Theory and troubleshooting using short modules packed with images to make learning quick and easy.
The potential relay is used in all capacitor start/capacitor run motors. They’re also used in capacitor start/induction run motors. However, it’s rare to see that style of a motor in residential and lite commercial in our industry.
The potential relay may also be called a start relay. Without it, the start capacitor could explode or destroy the windings in the motor within seconds. Using a start capacitor with a potential relay is known as a hard start kit.
There are two types of capacitors you’ll encounter in HVAC field. We have the run capacitor and the start capacitor. Both are measured in microfarads which you will see labeled as uf, µF, or mfd.
Capacitors are used only in single-phase motors. You will not see a capacitor in a three-phase unit.
A thermocouple is a type of flame safety that prevents the gas valve from dispensing the gas into the burners without the presence of a flame. These are found mostly in older standard pilot appliances including (mostly older) furnaces, water heaters, and unit heaters. A typical thermocouple will produce upwards of about 30 millivolts. To give you an understanding of how small of a voltage that is, 1,000 millivolts is 1 volt.
Above images are two of the more common high limits found in residential and lite commercial furnaces
A high-limit switch is a bi-metal snap disc switch that warps when heated to break a heat cycle and used to protect the heat exchanger from overheating and burning out due to insufficient indoor air passing over it. A dirty filter or squirrel cage, undersized ductwork, or many supply air vents closed are among some of the reasons to set this switch off.
Flame rollout switches are mounted around your burners. They exist to sense when/if the flame were ever to pose a risk to the homeowner.
These are usually bimetal switches that warp when heated to break the heat cycle when they sense a flame rollout situation. They’re traditionally manually reset by pressing the button in the middle, although some use a heat-activated fuse which would mean you’d need to replace the whole part if it were ever to sense rollout.
Furnace control boards can be very intimidating for newer technicians without much experience working with them. They’ll typically have many wires running to/from them, flashy lights, bulky built-in relays, resistors, and capacitors. It all looks very confusing, but it’s really not.
Ignition control boards are typical in older systems. They control the gas valve, igniter, flame sensor, and the damper (when used). With a call for heat, the board will start the sparker and light the pilot. When it senses the pilot, it will continue to light the burners.
There are many different kinds of burners. We’re mainly going to focus on the inshot type burner as seen above. The inshot burners are in most modern furnaces. A combustion blower is used to pull the flames through the burner tubes into the heat exchanger.
Burners are used to prepare the air:gas mixture for combustion. Gas is shot through one side, and as gas flows through at a relatively high speed, it pulls primary air into the burners with it. The quick flow of gas pulling air in with it is called the venturi effect.
The igniter is what’s going to ignite the gas in the combustion chamber.
Two of the most common igniters are the silicon carbide and silicon nitride.
Silicon carbide is among the older of the hot surface igniters. They come in different styles, and most of them run off 120v. They are very fragile and crack easily if not handled carefully. There’s a myth that the oils in your skin will cause damage. It’s not true.
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.