Theory and troubleshooting using short modules packed with images to make learning quick and easy.
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)
Sheet metal box connected directly to the supply outlet of the HVAC appliance that directs the heated or cooled air to the ductwork.
Most units have multiple, or at least one return duct in the house moving conditioned room air to the HVAC unit. When air is forced through the supply ductwork, conditioned room air gets pulled throughout the structures return ductwork into the central return air plenum to be moved through the appliance to be heated or cooled.