1. How a heat pump heats and cools
From outside, a central air conditioner and an air-source heat pump look almost the same: an outdoor unit with a compressor, a coil and a fan. The difference is that a heat pump can run in both directions, and that is what adds wires and settings at the thermostat.
An air conditioner only moves heat one way: out of the house. In winter it sits idle while a furnace or electric air handler does the heating.
A heat pump uses the same refrigeration cycle but can reverse it. In winter it absorbs heat from outdoor air, even below freezing, compresses the refrigerant to raise its temperature, and releases the heat indoors. Because it moves heat rather than making it, it can deliver several times more heat than the electricity it uses, at least in mild weather.
Efficiency: coefficient of performance (COP)
Heat pump efficiency is measured as the coefficient of performance (COP):
COP = heat delivered ÷ electricity used
For comparison:
- Electric heat strips or baseboards: COP of 1.0. Every 1,000 watts of electricity gives 1,000 watts (about 3,412 BTU per hour) of heat.
- High-efficiency gas furnace: 95 to 98% AFUE, meaning 95 to 98% of the fuel’s energy ends up as heat in the home.
- Modern heat pump in mild weather (around 45°F): COP of roughly 3 to 4, and lower as it gets colder. For every 1,000 watts of electricity it uses, it delivers about 3,200 to 4,200 watts of heat (roughly 11,000 to 14,300 BTU per hour), most of it moved in from the outdoor air.
That is why a heat pump can heat for much less than electric baseboards or propane in mild weather, and why the thermostat’s job is to keep the heat pump doing as much of the work as possible and bring in backup heat only when needed.
2. The reversing valve: O or B
The part that makes a heat pump reversible is the reversing valve in the outdoor unit. It sends the hot gas from the compressor to the outdoor coil in cooling, and to the indoor coil in heating.
A 24 V solenoid moves the valve, and the thermostat controls it through the O/B terminal. Manufacturers don’t agree on which mode powers the solenoid, which is why thermostats have an O or B setting.
O (energized in cooling) vs. B (energized in heating)
| Setting | When the valve is powered | Unpowered position | Typical brands | What to select |
|---|---|---|---|---|
| "O" Setting | In cooling | Heating | Most brands, including Carrier, Bryant, Trane, American Standard, Lennox, Goodman and Amana | O (sometimes shown as “energized in cool”) |
| "B" Setting | In heating | Cooling | Mainly Rheem and Ruud | B (sometimes shown as “energized in heat”) |
The brand on the outdoor unit is a guide, not a guarantee: the installation manual for your model, or a quick test, settles it.
How to tell the setting is reversed
If the wiring is right but the O/B setting is wrong:
- In cooling mode the vents blow warm air.
- In heating mode the vents blow cool air.
No rewiring is needed: change the O/B setting in the thermostat’s equipment or installer settings, then allow up to five minutes for the compressor delay before testing again. Give heating a few minutes before judging; a heat pump takes a little while to blow warm air.
3. Auxiliary heat vs. emergency heat
Auxiliary heat (AUX) and emergency heat (E) usually run the same electric heat strips, but they are used very differently, and mixing them up can be expensive.
Why heat pumps need backup heat
The colder it is outside, the less heat an air-source heat pump can pull in, while the house loses heat faster:
- A standard (non-cold-climate) heat pump can lose roughly half of its heating output between 47°F and 17°F, the two temperatures manufacturers publish ratings at. Cold-climate models hold their output much better.
- At the same time, the home’s heat loss rises roughly in step with the temperature difference between indoors and outdoors.
Below the temperature where the two lines cross, the thermal balance point, the heat pump can’t hold the setpoint on its own, and backup heat has to make up the difference.
Auxiliary heat (AUX / W2)
Auxiliary heat is automatic. The heat pump keeps running, and the thermostat also switches on the AUX / W2 output to add the electric heat strips in the air handler until the house catches up. (On a dual-fuel system the backup is a furnace instead, and the rules are different: see section 5.)
The exact rules vary by brand and settings, but smart thermostats typically bring on aux heat when:
- The room is well below the setpoint, for example after you raise it by several degrees in the morning.
