, etc.) has a diagnostic LED light, especially on modern furnaces. Abnormal heating operation can often be indicated by the diagnostic LED light on the control board if the unit encounters an internal fault.
Occasionally, the furnace will go into a “hard” lockout and turn off the diagnostic LED. If this occurs, check the power supply to unit for proper voltage. Check all fuses, circuit breakers and wiring. Disconnect the electric power for five seconds. If the LED remains off after restoring power, replace the control board.
More often, the furnace will have an “external” lockout which occurs if the control determines that specific faults occur such as, but not limited to, a measurable combustion cannot be established within several consecutive ignition attempts, flame is established but lost during run, the flame rollout protection device opens, a drop in flame signal occurs, the primary limit switch opens, or if flame is detected with the gas valve de-energized. These situations occur sporadically with most gas furnaces.
The other faults that are often more common today concern the pressure switch.
A pressure switch is a safety device to shut down a furnace if proper combustion air flow is not being provided. All pressure switches in a non-condensing gas furnace are placed there for safety purposes. Generally, the single pole type of switch typically used allows electrical current to flow when the pressures are at an acceptable operational level based on the manufacturer design.
A pressure switch problem that often occurs is the pressure switch is closed prior to furnace operation. When the control board senses a call for heat from the thermostat (24VAC on the "W" terminal) one of the first things it does is check to see that the pressure switch is open or non-energized. This is the normal state for most pressure switches on furnaces. If the control board senses that the pressure switch is open it then sends power to the inducer blower and then checks to make sure that the pressure switch closes. If it is then closed after a short time period (based on furnace manufacturer design) the control board proceeds with the ignition sequence.
If the control board senses a closed pressure switch before it has energized the inducer, its program determines that something is wrong and for safety does not continue with the ignition process.
This situation could be caused by something that is causing the switch to stay closed. Blockage in the hose or hoses going to the switch could be the problem. You also could have a defective switch. It is possible (but less likely) that you have a short in the wiring or (even less likely) that you have a defective furnace control board. You can check that the pressure switch opens and closes with an ohmmeter when you blow or suck on the pressure hose. Of course, perform this check with power removed from the furnace.
Another more common pressure switch problem encountered that can arise with furnaces is that the pressure fails to close after the inducer has started to run. This blower is called an "inducer blower." When the furnace gets a call for heat, it starts up the inducer to start blowing air (either sucks it into the burners ahead of the flame or blows it out after the flame—dependent on the manufacturer design). Since the whole point is to create a "draft", the furnace control must determine when this has been accomplished. The pressure switch “tells” the control board that there's sufficient air movement to go ahead and fire up the ignitor and then open the gas valve. In most cases, the pressure switch (normally open when the system is off) will close due to negative pressure created by the inducer running in a short period of time. Different pressure ratings exist for different pressure switches. These ratings often appear on the side of the switch. This is so replacement of a bad switch can be exactly what is there originally.
A pressure switch that is stuck open can be caused by many issues such as a plugged vent, a heat exchanger surface /passage problem due to carbon deposits (usually on LP gas furnace more that natural gas), the inducer slows down due to age, a pressure switch hose is plugged with debris, or simply a bad pressure switch is now in the furnace.
It only takes a small restriction to keep the pressure switch from closing.
One of the first steps in troubleshooting the open switch is to test it to see if it is defective. Remember, electrical devices (switches in this case) typically wear out over time. Using a voltmeter, place one probe on a lead wire going to the pressure switch and "ground" the other probe. To "ground" is to attach the probe to metal that is part of the furnace. The meter, which you should have set to volts AC, should register approximately 24 or more volts (24 to 28 volts is normal). A two wire pressure switch you should be getting 24 or more volts between both leads to ground. If you do not get 24 or more volts with the furnace running then you have a pressure switch problem.
One of the most common problems involves the tubing (typically rubber) where the hose slips onto the draft inducer. It may attach at the top, bottom or center of the draft inducer via a small nipple device. This point will get clogged rather easily. A simple test is to pull the hose off the pressure switch and blow into it. You should be able to blow air into the inducer easily. You can also pull the tube from the pressure switch and listen for air flow when the draft inducer turns on as well.
If you cannot blow air into the inducer, or hear air flow when the inducer is running, take a stiff wire, a piece of coat hanger, or a drill bit to clear the inducer nipple. You simply place the wire, or bit into the nipple on the inducer and tap it gently with a hammer until it breaks free. In extreme cases, you may have to actually drill the debris out with a drill and bit, but not most of the time.
Phil Rains
copyright(c)2009
About the Author: Phil Rains is Master Trainer/Technical Developer for HVACReducation.net. He has over 35 years of HVAC and Refrigeration experience in installation, service, and training. He is NATE-certified in 5 areas, a member of ASHRAE and RSES, and ACCA EPIC-Certified in Residential and Commercial Design. He also holds a Universal Classification in EPA 608.More About PhilIf you are interested in an approved online program addressing the many facets of gas furnace troubleshooting you can contact HVACReducation.net.
HVACReducation.net provides online courses related to furnaces and their proper operation.
Hi Efficiency Heating and Cooling Unit Operating Characteristics
Blower Operation
1. The blower will operate continuously whenever the mode selector of the electronic thermostat is set in the cool mode and the fan selector is set to on.
