Theory and troubleshooting using short modules packed with images to make learning quick and easy.
Fan center controls generally are found in older standard furnaces. These are added to separate the heating side from the air conditioning side and added while air conditioning is installed. Newer furnaces have everything controlled via the control board.
Older atmospheric furnace fans are controlled by temperature and a temperature-controlled fan won’t work for air conditioning since the fan needs to run as soon as air conditioning is triggered on.
The plastic orifice shown in the image above is a snubber. Sometimes the appliance will experience sudden pressure changes such as wind against the exhaust or the burners igniting. The snubber is designed to only allow a designed amount of pressure to be pulled against the switch at one time.
The insert should not be removed to make the appliance function. If you remove the insert and the appliance works, you have another larger problem such as a plugged intake, plugged exhaust or drain.
Thermistors are used to sense/measure temperature. You will find them in ductless splits to detect evaporator coil or condenser temperature, temperature measuring devices such as a digital thermostat, ambient air temperature, PTC start relay, and supply or return air temperature. The name thermistor is a combination of “thermal” and “resistor.”
Improper condensate trapping can cause a lot of property damage. It’s necessary that a drain line is installed correctly to avoid damage to a residence or business.
Evaporators coils are installed to allow a blower to either push or pull air across the coil. The pressure, either positive or negative applied across the coil is also the same applied pressure to the drain line connected to the drain pan of the evaporator coil.
The thermostat is the command center for an HVAC system. The thermostat is what will switch the equipment on and off. Most people are familiar with the thermostat on the wall for their heating and cooling, but they’re in almost everything from water heaters to freezers and kitchen equipment.
Modern thermostats sense temperature using a thermistor. The thermistors resistance value changes depending on the surrounding temperature. A small microcontroller converts the measured resistance to temperature and decides based on desired settings if heating or cooling should be switched on.
You’ll run into these terminals commonly on older standing pilot furnaces. TH/TR is a common connection, but that doesn’t explain much in itself.
We’ll overview which each of these terminals is intended for:
TH – The 24v hot leg from the thermostat on a call for heat (R+W closing on the thermostat) to the gas valve (TH terminal) to open the solenoid to allow gas to flow, assuming the transformer is working and the high limit is closed (allowing for the flow of electricity)
The reversing valve is the heart of a heat pump’s ability to provide year-round comfort. It is the component responsible for seasonal switching, physically redirecting refrigerant flow to turn an air conditioner into a heater. For many technicians, however, the reversing valve is a source of frustration. It is often misdiagnosed and condemned prematurely when the real issue lies in the electrical system or the refrigerant charge. Understanding exactly how these valves function is the first step toward faster, more accurate service calls.