- The heat pump has run a long time without catching up.
- It is colder outside than the compressor lockout temperature you set, if you set one.
Most also let you set an outdoor temperature above which aux heat is not allowed. Our guide to aux heat running too much goes through those settings.
Emergency heat (E)
Emergency heat is a mode you turn on yourself when the heat pump can’t run. When it is on:
- The thermostat stops calling the compressor (Y), so the outdoor unit stays off.
- All heating comes from the backup source: heat strips, or the furnace on a dual-fuel system. Depending on the thermostat, that call goes out on E or on the aux heat wire.
Wiring AUX and E on a smart thermostat
Old heat pump thermostats often had a jumper between the AUX and E terminals, or separate AUX and E wires that end up on the same terminal at the air handler. On the thermostats we cover:
- ecobee Smart Thermostat Premium / Enhanced: no E terminal and no jumper. In heat pump mode, the auxiliary heat wire goes on W1 (a second stage on W2), and ecobee’s emergency heat mode uses that same aux output. If your E wire lands on the same terminal as AUX at the air handler, it can be capped.
- Google Nest Learning Thermostat: the aux heat wire goes on the W2/AUX connector. Nest also supports a separate E wire; the Nest app’s compatibility checker shows where it goes for your system.
- Honeywell Home T9: has separate AUX and E terminals. If you only have one wire for both, see the heat pump with emergency heat guide.
4. Defrost and backup heat
In heating mode, the outdoor coil runs colder than the outdoor air. In damp weather near or below freezing, moisture in the air freezes on it as frost.
Frost blocks airflow through the coil and cuts the heat pump’s output, so every heat pump has a defrost control that clears it automatically.
What happens during defrost
After a set run time (commonly 30, 60 or 90 minutes) when the coil is cold enough, or when sensors detect frost on “demand defrost” units, the defrost control:
- Stops the outdoor fan, so the heat stays in the coil.
- Switches the reversing valve to cooling, so hot refrigerant flows through the outdoor coil and melts the frost, usually within a few minutes. You may see steam rising from the unit.
- Would blow cool air indoors, because the heat pump is briefly cooling the indoor coil.
- Turns on backup heat: to avoid that, many defrost boards have a relay that sends 24 V to the air handler’s W2 / AUX backup heat input while defrost runs, through the wiring between the outdoor unit and the air handler. The heat strips (or furnace) warm the air until defrost ends.
5. Dual-fuel systems: never run both at once
In cold regions, many homes pair a heat pump with a gas, propane or oil furnace. This is a dual-fuel (or hybrid) system: the heat pump heats in milder weather and the furnace takes over when it gets cold.
The catch is where the indoor coil sits, which makes running both at the same time harmful to the equipment.
Why running both is a problem
In an electric air handler, the heat strips sit after the indoor coil, so they can run together with the heat pump. In a dual-fuel system it is the other way round:
- The furnace heat exchanger is in the furnace cabinet.
- The heat pump’s indoor coil usually sits on top of the furnace, so the air leaving the furnace passes straight through it.
Furnace air is typically well over 100°F leaving the heat exchanger. If the thermostat is set up as a heat pump with electric aux heat instead of dual fuel, it will treat the furnace as ordinary aux heat and keep the heat pump running (Y on) while the furnace fires (W on).
What happens then:
- In heating mode the indoor coil is where the refrigerant is meant to give up its heat and condense. Hot furnace air stops it doing that.
- Refrigerant pressure climbs until the heat pump’s high-pressure safety switch trips.
- Done repeatedly, this stresses the compressor and can shorten its life, and compressor replacements are expensive.
The fix: one or the other
A dual-fuel system must run either the heat pump or the furnace for heat, never both. (Some systems briefly allow both during defrost, under the equipment’s own control.)
Older installations did this with a relay kit (often called a fossil fuel kit) at the equipment. Thermostats that support dual fuel do it themselves once they are set up for it:
- Our wiring tool asks whether the backup heat is a furnace, and marks thermostats that don’t support dual fuel (such as the Amazon Smart Thermostat) as not compatible.