2. The blower will operate intermittently whenever the mode selector is set to cool and the fan selector is set to auto. A delay is programmed into the microprocessor to allow the blower fan to continue to operate for one minute after the condensing unit has been commanded off by the thermostat.
3. The blower will operate intermittently whenever the mode selector of thermostat is set to heat. The fan is commanded on 35 seconds after the main burner lights and remains on for a predetermined time set by dip switches after the main burner is commanded off.
Heating Cycle
1. When the thermostat is set for heat mode and the room temperature drops one degree below the set point the circuit between the “R” and “W” terminals on the sub base is closed. The microprocessor on the electronic circuit board (ECB) sees a command for heat on the input terminal to the “W” terminal of the thermostat and commands the relay of the induced draft fan to close starting the induced draft fan motor.
2. If the passages through the combustion air piping, vent piping, and heat exchanger passages are unrestricted, the operation of the induced draft fan generates enough air pressure to cause the pressure switch to close within about two seconds. The microprocessor on the ECB reads the input signal from the pressure switch terminals and commands the hot surface igniter relay contacts to close allowing the ignition device to begin heating.
3. After the igniter has been hearing for 15 seconds, the microprocessor on the ECB commands the gas valve relay contacts closed for 7 seconds, opening the valve. As the fuel-air mixture passes across the red hot surface of the igniter, combustion occurs and the entire burner lights.
4. The presence of burner flame allows current to flow (1.7ua) from the tip of the flame sensing rod, through the ionized gases of the flame, and into the grounded surface of the burner. The current can only flow out the top of the rod and into the flame.
5. It cannot flow from the flame into the top of the rod. Consequently, the ac voltage across the flame rod and burner is rectified into a pulsating DC current. As long as a rectified DC current is present at the flame rod terminals, the microprocessor knows that a flame exists and allows the gas valve to remain open.
6. After the flame rod safety circuit has proved that the main burner has been lit for 30 seconds, the ECB microprocessor will command the blower motor relay contacts close, turning on the fan. Since the unit is operating in the heating mode, the fan speed relay coil remains de-energized.
7. When the signal from the “W” terminal of the thermostat opens, indicating that the room is satisfied, the ECB microprocessor will command the gas valve relay contacts open and the gas valve will close.
8. The ECB microprocessor allows the induced draft fan motor to operate for 15 seconds after the main burner is commanded off to purge the combustion gases from the burner box and heat exchangers. The blower motor will remain operating for some set time after the main burner is commanded off.
Cooling Cycle
1. When the thermostat mode is set to cool and the control point rises one degree above the set point, the input signal to the Y terminal on the ECB comes on. The ECB’s microprocessor reads the change in the “Y” terminal’s signal and commands the condensing unit relay contacts to close. This action energizes the condensing unit’s contactor, starting the compressor and condenser fan.
2. The blower fan unit is commanded on when a signal is present on the “G” input terminal from the thermostat. The fan operates for 60 seconds after the signal on the “G” terminal goes open, Indicating the thermostat is satisfied.The condensing unit is commanded off when the signal on the “Y” terminal of the thermostat and the ECB goes open.
The legend for the Electronic Control Board
· HSI = Hot surface igniter
· IDM = Induced draft motor
· GV = Gas valve
· BLM = Blower motor
· EAC = Electronic air cleaner
· ACC = Air conditioning control
· FPE = Flame proving electrode
· FRS = Flame rollout switch
· HLS = High limit switch
· PS = Pressure switch
ELECTRONIC CONTROL BOARD
Humidifier
1. The optional 24V AC humidifier is commanded on by the humidifier control relay whenever the furnace is in the heating mode and its blower motor is commanded on.
2. The humidifier is commanded off 15 seconds after the furnace is commanded off at the end of the post purge cycle.
Safety Strategies
1. During the hearing mode if the flame rod does not detect a flame within the first 7 seconds that the gas valve is commanded open, the ECB microprocessor commands the gas valve closed. A 15 second waiting period occurs before an ignition cycle will be attempted again. The induced draft fan motor remains operating during this period to purge unburned fuel from the heat exchangers.
2. After a flame failure, a second attempt is made to start the furnace. The igniter is started and allowed to operate for 25 seconds before the gas valve is commanded open. If the burner fails to light, another attempt will be made before the ECB microprocessor locks out the ignition cycle. The unit can be reset by a service technician by opening the disconnect switch to the furnace for 60 seconds.
3. If there is a, momentary loss of fuel or flame or a short or an open circuit occurs in the flame rod circuit, the microprocessor will close the gas valve. After 15 seconds the ECB will restart the ignition process. If for some reason the unsafe condition does not clear, the unit will lock out, requiring a manual reset.
copyright(c)2009
Roger J. Desrosiers
About the Author: Roger is a contributing faculty member of HVACReducation.net. He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.More About Roger
Energy Auditors, One of the New GREEN Jobs, Gain Momentum Across the Country
Austin, Texas is one of the first cities in the country to require energy audits on building performance when sold. It is anticipated that most if not all large cities in the country will adopt similar green energy efficiency programs. The Austin City Council approved the Energy Conservation Audit and Disclosure (ECAD) ordinance to improve the energy efficiency of Austin homes and buildings that receive electricity from Austin Energy.
