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Thermostat replacement starts with the system, not the thermostat: a 24-volt furnace and air conditioner, a heat pump with auxiliary heat, a dual-fuel system, a boiler, a millivolt fireplace and line-voltage baseboard heat each need a different control. Replace it when the screen stays blank on fresh batteries, when it reads more than a degree or two off a thermometer held beside it, or when it calls for heat and no 24-volt signal reaches the furnace board. A $90–$150 diagnostic settles which side is at fault before you buy anything.
Six system types cover nearly every house we see, and the thermostat has to match. A gas furnace with central air uses a conventional 24-volt thermostat. A heat pump needs one with a reversing-valve terminal and auxiliary-heat staging, and a dual-fuel system needs one that locks the heat pump out below a set outdoor temperature and hands over to the furnace. Set up wrong, a heat pump thermostat runs the expensive backup heat all winter without any fault showing.
Boilers are usually two-wire, heat-only and slow to respond, so they want a thermostat with a cycle setting meant for hot-water heat. A millivolt gas fireplace generates its own small control voltage from the pilot and needs a millivolt-rated, battery-powered thermostat; a 24-volt model will not run it. Line-voltage baseboard thermostats switch 120 or 240 volts through heavy wires and are an electrician's job under ESA rules, not ours.
Many thermostats are replaced for faults that live elsewhere. A blank screen can be dead batteries, but it can also be a blown low-voltage fuse on the furnace board, a tripped condensate safety switch or an open furnace door switch, and a new thermostat fixes none of those. The deciding test is at the control board: with the thermostat calling for heat, we measure for 24 volts between W and C. Signal present and no heat means the furnace; no signal means the thermostat or its cable, which is the logic in is it my thermostat or my furnace.
Old round and rectangular dial thermostats often hold a glass bulb of mercury. They do not go in the garbage: they go to the heating industry's Thermostat Recovery Program, which collects them through contractors and wholesalers, or to a municipal household hazardous waste depot. We take the old one away for recovery when we fit the new one. This page covers like-for-like and programmable replacements; Wi-Fi models, app setup and room sensors belong to smart thermostat installation.
Signs the thermostat is the fault and not the furnace
- The screen stays blank after new batteries while the light on the furnace control board is on as normal
- The reading is more than a degree or two away from a thermometer held beside it
- The furnace short-cycles in step with the thermostat clicking, and the board shows no fault code
- The display says heat is on but there is no call at the board (thermostat says heat on, no heat)
- Buttons or the dial stop responding, or the schedule and clock reset themselves
How it works
We read the wires on the old thermostat by terminal letter, then look at the equipment: furnace, heat pump, boiler or fireplace, one transformer or two. That decides which thermostats are even candidates.
With the thermostat calling, we test for 24 volts at the control board, then bridge R to W at the board to see whether the furnace fires without the thermostat. If the furnace is the problem you learn that before paying for a control you did not need.
The new thermostat is mounted level, wired, and set for system type, stages and cycle rate in its installer menu. Heating, cooling, fan and any auxiliary heat are each run from the wall, and an old mercury unit leaves with us. To book, request a visit.
Common questions
Can I replace a thermostat myself?
On a 24-volt furnace and air conditioner, often yes: switch the furnace off, photograph the wires, label each by its terminal letter and not its colour, and move them across. Stop and call if you find thick wires joined with wire nuts, which means line voltage, or a heat pump, where terminal setup decides whether backup heat runs. Trouble after a swap is covered in furnace not working after a new thermostat.
Will any thermostat work with a heat pump?
No. A heat pump thermostat needs an O/B terminal to switch the reversing valve between heating and cooling, and separate control of auxiliary heat. A furnace-only thermostat wired to a heat pump either blows cool air in heating mode or runs the backup heat alone. Dual-fuel systems add a lockout so the gas furnace and the heat pump never run together.
What do I do with an old mercury thermostat?
Keep it intact and upright in a sealed bag or its own box, and keep it out of the garbage and the recycling bin. The mercury sits in a small glass bulb behind the cover. Hand it to the technician who replaces it, or take it to your municipality's household hazardous waste depot. If the bulb breaks, ventilate the room and do not vacuum the beads.
Why does my new thermostat short-cycle the furnace?
Three usual reasons: the system type or cycle rate is set wrong in the installer menu, the thermostat is power-stealing because there is no common wire, or it sits above a register or in the sun. A real furnace fault short-cycles with the thermostat still calling and usually leaves a flash code on the board. Our guide to furnace short-cycling separates the two.
Do you replace thermostats on boilers and gas fireplaces?
Yes for hot-water boilers, which normally take a two-wire heat-only thermostat with a slower cycle setting, and yes for millivolt gas fireplaces, which need a millivolt-rated battery model. Not for electric baseboards: those are line-voltage thermostats and belong to a licensed electrical contractor. Tell us the heating type when you call so the right control is on the truck.