Smart thermostat with a heat pump: O/B, aux lockout, and what to buy

A furnace thermostat only has to call for heat. A heat-pump thermostat also has to flip the reversing valve (the part that decides whether the outdoor unit is heating or cooling), stage electric strips or a furnace without running them all winter, and, if you kept gas, stop the compressor before the furnace lights. Buy a pretty Wi-Fi dial that only knows W and Y, and you get backwards seasons or a January bill that looks like baseboard heat.

If you are still choosing the outdoor unit, start with heat pump vs furnace. If the live question is whether the furnace stays, dual fuel is the cutoff conversation. Stay here when the box on the wall is the next purchase, or when the installer handed you a communicating stat and you want to know whether a Nest or ecobee is even legal on that board.

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What does the thermostat actually cost?

Angi’s 2026 smart-thermostat page puts the device at $130 to $300 on average, with a wider range of $175 to over $1,000 if you relocate the stat, run new wire, or buy several. If the old wall control has no common wire (C-wire, the extra conductor that feeds the screen and Wi-Fi all the time), they add $120 to $200 to pull one. A non-programmable stat is $15 to $50. A programmable one is $20 to $150. The smart box is the premium because it stays online, not because copper got expensive.

Angi: what is a smart thermostat

Angi: C-wire installation cost

ENERGY STAR’s field data, not a brand brochure, is the savings number worth using: certified smart thermostats show about 8% less heating runtime and 10% less cooling runtime, which they translate to roughly 8% of heating and cooling bills, or $50 per year on average. Climate, occupancy, and how aggressively you used to leave the house at 72°F all move that. Homes that are always occupied save less. Variable-speed equipment often wants the manufacturer’s own controller anyway.

ENERGY STAR: smart thermostat FAQs

Do not let a $200 wall control “pay for” a $15,000 outdoor unit. The thermostat is how you keep the strips off. The cold-climate spec sheet is how you know the compressor can actually heat at 5°F.

Why a heat pump needs a different thermostat

The reversing valve is the one extra output. Honeywell’s own explainer: they use an O terminal to energize the valve in cooling and a B terminal to energize it in heating. Most residential brands energize O in cooling. A smaller group (Rheem, Ruud, and some Bosch air-source boards) energize B in heating. Land the orange or blue wire on O/B, then pick O or B in the installer menu. If you get that wrong, the house cools when you asked for heat.

Honeywell Home: changeover valve

Auxiliary heat is the second extra output. Those are the electric strips in the air handler, or the furnace on a dual-fuel job. They are meant to help when the compressor cannot keep up, or to take the house during defrost. They are not meant to be your everyday heat. DOE’s federal buying guidance says it in one paragraph: resistance elements are inefficient, so keep their use to a minimum, and install a thermostat designed for heat pumps that ramps the setpoint slowly so a big morning recovery does not slam the strips on.

DOE FEMP: purchasing residential air-source heat pumps

Emergency heat is a third mode, and it is not the same as aux. Aux can run with the compressor. Emergency heat is the “compressor is dead, heat the house with strips only” switch. If your smart stat has no E (or no way to lock the compressor off), you do not have a service mode for a failed outdoor unit.

A conventional furnace thermostat has no O/B and no honest aux lockout. It will either refuse to wire, or it will treat W like a gas valve and run expensive heat whenever the room is a degree short.

What settings actually keep the strips off?

Aux heat max outdoor temperature (ecobee’s name; Nest and Honeywell say lockout or compressor balance). Above that outdoor temperature, the strips stay off. Ecobee’s published default on Smart Thermostat Premium is 35°F. That default is a mild-climate guess. If you paid for a cold-climate heat pump that still has capacity at 5°F, 35°F is how you idle the machine you bought. Set lockout from the extended performance table and the written load for this house, the same numbers dual fuel wants on the quote, not from the cardboard insert.

ecobee Smart Thermostat Premium

Compressor lockout is the opposite number on dual fuel: below this outdoor temperature the heat pump stops and the furnace takes over. Put switchover near the capacity balance point, not fifteen degrees warmer because 40°F felt safe. The dual-fuel guide is the cutoff math.

Recovery. A 10°F morning jump on a dumb stat is a strip-heat event. A heat-pump thermostat should stretch that recovery so the compressor does the work. If the installer left “aggressive recovery” on, turn it down before the first cold snap and watch the runtime graph.

O versus B again, after the first cooling call. If the supply air is warm in July, you have the valve backwards. Flip the software setting before you accuse the outdoor unit.

ENERGY STAR also reports average resistance-heat use for heat-pump installs as a certification metric. The label is how you know the vendor at least measured strip runtime, not how you know your house will match the national mean.

Will this 24-volt smart stat talk to my equipment?

Ducted, 24-volt, single- or two-stage heat pump. This is the Nest / ecobee / Honeywell T9 job. Google’s Nest Learning Thermostat (4th gen) lists heat pump up to 3 stages with 1 or 2 stages of auxiliary or emergency heat (O/B, AUX, AUX2, E). Ecobee’s Premium lists heat pumps at 2H/2C plus 2-stage AUX. Honeywell’s T9 listing is forced air, hot water, steam, and heat pumps with electric backup, and it will not run 120/240-volt baseboard.