The energy auditor—is listed by the US Department of Labor as one of the new green jobs where growth is anticipated. Energy Auditors, also known as Building Performance Assessors, Home Energy Raters, and perhaps other titles, use equipment such as blower doors and infrared cameras, as well as visual walk through checklists to evaluate a building’s performance levels. As more cities follow Austin’s lead, energy auditors will be in demand.
The research indicates that this line of work is already growing as the interest and urgency in preserving the global environment increases. The International Council for Local Environmental Initiatives (ICLEI-Local Governments for Sustainability) has over 1107 cities, towns, counties, and their associations worldwide that comprise a growing membership. It is an international association of local governments and national and regional local government organizations that have made a commitment to sustainable development. To view a list of their global members, visit:
http://www.iclei.org/index.php?id=772
One of their programs is the Eco-Efficient Cities initiative that helps local governments develop strategies to address air quality, energy efficiency, water resources management, waste stream management, eco-mobility, and others in an integrated manner.
NATIONALLY
Here in the United States we hear a lot about Energy Star. Energy Star is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy helping us all save money and protect the environment through energy efficient products and practices. One concern about home performance is that a home can cause twice the greenhouse gas emissions of a car. Energy Star reports that in 2008 alone, Americans saved enough energy to avoid greenhouse gas emissions equivalent to those from 29 million cars—all while saving $19 million on their utility bills. Their web page
http://www.energystar.gov/index.cfm?c=home_improvement.hm_improvement_index
is devoted to home energy efficiency improvements; they all start with a comprehensive home assessment. If you’re wondering about home energy raters in your area, Energy Star provides an online list of qualified rater/partners by state.
STATES
The state of New Jersey instituted a statewide Clean Energy Program – recognized as a national model—that offers financial incentives, programs and services for New Jersey residents, business owners and local governments to help them save energy, money, and the environment. Their Residential Home Performance Program that offers financial incentives on energy efficient improvements begins with the home energy audit.
New York has been addressing energy efficiency for many years. Their New York State Energy Research and Development Authority (NYSERDA) helps to coordinate the efforts of stakeholders across the state to help utility customers solve their energy and environmental problems while developing new, innovative products and services. The Home Performance with ENERGY STAR® Program begins with a BPI Accredited Home Performance contractor who performs an assessment of the home.
The state of Virginia has implemented the Residential Energy Efficiency Rebate Program that will provide $7 million for energy efficiency improvements and retrofits made by homeowners for replacement of major systems equipment such as: central air conditioning units, heat pumps, furnaces, boilers, water heaters, window replacements, insulation, and programmable thermostats. Homeowners will be eligible to receive up to $250 for the cost of an energy audit conducted by a certified auditor.
The California Public Resources Code Section 25942 directs the energy Commission to adopt a statewide California Home Energy Rating System (HERS) Program for residential dwellings.
Southern California has adopted a Home Performance Program in league with Southern California Edison. This program finds, screens, trains, and mentors qualified HVAC and remodeling contractors to deliver comprehensive home performance improvement packages tailored to the needs of each existing home and its owner. To verify the program’s results they will perform a checklist walkthrough inspection of the reported job scope plus physical tests of each job’s key quantifiable measures, particularly duct sealing, airflow, combustion safety, and envelope leakage reduction.
Missouri passed Senate Bill 1181 into law in 2008. It includes a tax deduction for qualified home energy audits and the recommendations of those audits beginning with the 2009 tax year. For taxpayers to qualify for the deduction, the home energy audit must be performed by an energy auditor certified by the Missouri Department of Natural Resources. Missouri certification includes either the Building Performance Institute (BPI) or Residential Energy Network (RESNET) certification, or a similar alternative program.
Oregon offers home owners a state income tax credit for making their homes more energy efficient and helping preserve Oregon’s environment. Their standards may be more stringent than the federal government’s EnergyStar Program. Oregon Utility companies also offer rebates and incentives for improving home efficiencies. A non-profit organization, Energy Trust of Oregon, helps individuals identify and qualify for all of the energy savings programs that starts with a Home Energy Review.
CITIES AND COUNTIES
At the local government level, momentum is increasing as well. At the end of September, 2009 in Los Angeles, California at the Governors’ Climate Summit 1,000 mayors across the United States signed a pact to reduce greenhouse gas emissions. "We didn't just sign on. I can tell you, we've been working hard to meet those goals," Mayor Antonio Villaraigosa said. Some of the accomplishments already gained by this group of mayors include:
· Seattle was able to reduce its 1990 carbon footprint by 8% in 2005, largely through voluntary emissions reductions by households and businesses.
· Los Angeles reached the 7% Kyoto target in 2008, four years ahead of schedule, in part through an aggressive program in energy efficiency that included light bulb and street light replacements, mandatory green building standards, and a transition to alternative fuel on buses, trash trucks and other city vehicles.
· Boston has increased its solar capacity by 300%.
· Philadelphia has adopted a plan to retrofit 100,000 homes with energy-saving features over the next seven years.
· Cleveland has set a standard of converting to 25% renewable electricity.
Upon looking more closely at a number of cities across the country it is evident that the momentum for energy efficiencies and energy audits is growing.
Montgomery County, Maryland introduced an act to require that a home energy audit be conducted as part of a home inspection completed in connection with the sale of a single family residential building. It goes on to define what a “Qualified home energy performance rater” is: certified by RESNET as a home energy performance rater; or meets other equivalent requirements approved by the Director of the Department of Environmental Protection.
