Friday, December 25, 2009

Gas Furnace Troubleshooting Tips

by Phil Rains

As technicians, during colder months you will be involved with troubleshooting gas furnaces that are not performing as expected or desired. There are several troubleshooting tips that will assist you in assuring the furnace works correctly. We will discuss a few of these in this article. Always follow the furnace installation and operation instructions.

First, you must assure that the furnace is the correct size for the structure heat loss load. This step is often overlooked since the furnace is already installed and not working correctly. But, you should make sure that the furnace is sized to provide 100 percent of the design heating load requirement plus any margin that occurs because of furnace model size capacity increments. Heating load estimates can be made using approved methods available from several sources like the Air Conditioning Contractors of America (Manual J). Most furnaces are of sufficient output capacity to meet most loads if installed by competent technicians, but, always determine if you have enough output capacity with the furnace you are troubleshooting to meet the structure heating needs. Also, excessive oversizing of the furnace could cause the furnace and/or vent to fail prematurely. Never oversize the furnace greater than 140% of the heat loss load.



Always verify several visual aspects of the installation (that are commonly performed during gas furnace start up procedures) such as: type of fuel, power supply, control board connections and fuses, gas supply piping size and installation, and venting size, material and condition. Use the furnace installation and operation instructions as reference for all these checks. Make corrections as necessary.
Check the gas piping system from the gas meter to the furnace connections. All gas piping fittings and connections should be checked by an approved method such as a combustible gas leak detector, soap and water, or an equivalent non-flammable solution. NEVER check for gas leaks with a match or an open flame of any kind. Repair or replace any part(s) of the system that may be found defective.



Check inlet pressure for proper operation. First, make sure that the power is off, turn off the gas supply valve at the furnace and remove the plug from the line inlet pressure tap (usually on the gas valve). Connect a manometer or a “Magnehelic” gauge to the inlet pressure tap, turn on the gas supply and read the pressure on the gauge or manometer (without the furnace operating). The results of this test will be the most accurate when all other gas appliances are operating. Check with the equipment distributor or the installation and operation instructions for the particular furnace you are working on to determine the recommended supply pressures as well as the minimum and the maximum supply pressures for Natural and L.P. gas. Adjustments cannot typically be made to inlet pressure. If modifications are necessary, contact the gas supplier.



Check outlet manifold pressure for proper operation. First, make sure that the power is off, turn off the gas supply valve at the furnace and remove the plug from the gas valve outlet pressure tap (manifold pressure tap). Connect a manometer or a “Magnehelic” gauge to the pressure tap, turn on the gas supply and energize the furnace calling for heat. After the furnace fires read the pressure on the gauge or manometer. The results of this test will be the most accurate when all other gas appliances are operating. Check with the equipment distributor or the installation and operation instructions for the particular furnace you are working on to determine the recommended supply manifold pressure(s) as well as the minimum and the maximum manifold pressures for Natural and L.P. gas. You can typically make adjustments to manifold pressure on the gas valve as necessary by following the installation and operation instructions.



You may have to check and/or change orifice sizes and possibly replace them. Always pay attention to the possibility of electrical shock, fire or explosion possibilities. Failure to properly install orifices could result in death, personal injury and/or property damage. Turn off electrical power (at the disconnect) and the gas supply (at the manual shutoff valve in the gas line) when installing orifices. Changing orifices will require disconnecting the gas line from the gas valve, removing the manifold from the furnace, removing the orifices from the manifold, and replacing them with the properly sized orifices. Then you tighten the new orifices so they are seated and gas tight, and then reinstall the manifold and re-attach the gas line.



Adjust the pilot burner (if so equipped). If the furnace has a pilot flame to light the main burner, the flame should surround 3/8” to1/2”of the flame rod. To adjust remove the cap from the pilot adjusting screw on the gas valve and turn counterclockwise to increase or clockwise to decrease flame (in most applications) as required. Replace the cap after adjusting the screw. Always follow the furnace installation and operation instructions.

