Installed a New Thermostat and Now No Heat? Start Here
The furnace worked. You swapped the thermostat. Now it doesn't. That's not bad luck — the swap is the suspect, and this is one of the most common self-inflicted no-heat calls in the business. The good news: it's usually one of four small things, and none of them means you broke the furnace.
Safety first, every page: if you smell gas, stop — leave the house and call your gas company from outside.
1. A wire moved, broke, or went to the wrong terminal
Thermostat wires are thin and brittle. Pulling the old unit off often bends or snaps one, and terminal labels don't always match between old and new (the old thermostat's "W" jumper setup may not map one-to-one). If you took a photo of the old wiring before disconnecting — compare it now. If not, the wire colors are usually conventional (white to W for heat, red to R for power) but not reliably, which is why the photo matters next time.
With the furnace's power OFF, check that each wire is stripped, seated under its terminal, and actually the wire you think it is.
2. The furnace's 3-amp fuse blew during the swap
Here's the one most people never find: on the furnace's control board is a small automotive-style fuse (commonly 3A). If two thermostat wires touched each other — or a screwdriver — while the power was on during the swap, that fuse blew instantly. Everything looks fine, the new thermostat may even light up, and nothing runs.
A technician checks this in the first two minutes. If you're comfortable opening the panel with the power off, it's a plug-in fuse, sold at any auto parts store. If not — it's a quick visit, and now you can tell the shop exactly what to look for.
3. The new thermostat needs power the old one didn't
Many smart thermostats need a C wire (constant power). Without it, some won't run reliably — or at all — where your old dial thermostat was happy. If the new unit's screen is dead or it keeps rebooting, this is likely.
4. It's configured for the wrong system
In the new thermostat's setup menu: heating type matters. A heat pump configured as a furnace (or vice versa) can blow cold air on a heat call or nothing at all. Check the system-type setting against what you own.
The honest fallback
Put the old thermostat back on. If heat returns, you've proven it's the new install, not the furnace — no service call needed, just a better install. If heat does NOT return with the old unit, check that 3-amp fuse.
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