Originally Posted by phuz
I am trying to get some detailed info as to the inner workings of the heat pump controls. I will be removing my thermostat and doing all my HVAC control via an Allen Bradley PLC. I know the controls at the thermostat are just dry contacts, so this is going to be a pretty easy cut-over.
My main question is about the defrost cycle. I believe the thermostat has nothing to do with that, and it is, in fact, controlled by the main board in the air handler panel, and for this case, I won't even need to worry changing anything.
I am mostly trying to gain better control of when the heat comes on and when the auxiliary (electric) heat strips kick on. Going forward, I will be able to control/program a schedule to hopefully make the system more light on the electric bill. I just don't want to take away any of the important functionality such as the defrost cycle.
Thanks in advance.
The defrost is controlled entirely from you heat pump outside. When the thermostat is calling for heat and the temp sensor is below a set temp a timer runs on your defrost board. Every so many minutes of runtime below the temp sensor setpoint the unit defrosts. The electric heat being turned on etc is all controlled from outside & has nothing to do with the stat on any system i've ever seen.
The stat only controls the reversing valve (except for defrost, then it's controlled from the defrost board), the heat pump turning on/off and the aux heat turning on/off.