- Google Nest: during setup, tell it the backup heat is a gas or oil furnace (dual fuel). Nest switches from the heat pump to the furnace based on outdoor temperature and a balance point you can adjust.
- ecobee and Honeywell T9: in the equipment settings, set the heat pump’s auxiliary heat type to a fossil-fuel furnace (dual fuel), then set the outdoor temperature below which the compressor is locked out.
6. Thermal and economic balance points
On a heat pump or dual-fuel system, the switchover and lockout temperatures you set have a big effect on the heating bill. Set them too high and the backup heat runs more than it needs to; too low and the house can struggle to stay warm on cold mornings.
Two different balance points
- Thermal balance point: the outdoor temperature at which the heat pump’s output equals the home’s heat loss. For example, if a home loses 24,000 BTU/h at 32°F and the heat pump produces exactly 24,000 BTU/h at 32°F, the thermal balance point is 32°F. Below this temperature, the heat pump physically cannot keep up without help.
- Economic balance point (dual fuel only): the outdoor temperature at which heat from the heat pump costs the same as heat from the furnace. It depends on your electricity and fuel prices.
Working out the economic balance point
Compare the cost of 100,000 BTU of heat delivered by each:
Heat pump cost per 100,000 BTU = 100,000 ÷ (3,412 × COP) × electricity price per kWh
Furnace cost per 100,000 BTU = gas price per therm ÷ AFUE
Worked example (illustrative prices; use your own bills):
- Electricity: $0.13 per kWh. Natural gas: $1.40 per therm. Furnace: 96% AFUE.
- Furnace: $1.40 ÷ 0.96 = $1.46 per 100,000 BTU.
- Heat pump at 40°F (COP 3.4): 100,000 ÷ (3,412 × 3.4) × $0.13 = $1.12.
- Heat pump at 25°F (COP 2.2): 100,000 ÷ (3,412 × 2.2) × $0.13 = $1.73.
- Break-even COP: 100,000 ÷ 3,412 × $0.13 ÷ $1.46 ≈ 2.6.
With these prices, the heat pump is cheaper whenever its COP is above about 2.6. If its COP falls roughly in a straight line from 3.4 at 40°F to 2.2 at 25°F, that happens at about 30°F, so a switchover near 30°F would minimise cost, provided the heat pump can still heat the house at that temperature (the thermal balance point). With cheaper gas or pricier electricity the break-even COP rises and the switchover moves up. Your heat pump’s published performance data gives its real COP at each temperature.
7. Heat pump support by thermostat
What each thermostat we cover supports on a heat pump, from the manufacturers’ published specifications:
| Thermostat | Heat pump stages | Dual fuel | Separate E terminal | Power |
|---|---|---|---|---|
| Google Nest Learning (4th Gen) | Up to 2 compressor stages + 2 aux heat stages | Yes | Yes | Requires C-wire or Google Nest Power Connector. Google lists heat pumps among the systems that need one. |
| ecobee Smart Thermostat Premium | Up to 2 compressor stages + 2 aux heat stages | Yes | No (emergency heat uses the aux output) | C wire needed. Power Extender Kit (PEK) included in the box for 4-wire setups. |
| Honeywell Home T9 | Up to 2 compressor stages + 1 aux heat stage | Yes | Yes | C wire needed, or the Honeywell wire saver (K terminal). |
| Amazon Smart Thermostat | 1 compressor stage + 1 aux heat stage | No (Amazon lists dual fuel as not supported) | No | C wire needed, or Amazon’s C-wire adapter. |
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8. Testing and troubleshooting
If a heat pump doesn’t behave after a thermostat swap, these meter checks at the air handler’s low-voltage terminals show whether the thermostat is sending the right signals.
Meter checks
Set the meter to AC volts, keep the black probe on C, and touch the red probe to each terminal:
- R: about 24 to 28 V all the time. Zero means no power: check the fuse, service switch and door switch.
- Y / Y1: 0 V when idle; about 24 V when the thermostat calls for heating or cooling from the heat pump.