Houston, Texas – the Mayor’s Office of Environmental Programming has developed a home energy audit worksheet. Greenhoustontx.gov states that a home energy audit is often the first step in making your home more efficient. An audit can help assess how much energy your home uses and evaluate what measures you can take to improve efficiency. But remember, audits alone don’t save energy. You need to implement the recommended improvements.
Albuquerque, New Mexico – Executive Order no.20 established green building standards for city projects, including requirements to meet or exceed LEED Silver ratings. In 2007 the Albuquerque Energy Conservation Code – Volumes I and II were signed into legislation. It is the first comprehensive Energy Conservation Code in the State of New Mexico; and it reflects a concerted, combined effort between local government and those in the building, and building-related industries to develop a code acceptable to all. Mayor Martin Chavez states, “The revised building codes support green building targets and are essential to reduce the amount of greenhouse gases generated by buildings. It is estimated that the building industry generates 39% of carbon dioxide (CO2) emissions and 48% of all greenhouse gas (GHG) emissions in the United States.” Also, “. . . states otherwise should not be tied to the latest versions of ASHRAE and IECC standards. Climate change and energy independence are too urgent for localities to wait for Federal consensus on building codes.”
Alexandria, Virginia -- As part of the 2008 Eco-City Charter, the first of its kind in the region, the city adopted a new and progressive green building policy for commercial and residential buildings. The Charter outlines essential environmental sustainability principles and core values. To help them accomplish the goals, the city hired an energy manager. Their efforts have already been recognized as Alexandria just received Platinum Certification in the Virginia Municipal League Go Green Government Challenge, which encourages local governments to reduce energy usage and promote environmental sustainability.
Babylon, Long Island, New York – Mayor Steve Bellone founded The Babylon Project and the Long Island Green Homes project which has completed 120 deep retrofits with another 96 audited in the queue. On average, air infiltration has been decreased by 29%, diminishing CO2by 4.35t per house. The pilot program of 275 homes will be completed by the end of the year. Next year, they will be targeting 1,200 homes and aim to have them retrofitted by the end of 2010, thus reducing Babylon’s carbon footprint by 6, 416t.
Boston, Massachusetts – Because nearly 75% of the city’s green house gas emissions come from the energy demand of the building sector, they’ve developed a comprehensive green building strategy. In addition to the installation of solar power, they are coordinating energy efficiency programs for residents and businesses. Mayor Thomas Menino states, “I expect that the range of energy efficiency measures we are putting in place for existing buildings will be the most important.”
Burnsville, Minnesota -- The city has adopted a Sustainability Guide Plan that includes conducting energy audits on city facilities and retrofitting city facilities with energy efficient technology.
Charleston, South Carolina -- In order to reduce CO2 emissions from buildings, they have a contract with an energy services company. Much of their initial work has involved installing more energy efficient HVAC systems, lighting efficiency retrofits, efficiency control systems retrofits, and low flow water devices. In addition, during 2010 they hope to launch energy efficiency partnerships to bundle energy audits, efficiency upgrades and financing for all residential buildings, small commercial buildings, and many institutions, into a one-stop center. One thousand residential and small business buildings will be targeted for service in the pilot phase.
Denver, Colorado – “We’ve had a lot of success with our Residential Climate Challenge program,” says Mayor John Hickenlooper. They’ve worked with non-profit partners to go door-to-door in Denver’s neighborhoods to provide energy efficiency education and services. “We’re making homes more comfortable and building stronger communities.”
Miami, Florida – Next month, Miami will launch a Home Energy Challenge/ Reduce the Use program in partnership with the local non-profit Dream-in-Green for 50 homes. “We are currently looking for opportunities to scale this project to more homes in the community. This program is critically important because one of the most challenging and important parts of GHG mitigation is including homeowners, apartment dwellers, and small business owners in this effort.” Says Mayor Manuel Diaz
Philadelphia, Pennsylvania -- Released in April, 2009, Greenworks Philadelphia is a comprehensive strategy to lower greenhouse gas emissions and improve the quality of life for all Philadelphians. It includes 169 separate initiatives, of which one of the most important is the ongoing effort to weatherize and install more efficient heating systems in homes. It calls for the weatherization of 100,000 homes over the next seven years.
Pleasanton, California -- Pleasanton has established a collaborative partnership with the local utility to provide energy efficiency audits and a retrofit program; rebates are provided to make this more cost effective. They have established a Green Building Ordinance, and are moving through the steps of creating a special financing district that allows residents and business owners to finance energy efficiency upgrades, including solar, and have it added to their property taxes.
Cape Light of Barnstable, Massachusetts offers a Home Energy Audit for residential customers to see the potential for energy saving measures and to commit to install energy saving improvements with the help of generous program incentives. All audits come with a three-page report summarizing the findings, and a list of recommended mitigation measures.
During the weeks I spent researching and assembling this list of information, new energy efficient activities were emerging every day. The United States is already experiencing a surge in building performance activities that will result in a smaller carbon footprint. One of the keys to almost every residential program is an energy audit that scientifically identifies specific issues of a building’s energy performance, and then makes recommendations to improve any weaknesses. I would encourage you to participate as our country goes green.
