Furnace Not Heating? 7 Checks to Try Before Calling a Technician
By EasyToFix Homes Team
Few things are worse than waking up to a cold house in January. Before you pay for an emergency service call, work through these seven checks. Most no-heat problems come down to a wrong setting, a dirty filter, a tripped breaker or a dirty flame sensor, and every one of them is something a homeowner can check in under an hour. If you ever smell gas or rotten eggs, skip all of this: leave the house and call your gas utility from outside.
1. Check the thermostat settings
Make sure the thermostat is set to HEAT and the temperature is set a few degrees above the current room temperature. It sounds obvious, but an accidental switch to COOL, a dead battery or a schedule override causes a surprising number of no-heat calls. If the screen is blank, replace the batteries.
Check the fan setting as well. With the fan set to ON instead of AUTO, the blower runs constantly and pushes cool air between heating cycles, which can make the house feel cold even when the furnace works fine. Set it to AUTO.
2. Replace a dirty air filter
A clogged filter restricts airflow, which makes the heat exchanger overheat. When that happens, a safety limit switch shuts the burners off, so the furnace cycles on and off without ever warming the house. Hold the filter up to a light: if you cannot see light through it, replace it.
Standard 1-inch filters need replacing every 1 to 3 months during heating season. Homes with pets, recent remodeling or high dust may need monthly changes. A clean filter is also the cheapest way to extend the life of your furnace.
3. Check the breaker and the furnace switch
Find the furnace breaker in your electrical panel. If it has tripped, switch it fully off and then back on. Many furnaces also have a dedicated shutoff switch mounted nearby that looks like an ordinary light switch, sometimes with a red cover. Make sure it is on.
If the breaker trips again immediately, stop and call a technician. Repeated tripping signals an electrical fault, a shorted blower motor or a failing control board. Those are not DIY repairs.
4. Verify the gas supply
Check that the gas valve near the furnace is fully open, with the handle parallel to the pipe. If you have other gas appliances, see whether they work. If the stove and water heater work but the furnace does not, the problem is at the furnace, not the supply.
Older furnaces use a standing pilot light that can blow out. Newer furnaces use an electronic hot surface ignitor that glows orange before the burners light. If the ignitor does not glow and the burners never light, the ignitor may need replacing, which is a quick job for a technician.
5. Clean the flame sensor
The flame sensor is a thin metal rod in front of one of the burners. It tells the control board that the flame is lit. When it gets coated with soot, the furnace lights for a few seconds and then shuts off, which is one of the most common no-heat symptoms.
With the power off, you can remove the single screw holding the sensor and gently clean the rod with fine emery cloth or steel wool. Do not touch the rod with bare fingers. Reinstall it, restore power and test the furnace. A clean sensor solves this problem most of the time.
6. Clear blocked vents and the condensate line
High-efficiency furnaces vent through PVC pipes to the outside. If snow, leaves or a bird nest blocks the intake or exhaust, the pressure switch keeps the furnace from starting. Walk outside and make sure both pipes are clear, with nothing stacked against them.
These furnaces also produce water that drains through a condensate line. If the line or the condensate pump is clogged, a safety switch shuts the furnace down. Clear the trap, make sure the pump is powered and check that the drain hose runs freely to a floor drain.
When to call an HVAC technician
Call a professional if the burners will not stay lit after you cleaned the flame sensor, if the breaker keeps tripping, if you hear banging or screeching, or if your carbon monoxide detector alarms. A cracked heat exchanger can release carbon monoxide, which is odorless and deadly, so never ignore that alarm.
Furnaces typically last 15 to 20 years. If yours is older, needs frequent repairs or has a cracked heat exchanger, replacement is the safer and often cheaper choice. A certified technician can inspect the heat exchanger, test for carbon monoxide and give you an honest answer about repair versus replacement.