When the heat is not working in your house, the cause is usually a thermostat setting, a clogged air filter, a tripped breaker or switch, or a failed part in the furnace's ignition or safety system. The way the system fails tells you which one: a heater that does nothing has lost power or its signal, one that blows cold air has an ignition or heat pump problem, and one that shuts off after a few minutes has tripped a safety. Three quick checks at the thermostat, filter and breaker solve a good share of no-heat calls, and yearly maintenance prevents most of the rest.
First Checks When Your Heater Is Not Working
Start with the parts you can check safely in a few minutes.
Check the Thermostat Settings
Make sure the thermostat is set to Heat and the setpoint is at least three degrees above the room temperature. Replace the batteries if the screen is dim or blank, since many thermostats stop calling for heat before the display dies completely. A blank thermostat screen can also point to a tripped float switch or a blown fuse at the furnace. If you have a heat pump, confirm it isn't sitting in EM heat, which runs only the backup heat strips.
Look at the Air Filter
A clogged filter chokes airflow, and a furnace that can't move enough air overheats and shuts its burners off to protect itself. If you can't see light through the filter, replace it. A filter left months past its change date is one of the most common reasons a furnace quits on the first cold night.
Confirm the Furnace Has Power
Check the breaker for the furnace or air handler, and look for a switch that looks like a light switch on or near the unit. It gets flipped off by accident more often than you'd think. The blower door also has a safety switch, so make sure the panel is seated firmly. Reset a tripped breaker once. If it trips again, leave it off and call a technician.
If you smell gas or your carbon monoxide alarm sounds, skip the troubleshooting. Leave the house and call your gas utility or 911 from outside.
Furnace Not Turning On at All
A furnace that stays completely silent has either lost power or isn't getting the low-voltage call from the thermostat. Once you've ruled out the breaker and the service switch, the common causes are a blown fuse on the control board, loose thermostat wiring, a failed transformer or control board, or a tripped condensate switch.
That last one catches a lot of homeowners with high-efficiency furnaces. These units make water as they run, and a clogged drain line trips a float switch that shuts everything down. If water is pooling near the furnace, a clogged condensate line is the likely culprit.
Most furnaces have a small window on the blower door with a blinking status light. Count the blinks and compare them to the code chart on the inside of the panel door. Telling a technician that code saves diagnostic time.
Furnace Blowing Cold Air
A furnace blowing cold air is running the blower without lighting the burners, or it's lighting them and then shutting them down.
Check the simple one first. If the thermostat fan is set to On instead of Auto, the blower runs nonstop and pushes room-temperature air between heating cycles. Switch it to Auto.
If the fan is on Auto and the air is still cold, the ignition sequence is failing somewhere. A modern gas furnace checks that the vent is clear with a pressure switch, heats a hot surface ignitor, opens the gas valve and then confirms flame with a flame sensor. If any step fails, the furnace tries a few more times and then locks out, often for an hour. During that time the blower may keep running to cool the furnace, which is the cold air you're feeling. A cracked ignitor, a dirty flame sensor and a faulty pressure switch are the usual causes, and all of them are routine furnace repair work.
Heat Pump Blowing Cold Air
Heat pump air often feels cooler than furnace air, because a heat pump supplies air in the 90s while a gas furnace runs well above 110 degrees. Lukewarm air from a heat pump isn't automatically a problem. Short bursts of cool air during a defrost cycle are normal too.
Truly cold air is different. If the outdoor unit isn't running, the reversing valve is stuck in cooling mode or the system is low on refrigerant, the heat pump can't heat. Low refrigerant means there's a leak, and topping it off without finding the leak only buys a few weeks. Book heat pump repair and use emergency heat to stay warm until then.
Furnace Keeps Shutting Off After a Few Minutes
A furnace that starts, runs briefly and shuts off is short cycling, and it's almost always a safety device doing its job.
The most common cause is a dirty flame sensor. A thin layer of oxidation on the sensor rod keeps it from proving the flame, so the control board shuts the gas off a few seconds after the burners light. Cleaning or replacing the sensor is a quick fix during a service visit.
The second cause is overheating. When airflow is restricted by a clogged filter, closed vents or a weak blower, the heat exchanger gets too hot and the high-limit switch cuts the burners. Repeated overheating stresses the heat exchanger over time, so this one deserves attention even if the house still feels warm.
Less often, short cycling comes from a thermostat mounted near a heat source, a blocked exhaust vent or a furnace that's too large for the house.
Furnace Running but Not Heating the House
When the furnace runs for long stretches but the house won't reach the setpoint, look at the air side first. Open every register, move furniture off the return grilles and check that the filter is clean. Leaky or disconnected ductwork in an attic or crawlspace can dump heated air outside the living space, which often shows up as one or two rooms that never warm up.
If the system kept up last winter and doesn't now, something has changed, such as a weakening blower motor, a burner that isn't firing fully or a heat pump that's losing refrigerant. A technician can measure the temperature rise across the furnace to tell whether it's producing the heat it was built for.
One exception: during a hard freeze, a heat pump running nearly nonstop is normal. That's how it's designed to work in the cold.
Should You Turn Off Your Heat If It's Not Working?
Turn the system off at the thermostat and the service switch if you smell burning or gas, hear loud banging or grinding, see water leaking from the unit or have a breaker that won't stay reset. Otherwise, a furnace sitting in lockout won't hurt itself.
To reset a furnace once, turn off the service switch, wait about 30 seconds and turn it back on. If it locks out again, resetting it repeatedly won't fix the cause and can hide a safety problem.
How Long Can a House Go Without Heat?
A well-insulated house can hold a livable temperature overnight in mild weather, but the bigger risk in a freeze is your plumbing. Pipes in exterior walls, crawlspaces and garages can freeze once outdoor temperatures stay in the low 20s. If a repair can't happen quickly, open the cabinet doors under sinks on outside walls, let faucets drip and use space heaters only with plenty of clearance around them.
Repair or Replace Your Heating System?
Most no-heat calls end in a repair, not a replacement. Replacement starts to make sense when a furnace past 15 to 20 years old has a cracked heat exchanger, or a heat pump past 12 to 15 years needs a new compressor. Those repairs cost a large share of a new system, and the rest of the equipment is near the end of its life anyway. A good technician will show you both prices side by side before you decide on furnace replacement.
Get Your Heat Back On
MR. HVAC is family-owned and has repaired furnaces and heat pumps for more than 25 years. Our technicians track down the actual cause, explain it in plain terms and give you the price before any work starts, and our trucks carry the parts behind most common no-heat problems. If the checks above didn't bring your heat back, schedule heating repair and we'll get your home warm again.