Patricia (Patty) Leiser
Executive Assistant,
HVACReducation.net
About the Author:Like many of you, I have a passion for education and lifelong learning. I have a Bachelor of Science in Education/Business from the University of Idaho where I transferred after two years at Gonzaga University. I also studied with Berean Bible College and Riverside Community College. I bring to you 35 years of work experience in business and education. I have worked as a Classroom Aide, Special Ed Tutor, Preschool Director, Secretary, Office Manager, Assistant for the Dean of Education, Administrative Assistant to Professional-Technical Education, and Human Resources Coordinator for a school district. I believe the future of education is online, and employment opportunities are in the trades. I am honored to work with HVACReducation.net and dedicate myself to providing excellent service to our students, faculty, and staff.
On a personal note, I have a nice little office with a view of the forest from our cabin in the woods. I love the northwest because I can go right outside and enjoy hiking, cross country skiing, biking, canoeing, and camping—my favorite activities. I look forward to working with each of you.
Research and information for this article is taken from:
http://www.austinenergy.com/About%20Us/Environmental%20Initiatives/ordinance/index.htm
http://www.iclei.org/index.php?id=10509
http://www.latimes.com/news/nationworld/nation/la-na-mayors-climate3-2009oct03,0,4137038.story
http://www.njcleanenergy.com/residential/programs/home-performance-energy-star/benefits-and-incentives
http://docs.google.com/gview?a=v&q=cache:MKLwGCRL0ckJ:www.naco.org/cffiles/ggi/green_counties/documents/Montgomery%2520County%2520MD%2520Home%2520Energy%2520Audit%2520Policy.pdf+home+energy+audit+laws&hl=en&gl=us&sig=AFQjCNGtJ--cJUsHbypKgarjFgrhtrY0Cg
http://www.energy.ca.gov/2008publications/CEC-400-2008-011/CEC-400-2008-011-CMF.PDF
http://www.takecareoftexas.org
http://www.greenhoustontx.gov
http://www.getenergysmart.org/SingleFamilyHomes/ExistingBuilding/HomeOwner.aspx
http://www.energytrust.org/
http://www.virginia.gov/eerebates
http://www.capelightcompact.org/home_energy_audit.html
Sunday, November 1, 2009
HVAC/R Electronics # 6
Ventilating Control Program
Sequence of Operation:
1. Supply fan starts and enables return fan start and system controls.
2. SA smoke detector stops supply fan when smoke detected.
3. RA smoke detector stops supply fan when smoke detected.
4. Controller stops fan when low temperature detected.
5. SA high static pressure control stops fan when unsafe pressure exists.
6. Automatic fan system control subject to commandable on-off-auto software
point.
7. Control program turns supply, return, and exhaust fan on and off dependent
upon optimized time schedule, unoccupied space temperatures, and occupant
override requests.
8. Occupant override switch provides after hours operation when pressed.
9. Duration of operation for override request.
10. Space temperature (perimeter zone) inputs to optimum start-stop, unoccupied
purge, and low limit programs.
11. Set-point at which unoccupied low-limit program executes.
12. OA temperature input to optimum start-stop program.
13. Return fan operation enables exhaust fan control program.
14. Exhaust fan status (operator information).
15. Warm-up mode status (operator information).
16. Supply fan load (VAV type systems-operator information).
17. Return fan load (VAV type systems-operator information).
Air handling system shall be under program control, subject to supply air (SA) and return air (RA) smoke detectors, SA high pressure cut-out, and heating coil leaving air low-temperature limit control; and shall be subject to system software on-off-auto function.
Supply fan shall be started and stopped by an optimum start-stop seven-day time schedule program, an unoccupied low space temperature limit program, or by an occupant via push button request. The push button shall be integral with the space temperature sensor. Any push button request shall provide sixty minutes (operator adjustable) of full system operation. Return fan shall operate anytime the supply fan proves flow (via a current sensing relay). The exhaust fan shall operate during scheduled occupancy periods and during occupant requested after-hour periods anytime the return fan proves flow.
In figure 1 we see a supply air control loop with the sequence of operation.
Sequence of Operation:
The DDC controller uses a temperature sensor mounted in the supply air duct to modulate control valves or mixing dampers to maintain a supply air temperature set-point. In most systems that employ a heating and cooling coil, the hot water valve and the chilled water valve should be modulated in sequence.
When the supply air temperature falls below set-point, the hot water valve begins to modulate open and consequently, the cooling valve begins to modulate closed. If the supply air temperature continues to fall below the set-point, the heating valve will open fully and the cooling valve will close completely.
When the supply air temperature rises above set-point, the hot water valve begins to modulate closed and consequently, the cooling valve begins to modulate open. If the supply air temperature continues to rise above the set-point, the heating valve will fully close and the cooling valve will open completely.
A temperature sensor located in the mixed air stream (between the unit filters and the coils) is used to provide mixed air low limit control. When the temperature sensed by this element falls below the setpoint, the outside air damper fully closes, the return air damper fully opens, the exhaust air damper closes to a minimum position, and the valves on all coils will fully open. This sequence should always be used on systems with wetted coils.
When the unit fan is turned off, the outside air damper fully closes, the return air damper fully opens, the exhaust air damper fully closes, and all control valves return to their “normal” positions.
Design Considerations:
Control Valves:
· Avoid using spring return actuators on control valves for wetted coil
applications.
