Why the heat pump or conventional system type setting matters
The system type setting tells the thermostat what equipment it controls: a conventional system such as a furnace, a heat pump, radiant heat, or cooling only. Everything else follows from it, including which modes appear, how the thermostat switches between heating and cooling, and whether compressor and backup heat settings exist. If it is wrong, the equipment is controlled incorrectly.
What does the system type setting tell the thermostat?
It tells the thermostat what kind of heating and cooling equipment is on the other end of the wires. A thermostat cannot see the equipment. It only knows what it was told during setup, and it uses that answer to decide which signals to send, which modes to offer and which settings to show you.
On a Honeywell Home T6 Pro Programmable, this is installer option 200, "Heating System Type", with four choices: Conventional Forced Air Heat, Heat Pump, Radiant Heat, or None (Cool Only). The installation guide shows Conventional Forced Air Heat as the factory value. A second option, 205, narrows it down to the equipment itself. For a heat pump that means "Air to Air Heat Pump" or "Geothermal Heat Pump"; for conventional forced air it includes choices such as oil or electric forced air.
Google says the same about Nest thermostats: during setup the thermostat asks what type of system you have and the fuel it uses, and if you make a mistake, the thermostat won't control your system correctly.
Why does a heat pump need its own setting?
A heat pump heats and cools with the same compressor, and it switches direction with a reversing valve. A furnace and an air conditioner are two separate machines. That one difference changes several things the thermostat has to handle:
- The changeover signal. The thermostat must tell the heat pump which direction to run. The T6 Pro asks this in option 218, "Reversing Valve O/B", with O (energized in cool) as the factory value and B (energized in heat) as the alternative. On a Nest, Google calls it the heat pump orientation.
- Backup heat. The T6 Pro asks how many stages of backup heat the heat pump has (option 221). The answer decides whether auxiliary heat (AUX), emergency heat and the AUX tuning options exist at all.
- Compressor timing. Because the compressor heats as well as cools, compressor protection applies in winter on a heat pump. Honeywell's T6 Pro manual says the display flashes "Cool On", or "Heat On" if you have a heat pump, while that delay runs.
- Cycle rate. Honeywell's T6 Pro has separate cycle rate options for compressor stages and for heating stages, so the cycles per hour (CPH) defaults follow the equipment type.
What goes wrong when the setting does not match the equipment?
The thermostat then behaves correctly for equipment you do not have. These are the symptoms manufacturers document:
| What you notice | Setup cause the manufacturer gives | Manufacturer |
|---|---|---|
| Cool air in heat mode, warm air in cool mode | Heat pump orientation set to O when the equipment needs B, or the reverse | Google Nest |
| Heat pump keeps running when the app says the system is off | Reversing valve activation set to seasonal when the equipment needs each cycle | Google Nest |
| Brief burst of cold air at the end of a heating cycle, or a loud whoosh | Activation set to each cycle when the equipment needs seasonal | Google Nest |
| Heat Pump Balance missing | Thermostat not set up as a heat pump with AUX heat, or set up as dual fuel | Google Nest |
| No emergency heat mode | Thermostat not configured for a heat pump with backup heat | Honeywell Home (T9) |
| AUX or compressor thresholds missing | Thresholds shown depend on the equipment configuration | ecobee |
The first row is the most common complaint, and it has its own page: heat pump blowing cool air while the thermostat says heat.
Who should change it?
The person who installed the thermostat, or a licensed HVAC technician. The setting is part of installer setup on Honeywell Home thermostats, and ecobee keeps its equipment thresholds on the thermostat only, with a warning to contact ecobee Support first if you are unfamiliar with your equipment. Google does give Nest owners the orientation steps in its heat pump troubleshooting article, but it also says to contact a local professional if those do not solve the problem.
What you can safely do yourself is notice the clues. If your thermostat offers emergency heat but you have a furnace and a separate air conditioner, or offers nothing about AUX when you know you have a heat pump with backup heat, the setup answers are worth questioning.
How does the system type affect day-to-day settings?
Quite a lot. A heat pump thermostat gains controls a furnace thermostat never shows: outdoor lockouts, AUX timing, and on a Nest, Heat Pump Balance. Google says forced air systems without a heat pump get Early-On instead. For a side-by-side list of what changes, see heat pump vs furnace thermostat settings. If the thermostat is set up correctly and you are only seeing emergency heat (EM heat) on the screen, see why a thermostat says emergency heat.
Frequently asked questions
- How do I know which system type my thermostat is set to?
- Look at what it offers. An "Em Heat" or emergency heat mode, Heat Pump Balance on a Nest, or AUX settings all mean it is set up as a heat pump with backup heat. A flashing "Heat On" during the compressor delay on a Honeywell Home T6 Pro also points to a heat pump.
- Can I change the system type myself?
- On most thermostats it sits in installer setup, and getting it wrong changes how the equipment is driven. Google says a Nest that has the wrong system information will not control your system correctly. If you think the setting is wrong, ask the installer or a licensed HVAC technician to check it.
- What is O or B on a heat pump setting?
- It tells the thermostat how the heat pump switches between heating and cooling. Some equipment expects the signal in cooling (O), some in heating (B). Get it backward and Google says the heat pump blows cool air in heat mode and warm air in cool mode.
- What is a dual fuel system?
- A heat pump paired with a separate indoor heater, such as a furnace, as backup. Google says Nest thermostats treat that backup as alternate heat that never runs while the heat pump is heating, and dual fuel systems cannot use Heat Pump Balance.