Nest Learning Thermostat 4th gen specs

Communicating / inverter ducted. Many variable-speed brands want their own wall control on a data bus, not a 24-volt Nest. ENERGY STAR says it outright: variable-capacity equipment generally performs best with the manufacturer-recommended thermostat, ENERGY STAR or not. If the bid says “Infinity,” “iComfort,” or “communicating only,” the pretty third-party stat is a spare. Ask whether a 24-volt fallback mode exists. If it does not, buy the OEM control and stop shopping Amazon.

Ductless mini-splits. The wall unit already has a controller. A hallway Nest cannot speak most indoor heads. People use an IR blaster (Sensibo, Cielo) that pretends to be the remote. That is a different product, and it does not give you aux lockout because there are no strips. Mini-split for one room is that fork.

Dual fuel. You need outdoor temperature, a written switchover, and a thermostat that will not run the compressor and the furnace at once except during the changeover the manufacturer allows. A cheap 1H/1C Wi-Fi stat is not that.

Which box should I actually order?

Photo the terminals on the old stat before you buy. Count C, O or B, Aux/W2, and E. Then match the listing, not the color of the glass.

The ecobee Smart Thermostat Premium is the 24-volt heat-pump pick when you want a remote room sensor in the box, a Power Extender Kit if C is missing, and installer menus that name aux max outdoor temperature. Ecobee’s site lists $259.99 regular, recently $229.99 on sale, heat pumps at 2H/2C plus two-stage AUX, and a PEK in the box. Their 26% / $284 savings line is an internal 2021 analysis against a 72°F hold. Use ENERGY STAR’s $50-class field number for planning, and treat $284 as a marketing ceiling.

The Google Nest Learning Thermostat (4th gen) is the pick when you already live in Google Home, you want emergency heat as a named mode, and you can live with the compatibility checker. Google lists $279.99, most 24V systems including heat pump, and O/B plus AUX, AUX2, and E. Their listing says no C-wire is required in most homes. Run g.co/nest/works against a photo of your wires anyway, because “most” is not your air handler.

The Honeywell Home T9 is the pick when you want a touchscreen, a room sensor, and a brand HVAC techs already carry. Amazon’s T9 listing says heat pumps with electric backup, C-wire required, and no 120-240V baseboard. List price on that pack is $209.99.

None of these is the communicating OEM control. If the outdoor unit warranty names a specific thermostat, that piece of paper wins.

Check ENERGY STAR’s rebate finder with your zip before you pay list. Utility rebates are often the ENERGY STAR model, not “any Wi-Fi dial.”

What should be on the quote (or your own install notes)?

1. System type in writing: heat pump, heat pump plus strips, or dual fuel, with the outdoor model.
2. O or B, copied from the outdoor unit’s diagram, not guessed from wire color.
3. Aux lockout and, on dual fuel, compressor lockout, both in °F, tied to a load and a capacity table.
4. Whether C-wire is present, PEK/power connector, or a new conductor at Angi’s $120 to $200.
5. Emergency heat: how you turn the compressor off if the outdoor unit dies in January.
6. Whether the wall control is 24-volt third-party or the brand’s communicating stat, and what that does to the warranty.
7. Outdoor sensor: built-in internet weather is a fallback, not a substitute, if lockout is load-bearing.
8. Who sets the installer menu after the wires land. A box left on furnace defaults will heat with strips.

If you are swapping the stat yourself, kill power at the air-handler switch, photograph every terminal, and label the wires. A labeled photo is the whole kit if you have to put the old control back.

A dirty filter will still ice a coil no matter how clever the lockout is. Filter size and MERV is the pad in the slot, not the app.

How does this go wrong?

The new Nest is wired as a furnace. Heat and cool are swapped, or the strips run whenever the room is 1°F low.

Lockout stays at 35°F on a unit that still heats at 10°F, so January is electric resistance with a heat pump statue in the yard.

Someone puts a hallway smart stat on a mini-split and wonders why the head ignores it.

The communicating inverter is paired with a 24-volt Amazon special, the outdoor board throws a control fault, and the warranty conversation starts with “which thermostat.”

C-wire is missing, the PEK is skipped, and the stat reboots or steals power from the reversing valve until the fuse opens.

Recovery is set to jump 8°F at 6 a.m., which is a strip-heat schedule with extra Wi-Fi.

How do I decide this week?

1. Photograph the old terminals and write down whether you see C, O or B, Aux/W2, and E before you put a box in the cart.
2. Ask whether the outdoor unit is 24-volt conventional or communicating, and stop shopping Nest/ecobee if the bid names an OEM data-bus control.
3. If the system is ductless, do not buy a hallway 24-volt stat. Use the head’s controller or an IR bridge, and keep aux lockout out of the conversation because there is no strip pack.
4. On a ducted heat pump, pick a stat that names O/B and aux lockout, then set lockout from the capacity table, not from 35°F because that was the default.
5. Budget Angi’s $130 to $300 for the device and $120 to $200 if you need a C-wire pulled, and look up the ENERGY STAR rebate for your zip before you pay list.
6. After it boots, run one cooling call and one heating call the first afternoon, and confirm the supply air matches the mode before you trust it overnight.