· When selecting two-way valves for control
of wetted coils:
· For hot water coils, have the valve configured in the “normally” open position.
· For chilled water coils, have the valve configured in the “normally” closed
position.
When selecting three-way valves for control of wetted coils:
· For hot water coils, have the valve piped such that when the valve is in the
“normal” position, the water flows through the coil.
· For chilled water coils, have the valve piped such that when the valve is
in the “normal” position, the water bypasses the coil.It is recommended that
mixing valves be used in all three-way applications unless otherwise
specified.Also be cautious to not pipe globe valves that are designed
for mixing applications for diverting service. The fluid flow will
cause a“hammering”effect and severe noise and damage will follow.
In Electronic #5 I discussed how to program a control loop, following are some of the most common loops that can be controlled that you will find on sophisticated controls.
1. Discharge Air Temperature
2. Mixed Air Temperature
3. Hot Cold Deck Temperature
4. Cold Deck Temperature
5. Humidity or Dew Point Control
6. Indoor Air Quality Control
7. Ventilation Control
8. Supply Fan Static Pressure Control
9. Supply Fan Start/Stop Control
10. Return Fan Start/Stop Control
One of the last steps is to connect the analog inputs and outputs to an 8x computrol controller as shown in figure 1 below.
From this diagram you can see that the DDC is powered with 24 volts supply at the terminal strip in the lower left side. The discrete (on-off) inputs are called binary inputs. They are connected to any terminal that you designate as a binary, because you have the option to designate any of the terminals you like, which is a feature of these controls. Therefore point 1 thru 8 can be an analog input or output or it can be a binary input or output.
One of the main advantages of a DDC system is that it can be connected to a network and be controlled remotely. In some cases the control is setup in a special room, where the HVAC technicians for a large building or campus can monitor the entire system from a single console. Another feature of the network is that its data can be transmitted over dedicated telephone lines or connected through the Internet so that the data are available worldwide. Other features such as energy management, security, fire control and other essential functions can be controlled over these networks.
Conclusion:
This brings me to the end of this series of discussions on the wonderful world of DDC Controls and some of the attributes and marvelous things that can be done with these controls. My intention was not to teach you all about these controls but more of a fundamental introduction. If I aroused your interest into looking further into how these controls work then I will feel that I have done some good for the betterment of this industry. If you are interested in learning more about DDC Controls I urge you to peruse the following web sites:
DDC-Online
Computrols.com
copyright(c)2009
Roger J. Desrosiers
About the Author: Roger is a contributing faculty member of HVACReducation.net He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.Lean More About Roger!
Saturday, October 24, 2009
Air-Source Heat Pump (ASHP) Auxiliary-Supplemental Heat Sizing
Outdoor temperatures lower than the structure “balance point” requires an additional heat source since the ASHP refrigeration cycle alone will not produce enough heat for indoor comfort needs during winter months. This additional heat is often called either auxiliary or supplemental heat.Typically, around 30 - 40 degrees the ASHP will generally be able to produce the same amount of heat with the compressor and refrigerant cycle alone that the structure is losing (this is considered the structure "balance point”).
Every structure has a distinctive “balance point” which can be calculated.If the ASHP is located where the heating requirements are very much greater than the cooling requirements, there will be a need for this additional heat to help meet the load of the structure during heating. This is required as it is harder to extract heat from colder air.
Since the indoor coil of an "all electric heat pump" can be located upstream of the electric heating elements used as auxiliary or supplemental heat and emergency heat, the ASHP and the backup heating can operate simultaneously in the heating mode. These heaters, of course, will not, and should not, energize during the cooling mode. If additional heat is required due to unexpected heat loss above the “balance point,” the heaters energize briefly, controlled by the thermostat.
Note in the diagram shown that the RA (return air) enters the indoor coil, not the heated air from the heating elements. An ASHP is always sized for the cooling load of the structure so that it has a long run cycle and can properly dehumidify the space.
When installing an ASHP, choosing the right auxiliary/supplemental heating system is almost as important as proper heat pump sizing.
Unfortunately, in the past and even now, it has become common practice by some contractors and/or technicians to install an ASHP with larger electric heat strips than necessary. But remember, any time the temperature is below the structure “balance point” the auxiliary/supplemental heat may come on, called for by the “second heating stage” of the indoor thermostat.
Most ASHPs operating in the heating mode around these temperatures will run most of the time. This is actually normal operation, At or below the “balance point” the heat is leaving the structure faster than the ASHP compressor can transfer it into the conditioned space. This is a function of 1) the temperature difference between indoors and outdoors, 2) the insulating value of the structure envelope, and 3) the amount of air leakage into or out of the structure. Without supplemental heating, the indoor temperature will fall below the thermostat setting.
You should always calculate the proper size of the heaters to meet the entire structure heat loss in Btu/h from the indoor design temperature (say 70 degrees) down to the typical outdoor design temperature for the locale (for example 0 degrees). This will provide enough electric heat to meet the indoor thermostat setting alone without the compressor. However, remember that the electric heaters operating alone are much less efficient than the heat pump compressor during heat pump heating. The heat pump should be configured to provide heating throughout the heating needs (say from 65 degrees outside down to the outdoor design temperature) to achieve the efficiency of the electric heat pump compressor, with or without the heaters. Basically, always allow the compressor to provide heat during the heating cycle and never wire the system to turn the compressor off at some specific outdoor temperature above the outdoor design except if combined with a gas furnace (an add-on or dual fuel configuration).