- O/B:
- O systems: about 24 V in cooling, 0 in heating (many thermostats keep it on for as long as cooling mode is selected).
- B systems: about 24 V in heating, 0 in cooling.
- AUX / W2: 0 V while the heat pump runs alone; about 24 V when the thermostat calls for aux heat (raise the setpoint well above room temperature to test).
Three common heat pump problems
1. The outdoor unit runs but the indoor blower doesn’t
Check that the thermostat is set up for a heat pump (or electric heat), so it turns on the fan (G) with the compressor. A gas furnace runs its own blower in heating, so a thermostat set up for a furnace may leave G off. Check the G wire at both ends too.
2. Cool air for a few minutes now and then in winter
That is usually defrost. If it is noticeably cold, the backup heat may not be coming on during defrost: on many systems that depends on the W2 wire between the outdoor unit and the air handler. Have it checked if it bothers you; the defrost itself is normal.
3. The outdoor unit doesn’t start right away
Smart thermostats have a compressor protection delay, typically up to about five minutes after the compressor last ran or after a power cut, and many show a “waiting” message. Wait it out before troubleshooting further.
9. Frequently asked questions
How do I know whether my heat pump reversing valve is O or B?
Start with the brand of the outdoor unit: most (Carrier, Bryant, Trane, American Standard, Lennox, Goodman, Amana) use O, energized in cooling; Rheem and Ruud use B, energized in heating. The installation manual for your model confirms it. If cooling blows warm air or heating blows cool air after setup, switch the setting. See the O/B guide.
What is dual-fuel (hybrid heating) and why does it need special thermostat wiring?
Dual fuel pairs a heat pump with a gas, propane or oil furnace. The heat pump’s indoor coil usually sits right after the furnace, so running both at once overheats the coil and trips the heat pump’s high-pressure safety. A thermostat set up for dual fuel runs one or the other: the heat pump above a switchover temperature, the furnace below it.
Can I wire Auxiliary Heat and Emergency Heat to the same terminal on Nest or ecobee?
Often AUX and E run the same heat strips and are joined at the air handler. On the ecobee Premium, the aux wire goes on W1 in heat pump mode and emergency heat uses the same output, so a separate E wire can be capped if it lands on the same terminal as AUX at the air handler. On the Nest Learning Thermostat, aux goes on W2/AUX and a separate E wire is supported. No jumper is needed on either.
Why does my heat pump turn on auxiliary heat during the defrost cycle in winter?
In cold, damp weather frost builds up on the outdoor coil. To melt it, the defrost control switches the heat pump to cooling for a few minutes and stops the outdoor fan. So that the house doesn’t get cold air meanwhile, many defrost controls also turn on the backup heat through the W2 wire to the air handler. It is normal.
What is the thermal balance point of a heat pump?
It is the outdoor temperature at which the heat pump’s heating output equals the home’s heat loss. Above it, the heat pump can keep up alone; below it, backup heat is needed. It depends on the heat pump and the house: often around 25 to 35°F for a standard heat pump, and much lower for cold-climate models in a well-insulated home.
What is the economic balance point on a dual-fuel heat pump system?
On a dual-fuel system, it is the outdoor temperature at which heat from the heat pump costs the same as heat from the furnace, given your electricity and fuel prices. Cheap gas and expensive electricity push it higher (switch to the furnace sooner); cheap electricity pushes it lower. Section 6 has a worked example.
Why does my heat pump blow cool air instead of hot air when heating?
Heat pump air is cooler than furnace air, often around 90 to 105°F at the vents instead of well over 120°F, and air near skin temperature can feel cool on your hand even though it is heating the room. If the air is actually cold, check the O/B setting, and remember that defrost briefly blows cooler air.
Does a heat pump require a C wire for smart thermostat installation?
In practice, yes. ecobee, Honeywell and Amazon thermostats need a C wire (or their adapter) on any system, and Google lists heat pumps among the systems where Nest needs a C wire or the Nest Power Connector. A typical heat pump uses R, C, Y, G, O/B and an aux heat wire. See the heat pump with C wire diagram and the no C wire version.