Thermostat says heat is on, but there is no heat
A screen that says "heating" only means the thermostat is asking. It does not mean the equipment heard the request or answered it.
Start with the fact that changes how you look at this. The word "Heat" or "Heating" on the display describes what the thermostat is requesting, not what your furnace or heat pump is doing. Plenty of thermostats show that message the moment the room is below the setpoint, whether or not anything downstairs is listening.
So the useful question is not whether the thermostat is on. It is whether the equipment responds when the thermostat calls. Stand near the furnace or the indoor unit, raise the setpoint a few degrees above room temperature, and listen. Within a minute or two you should hear something start.
Check the mode and the fan setting first
The cheapest causes come first. Confirm the mode is set to Heat and not Auto, Cool or Off, and that the setpoint is clearly above the room reading, not just one degree. Check the fan switch too. If it is set to On, the fan can blow air constantly, and that air will feel cold whenever the burner or heat source is not running.
Some thermostats also have a hold or vacation setting that overrides your schedule. The screen can say heat is on because the schedule says so, while a temporary hold quietly keeps the setpoint low.
Nothing starts at all
If the thermostat calls and the furnace or heat pump does nothing, the fault sits between the two or in the equipment. That can be a tripped breaker, a furnace switch that was turned off, a blown low-voltage fuse on the control board, or a wiring fault. A thermostat with weak batteries can also show a full display but fail to send a solid signal.
Check the breaker for the furnace or air handler and the service switch near the unit, which looks like an ordinary light switch. Replace the thermostat batteries if it takes them. That is the limit of what is worth doing yourself.
The equipment runs but the air is not warm
This is a different problem wearing the same symptom. If the blower runs but the air is lukewarm or cold, a gas furnace may be locking out on a safety control. And a heat pump may be in a defrost cycle or struggling in cold weather. Those are equipment faults, and the thermostat is not the cause.
A heat pump in defrost can briefly blow cooler air and then recover. If it stays cool for long stretches, it needs a technician. The same goes for a furnace that starts, runs briefly and shuts down again.
Do not treat a wiring fault as a DIY job
It is tempting to pull the thermostat off the wall and start touching wires. Skip that. Miswired low-voltage connections can damage the control board, and a repair that seems to work can leave a safety circuit bypassed. Replacing the thermostat again and again will not fix a fault that lives in the wiring or the equipment.
If you smell gas at any point, or a carbon monoxide detector alarms, leave the house and call your gas utility or 911 from outside. Do not try to investigate the source.
When to call a licensed HVAC technician
Call one when the mode, setpoint, batteries and breaker all check out and the equipment still does not start, or when it starts and will not produce heat. The fault is in the wiring, the control board or the equipment itself, and all three are a licensed technician's work.
Frequently asked questions
Why does my thermostat say heat is on when my house is cold?
The message shows that the thermostat is calling for heat, not that the furnace or heat pump is delivering it. The call may not be reaching the equipment, or the equipment may be running but not producing heat.
Can a thermostat be fine while the furnace is the problem?
Yes. If the display, setpoint and batteries are all normal and the equipment does not start when you raise the setpoint, the fault is usually in the wiring, the control board or the furnace itself.
Is cooler air from a heat pump always a fault?
Not always. A heat pump can blow cooler air for a short time during a defrost cycle. If the air stays cool for long stretches, have it checked.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Smart Thermostats