HVAC heat not working
A furnace and a heat pump fail in different ways for different reasons, even though both get described with the same phrase. The fix depends on which one is actually in your house.
The first question is which system you actually have
"HVAC heat not working" covers two genuinely different pieces of equipment, and the fix looks nothing alike depending on which one sits in your house. A furnace burns gas, or in some homes uses electric resistance, to make heat directly. A heat pump moves heat from outside air into the house instead, using the same equipment that cools it in summer. Fixing the wrong one wastes time, and on a heat pump, risks handling a component that was never the actual cause.
If you are not sure which you have, look outside. A single outdoor unit that also runs in summer for air conditioning is almost always a heat pump. A furnace is indoors only, typically in a closet, basement, or attic, with no outdoor cabinet doing any of the heating work.
Furnace and heat pump failures do not look the same
| What you notice | More likely on a furnace | More likely on a heat pump |
|---|---|---|
| No warm air at all, blower not running | Ignition failure or a tripped safety switch | A power problem: breaker, disconnect, or a lockout |
| Blower runs, but the air is only lukewarm | A cracked heat exchanger or a failing ignition component partway through its cycle | Often normal: heating-mode air runs cooler than furnace air and is not meant to feel hot at the vent |
| System runs constantly and the house still is not warm | Undersized equipment or a dirty filter starving airflow | Extreme cold pushing past what the heat pump alone can deliver, or backup heat not engaging |
| Outdoor unit frosted over | Not applicable: a furnace has no outdoor coil | Often normal, from the defrost cycle; ice that never clears is not |
What both systems share
Regardless of which you have, the same three checks come first. Confirm the thermostat is set to heat rather than cool or off, confirm the setpoint is actually above room temperature, and check the breaker panel for anything tripped. A blank thermostat screen or a home that lost power recently points at electrical service well before it points at the furnace or heat pump itself.
A dirty filter affects both systems too. Restricted airflow starves a furnace's heat exchanger and can trip a safety switch that shuts the burner down. And it can reduce a heat pump's heating output in a similar way. Replacing a visibly dirty filter costs a few dollars and rules out one of the more common causes before you call anyone.
Where a furnace usually fails
Most furnace no-heat calls come down to a short list: the igniter failing to light the burner, a flame sensor too dirty to detect that the flame caught, a clogged filter tripping a high-limit safety switch. Or a thermostat problem shared with any system. See furnace won't turn on for the full sequence, including which of those a homeowner can safely check.
Where a heat pump usually fails
Heat pump no-heat calls lean electrical more often: a tripped breaker or disconnect, a lockout after failed start attempts. Or a defrost cycle that is not actually the problem it looks like. See heat not turning on in the house for the order to check them in. And heat pump not heating if the system runs but the air never gets warm.
A third, less common case: electric resistance heat
Some homes, especially in mild climates, heat with electric resistance coils built into the air handler and nothing else. There is no gas burner and no outdoor heat pump unit doing the heavy lifting. If your system has no gas line and no outdoor unit that runs in winter, this is likely what you have.
These systems fail more like a large space heater than either a furnace or a heat pump: a tripped breaker, a failed heating element. Or a limit switch are the usual suspects, and there is no ignition sequence or defrost cycle to rule in or out.
What not to do, on either system
- Do not keep resetting a breaker that trips again immediately.
- Do not open a furnace's burner compartment or a heat pump's outdoor electrical compartment yourself. A capacitor and a gas burner assembly both carry real risk, for different reasons.
- Do not keep troubleshooting past a gas smell or a carbon monoxide alarm. Leave the house and call the gas company or 911 first, and worry about the furnace afterward.
When it stops being a homeowner question
The thermostat, the filter, and the breaker panel are the parts of this that belong to you. Once those check out and the system still is not producing heat, a licensed HVAC technician needs to look at it. Which kind of technician call you make depends on the system type you identified at the start.
Frequently asked questions
Does "HVAC" mean furnace or heat pump?
Neither specifically. HVAC is the umbrella term for heating, ventilation and air conditioning equipment in general, so "HVAC heat not working" could describe a furnace, a heat pump, or in some homes an electric resistance system built into the air handler. Identifying which one you have is the first step, not an afterthought.
I have a furnace for heat and a separate air conditioner. Does any of this apply to me?
The thermostat, filter and breaker checks apply to any forced-air system. The specific failure causes differ, though: a furnace-and-AC setup does not run its outdoor unit during heating, so a silent outdoor unit while the furnace runs is not a relevant sign on your system.
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. Air-Source Heat Pumps
- ENERGY STAR. Energy Savings at Home: Heating & Cooling