Quick answer
You need a new thermostat with a heat pump if the old one has no O/B terminal for the reversing valve, no AUX or W2 terminal for backup heat, or no C-wire. A basic furnace and AC thermostat fails that test. Dual-fuel systems also need a control that sets the switchover temperature, and inverter systems from Mitsubishi, Daikin and Carrier often require the manufacturer's own controller.
Do I need a new thermostat with a heat pump? If the one on your wall was installed for a furnace and air conditioner, probably yes, and its back plate will tell you in a minute. A heat pump heats and cools with the same outdoor unit, so the thermostat has to tell it which direction to run and when to bring on backup heat. A conventional thermostat has no terminal for either instruction.
The three connections a heat pump thermostat needs
- O/B, the reversing-valve terminal. It switches the outdoor unit between heating and cooling. Most brands energise the valve in cooling (O); Rheem and Ruud energise it in heating (B). Set the wrong way, the system cools when you ask for heat.
- AUX or W2, and E on some models: auxiliary heat. This brings on the electric heat strips or the furnace when the heat pump cannot keep up or is defrosting. Left unwired, the house falls behind on the coldest nights and the vents blow cool air during defrost.
- C, the common wire. Heat pump thermostats switch more circuits and nearly all need steady 24-volt power. Many houses built before the 2000s have only four conductors in the wall, already used by R, W, Y and G.
Pull the thermostat off its base and count. Wires on R, W, Y and G only means the thermostat cannot run a heat pump, and the cable needs a spare conductor or a new run from the air handler. A thermostat that is already heat-pump capable says so in its installer menu, where the equipment type can be changed from conventional to heat pump.
Dual-fuel systems: what decides when the gas furnace takes over
On a hybrid system the control needs one more ability: locking out the heat pump and starting the gas furnace below a set outdoor temperature, without ever running both together. That takes an outdoor temperature reading, from a wired sensor at the outdoor unit or an internet weather feed, and a dual-fuel setting. The switchover point is chosen from the heat pump's capacity in the cold and the price of gas against electricity, so it is set at commissioning and not left at a factory default; the glossary explains the dual-fuel changeover temperature.
A thermostat without dual-fuel logic treats the furnace as ordinary auxiliary heat and can fire it on top of the heat pump. The heat pump's indoor coil sits in the furnace's hot supply air, so running both overheats the refrigerant circuit and trips it out. Some systems handle the switchover in the furnace control board or a separate dual-fuel kit, and then a simpler thermostat is fine. Your installer should be able to say which device is doing that job.
Inverter and communicating systems that need their own controller
- Mitsubishi: ducted and ductless units talk to Mitsubishi's own wall or wireless controllers. A conventional 24-volt thermostat works only through a thermostat interface adapter, and then the unit receives on and off calls in place of the room data it uses to modulate smoothly.
- Daikin: inverter models pair with Daikin's communicating thermostat. Conventional thermostats work on some models and through an adapter on others, with coarser control.
- Carrier Infinity: the furnace or fan coil, the outdoor unit and the wall control share a data connection over four wires. A Nest or ecobee cannot be dropped in without giving up the communicating features.
The rule: if the indoor and outdoor units are joined by a communicating cable instead of conventional thermostat wires, keep the manufacturer's control. If you want a particular smart thermostat, say so before the equipment is chosen. Models that suit conventional heat pumps are compared in best thermostats for heat pumps.
What a thermostat change adds to a heat pump install
The thermostat is normally a line in the heat pump quote and should be named there by model. A 3-ton cold-climate heat pump installs for $9,500–$15,000, and the control is a small part of that; the labour question is the cable. Smart thermostats can qualify for a rebate through the Home Renovation Savings Program: see the smart thermostat rebate page for the current amount.
ZK Mechanical wires and sets up thermostats for conventional, dual-fuel and communicating heat pumps, including the O/B setting, auxiliary lockouts and switchover temperature. For smart thermostat installation in Burlington, Oakville, Milton, Hamilton or Mississauga, call (647) 801-1252 or book online. Further buying decisions are covered under costs and buying questions.