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.

Sunday, December 20, 2009

Capacitor Start – Capacitor Run Motor

by Roger Desrosiers

Capacitor Start-Capacitor Motors or (CSR) are used almost exclusively on Hermetic and Semi-Hermetic motors compressors. Rarely will this motor be used on an open type motor because of the cost of the components necessary to produce it. Most open–type motors do not use a starting relay, but use a centrifugal switch instead. Open type motors are usually built as permanent split-capacitor or capacitor start motors.


The CSR motor begins operation on a phase displacement between the start winding and the run winding, which allows rotation to begin. The run capacitor adds a small amount of starting torque but its main function is to increase the running efficiency of the motor. The run capacitor is wired in the circuit to provide the most efficient phase angle between the current and voltage when the motor is running. The run capacitor is in the circuit any time the motor is running. Both the start and run capacitors are wired in series with the start winding but are in parallel with each other. Figure 1 shows a schematic diagram of the motor with the starting components.

Sunday, December 13, 2009

Gas Furnace Start Up Tips

by Phil Rains

As technicians, during colder months you will be involved with the start up of gas furnaces for proper heating of homes. This will involve both visual inspections and effective start up procedures. We will discuss a few of these in this article.

Prior to the start-up of a gas furnace there are several things that you should verify. These are usually items that can be accomplished by visual inspection.

First, verify the type of fuel being utilized. As simple as this may sound, it is an important item to check. The furnace nameplate will typically detail the type of fuel that should be used. Almost all manufacturers today will produce furnaces that are set-up to use natural gas. These furnaces can easily be converted to LP gas operation by installing the correct LP conversion kit. Part of this kit may be a sticker or a new rating plate that identifies the converted furnace as an LP gas furnace.

Sunday, December 6, 2009


by Roger Desrosiers

Many refrigeration compressors serviced today have positive-displacement oil pumps to help lubricate the internal compressor parts. Most compressors that have positive oil pumps also have a control that senses oil pressure and acts as a safety device whenever the oil pressure falls below a certain threshold level.

It is the action of the oil safety control we will discuss in this article.

There are several types of oil safety control devices on the market today. The two basic controls we are most familiar with are the mechanical differential control and the pressure-sensing electronic control.

The mechanical control uses tubing that senses the suction pressure of the compressor and the outlet oil pressure of the pump. The electronic control has a special pressure sensor that mounts in the outlet of the pump and connects only with an electrical cable.