Perform a main burner check. Allow the furnace to run approximately 10 minutes then inspect the main burner flames. Check for stable and blue flames. Dust may cause orange tips or wisps of yellow, but the flames must not have solid, yellow tips. Flames should extend directly from the burner into the heat exchanger, and should not touch the sides of the heat exchanger. If any problems with the main burner flames are found, it may be necessary to re-adjust gas pressures, or check for drafts.


When troubleshooting gas furnaces, you will be dealing with three areas of concern that all can have individual problems, and affect each other. These three areas are: air flow, fossil fuel, and electricity. We have discussed simple checks for gas and electrical already.

For air flow, perform a temperature rise check and determine the actual air flow (Cubic Feet per Minute) crossing the heat exchanger during heating operation. Temperature rise can be checked by placing a thermometer in the return air duct within 6 inches of the furnace. Place a second thermometer in the supply duct at least two feet away from the furnace to prevent any false readings caused by radiation from the furnace heat exchanger. Make sure that the filter is clean and that all registers and/or dampers are open.


Operate the furnace for 15 minutes before taking temperature readings. Subtract the return air temperature from the supply air temperature. The result is the temperature rise. Compare the result with the allowable rise listed for the model (size) you are checking in the installation and operation instructions or the unit data plate. The speed is set at the factory but it may sometimes be necessary to adjust the speed selection. The speed taps that the manufacturer sets from the factory are typically based on a nominal 400 CFM per ton cooling and the basic mid range on the temperature rise for heating. If the rise is not within the specified range, it will be necessary to change the heating blower speed. If the rise is too high, it will be necessary to increase the blower speed. If the rise is too low, it will be necessary to reduce the blower speed.


Since the manufacturer cannot establish the static pressure that will be applied to the unit, it is the responsibility of the technician to select the proper speed taps on PSC blower motors for the application when the unit is installed, or when servicing in the field.

In most newer model furnaces if it is necessary to change the blower speed, the adjustments can usually be easily made on the Fan Control Board. Many of today’s furnaces have integrated furnace controls of some kind and this feature will have a position for all blower speeds and you must simply relocate the heating speed wire for the blower motor, or adjust switch positions.

Determine the CFM of the running furnace in the heating mode. Using the thermometers already inserted to check temperature rise, calculate the CFM with the following formula. The most important part of CFM determination is that the temperature rise is within the acceptable range with the CFM moving. If you are unsure, typically an airflow setting that will allow a rise between 40° and 70° will usually be acceptable. Ideally, 3 degrees above mid-point is recommended for optimum performance.









Many furnace manufacturers have modified the ignition systems to achieve higher efficiencies expressed in Annual Fuel Utilization Efficiency (AFUE). New types of ignition systems have and are being developed to meet the following needs: higher efficiency requirements of the government , conservation of fossil fuels, convenience in remote locations, less annoying shut downs, greater electronic control use, and higher safety ratings.

Typical ignition systems now include: standing pilot, spark to pilot, hot surface to pilot, and hot surface ignition.

Upon initial start-up the heat to light the burners (direct ignition) or the pilot (intermittent ignition devices) may be provided by either a spark device or a hot surface device. After lighting the pilot or the burners the heat is provided by the burning of the fuel gas. The fuel used in the furnace may be either Natural Gas or LP gas. Most furnaces are shipped set up for natural gas and must be converted for LP gas operation. The fuel is delivered to the burners through the supply gas piping to a Combination Gas Valve (combination in that it contains the flow regulating feature as well as a pressure regulator), to the burner orifices and then to the burner itself. The oxygen (air) needed for combustion is supplied by the operation of the vent motor assembly (also referred to as the combustion blower motor or exhaust blower). A negative pressure is created in the heat exchanger by the operation of the combustion blower and the positive pressure of the air surrounding the furnace pushes air into the burner box where it is drawn into the burner inlet (primary air) and mixes with the flame at the burner outlet to provide complete combustion (secondary air).

The best way to troubleshoot ignition systems is to determine which type the furnace has and follow the appropriate installation and operation criteria for the furnace you are dealing with. Most will include detailed troubleshooting steps for the ignition system installed. If not available, contact the equipment distributor or the manufacturer for more information. We will discuss troubleshooting some of these systems in future articles.

Phillip A. Rains
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.
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