How to Choose a Heat Pump Thermostat
Heat pump control is where generic smart thermostats usually fail. Three things decide whether a control will work: the reversing valve convention, the number of stages, and how auxiliary heat is handled.
B and O terminals
A heat pump reversing valve is energised either in heating or in cooling, depending on the manufacturer. A control with a single combined terminal only covers one convention. Every heat pump model in our range provides separate B and O terminals, so either convention wires without an external relay.
Counting the stages
A 2 heat 1 cool control covers a heat pump with one stage of auxiliary heat. Buildings with two compressor stages and auxiliary heat need 3 heat 2 cool, which our universal models such as STN855 and STN805 provide.
Controlling auxiliary heat
Auxiliary electric heat is the expensive part of running a heat pump. Our heat pump models offer a second stage swing adjustment set separately from the first, so the backup only runs when the compressor genuinely cannot hold the room.
Compressor protection
After a power interruption a compressor restarted immediately can be damaged. A selectable 5 minute compressor delay is available across the heat pump range and should normally be left on.
Emergency heat
Where the specification calls for a dedicated emergency heat position, the S725 carries an E terminal alongside RH, RC, G, Y, C, O, B and W.
Questions On This Topic
What happens if I wire O instead of B?
The valve energises in the wrong season, so the system heats when it should cool. Check the equipment documentation and use the matching terminal.
Can one thermostat cover heat pump and conventional systems?
Yes. Our universal models cover up to 2 heat 2 cool conventional and up to 3 heat 2 cool heat pump, selected in the installer menu.
Should the compressor delay ever be switched off?
Rarely. It exists to protect the compressor and adds at most a five minute wait after an interruption.
Need This Checked Against Your System?
Send the equipment details on WhatsApp and we will confirm which of our models matches before you order.