The best way to provide effective auxiliary/supplemental heating is to have multiple levels of heat if necessary. These banks of heat then energize as necessary. Most manufacturers provide heaters in sizes that meet most structure heating needs (5, 10, 15, 20 kW banks, etc.).
Emergency heat is always necessary with ASHPs if the need arises. This heating is actually provided by the auxiliary/supplemental heaters as well. This allows the occupant to energize the heaters and the indoor blower to meet the heating requirements of the particular structure as a backup if malfunctions occur with the ASHP refrigerant cycle.
Homeowners can energize the emergency heat mode typically with a switch on the indoor thermostat. In fact, most ASHP thermostats are now electronic and equipped with these selector switches or other means of energizing emergency heat. Always study the particular thermostat specifications for the model you use to assure the emergency heat can operate properly if necessary.Emergency heat relays should always be used in conjunction with outdoor thermostats to allow electric heat operation in the event the heat pump compressor and refrigerant cycle become inoperative. An outdoor thermostat disallows the second stage (if provided) of electric heat above a selected outdoor temperature. If the outdoor temperature falls below the setting on the outdoor thermostat, this additional heater stage will come on. When the outdoor air temperature rises, and the outdoor thermostat set point is reached, the system will revert back to first stage electric heating.
Codes are very strict concerning electric heaters in ASHPs. The code governs where the electric heaters are to be installed, how they must be wired, and, in some cases, how much additional heat must be added to the ASHP to compensate for capacity loss during the heating mode. If the ASHP fails, the code specifies how much heat must be added to serve as both supplemental heat and emergency heat. It is imperative that you as the technician check the governing codes and follow them exactly.
Copyright © Phil Rains
About the Author: Phil Rains is Master Trainer/Technical Developer for HVACReducation.net. He has over 35 years of HVAC and Refrigeration experience in installation, service, and training. He is NATE-certified in 5 areas, a member of ASHRAE and RSES, and ACCA EPIC-Certified in Residential and Commercial Design. He also holds a Universal Classification in EPA 608.
Monday, October 19, 2009
HVAC/R Electronics # 5-DDC Controls
The microprocessor unit (MPU) in the controller provides the computation. Therefore, the term digital in DDC refers to digital processing of data and not that HVAC sensor inputs or control outputs from the controller are necessarily in digital format. Nearly all sensor inputs are analog and most output devices are also analog. In order to accept signals from these I/O devices, A/D and D/A converters are included in the microprocessor-based controller. The figure below shows several inputs and outputs. The microprocessor usually performs several control functions.
DDC provides more effective control of HVAC systems by providing the potential for more accurately sensed data. Electronic sensors for measuring the common HVAC parameters of temperature, humidity and pressure are inherently more accurate than their pneumatic predecessors. Since the logic of a control loop is now included in the software, this logic can be readily changed. In this sense, DDC is far more flexible in changing reset schedules, set points and the overall control logic. Users are apt to apply more complex strategies, implement energy saving features and optimize their system performance since there is less cost associated with these changes than there would be when the logic is distributed to individual components. This of course assumes the user possesses the knowledge to make the changes.
Elements of a Direct Digital Control System
8X DDC Controller: Figure 1
Figure 1 above is a typical state of the art DDC controller in use today.
Any point can be configured through software to be Analog In, Analog Out, Binary In, or Binary Out – no jumpers with Bright on-board LEDs assist in troubleshooting. You can directly connect a normal web browser for simple management. It's high speed communications allow the ultimate in flexibility and snappy response, 3 decimal rotary switches (0-9) allow simple addressing – no hex, no binary and all of the electronics are on one easily replaceable brain board for quick repairs.
POINTS
All field devices and any logic or calculations associated with those devices are points. The word "points" is used to describe data storage locations within a DDC system. Data can come from sensors or from software calculations and logic. Data can also be sent to controlled devices or software calculations and logic. Each data storage location has a unique means of identification or addressing. A point can be an actuator, a temperature sensor, a control sequence or any other quantity or status that can be monitored or controlled. We recommend naming your points based on their function to make it easier for the operator. For example, if you have a temperature sensor that reads the outside air temperature, name the point “Outside Air Temp.” There are two categories of points: Hardware and Software
HARDWARE POINTS
Hardware points are points that can be physically wired or connected through a wireless sensor to the terminal strip of a controller. They include field devices such as relays, actuators and sensors. Their function is to transmit data back to the controller or physically carry through a building automated control command.
There are four main types of hardware points. They are analog inputs, analog outputs, binary inputs, and binary outputs. Binary points have only two states such as ON/OFF, OPEN/CLOSE, or START/STOP. Analog points on the other hand, represent a range of measurement such as a temperature of 0°F to 110°F, a pressure of 1psi to 5psi, or a flow rate of 100 CFM to 200 CFM.
Whether a point is binary or analog, it must be either an input or an output. Points that monitor the status of a field device are inputs. Field devices send their condition or quantity to an input on the controller.
Points that control the status of a field device are outputs. The user can either control outputs manually, or allow for automatic control based on schedule, logic, PID, or other software outputs programmed in the building automated control.
SOFTWARE POINTS
Software points include calculations, points of reference, and logic statements. They are intelligent points that are not physically connected to the controller. Instead, they gather data and send commands to hardware points. An example of gathering data is the average supply temperature of all AHUs in the building. An example of sending commands is, "if Outside Air Temp is less than 50°F, then start VAV heat strips.
CONTROLLER CONFIGURATION
- The microprocessor
- A program memory
- A working memory
- A clock or timing devices
- A means of getting data in the basic elements of a microprocessor-based (or microprocessor) controller. (Fig. 3)
In addition, a communications port is not only a desirable feature but a requirement for program tuning or interfacing with a central computer or building management system.
Timing for microprocessor operation is provided by a battery-backed clock. The clock operates in the microsecond range controlling execution of program instructions.
Program memory holds the basic instruction set for controller operation as well as for the application programs. Memory size and type vary depending on the application and whether the controller is considered a dedicated purpose or general purpose device.
Dedicated purpose configurable controllers normally have standard programs and are furnished with read only memory (ROM) or programmable read only memory (PROM.)
General purpose controllers often accommodate a variety of individual custom programs and are supplied with field-alterable memories such as electrically erasable,programmable, read only memory (EEPROM) or flash memory. Memories used to hold the program for a controller must be nonvolatile, that is, they retain the program data during power outages.
Fig. 3. Microprocessor Controller Configuration for Automatic Control Applications.
All input signals, whether analog or digital, undergo conditioning (Fig. 3) to eliminate the adverse affects of contact bounce, induced voltage, or electrical transients. Time delay circuits, electronic filters, and optical coupling are commonly used for this purpose. Analog inputs must also be linear zed, scaled, and converted to digital values prior to entering the microprocessor unit. Resistance sensor inputs can also be compensated for lead wire resistance...
Performance and reliability of temperature control applications can be enhanced by using a single 12-bit A/D converter for all controller multiplexed inputs, and simple two-wire high resistance RTDs as inputs.
A/D converters for DDC applications normally range from 8 to 12 bits depending on the application. An 8-bit A/D converter provides a resolution of one count in 256. A 12-bit A/D converter provides a resolution of one count in 4096. If the A/D converter is set up to provide a binary coded decimal (BCD) output, a 12-bit converter can provide values from 0 to 999, 0 to 99.9, or 0 to 9.99 depending on the decimal placement. This range of outputs adequately covers normal control and display ranges for most HVAC control applications. D/A converters generally range from 6 to 10 bits.
The output multiplexer (Fig. 3) provides the reverse operation from the input multiplexer. It takes a serial string of output values from the D/A converter and routes them to the terminals connected to a transducer or a valve or damper actuator.
The communication port (Fig. 3) allows interconnection of controllers to each other, to a master controller, to a central computer, or to local or portable terminals.
Types of controllers:
DDC can be designed for system level or zone level control like that shown below.
Zone level
Zone-level controllers can be applied to a variety of types of HVAC unitary equipment. Several control sequences can be resident in a single zone-level controller to meet various application requirements. The appropriate control sequence is selected and set up through either a PC for the system or through a portable operator's terminal. The following two examples discuss typical control sequences for one type of zone-level controller used specifically for VAV air terminal units.
VAV sequence of operation
In a pressure independent VAV cooling only air terminal unit application the zone-level controller controls the primary airflow independent of varying supply air pressures. The airflow set point of the controller is reset by the thermostat to vary airflow between field programmable minimum and maximum settings to satisfy space temperatures. On a call for less cooling, the damper modulates toward minimum. On a call for more cooling, the damper modulates toward maximum. The airflow control maintains the airflow at whatever level the thermostat demands, and holds the volume constant at that level until a new level is called for. The minimum airflow setting assures continuous ventilation during light loads. The maximum setting limits fan loading, excessive use of cool air, and/or noise during heavy loads.
System-Level Controller
System level controllers have more capacity and are more flexible then zone level controllers. System level controllers are used in central chiller and boiler plants, equipment rooms, and built up air handlers. Control sequences usually contain customized programs written to handle the specific application. The application of the controller must allow both the number and mix of inputs and outputs to be variable. The number of inputs and outputs required for the system level controller is usually not predictable.
Programming a DDC Loop
Most DDC systems use tables similar to the one shown below to set up a loop control for each part of the system. From this diagram you can see the loop is separated into several sections. The first section is the controller, which takes in a set point from an operator /programmer. It also takes in a feedback signal from a sensor and sends it to a control algorithm, which compares it to the set point. Any error found is the difference between the set point and the signal from the sensor also called the process variable. The output signal from this area goes through the digital/analog convertor. The corrective signal is sent as an analog signal from the controller to the final control element. (Chilled water/hot water valve etc.) For example, this loop could be controlling the temperature of a hot deck by modulating a hot water valve. The sensor could be an RTD thermostat, which would send back a feedback signal to the controller so the actual temperature in the hot deck can be compared to the set-point. If the set-point is 80* and the RTD says its 78* the controller would determine that the system needs to be warmer.
Simple Control Loop.
In Electronic # 6 we will talk about sequence of operations of various applications and web sites you can look up the gain more knowledge on DDC Controls. In the mean time: Be happy in your work and learn a lot!
copyright(c)2009
Roger J. Desrosiers
About the Author: Roger is a contributing faculty member of HVACReducation.net He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.
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