Furnace Not Working? What to Check Before You Call
Nine safe checks that fix more no-heat problems than you would expect, plus the warning signs that mean you should stop and call a technician.
Stop first if you notice any of these
Do not troubleshoot. Leave the house and make the right call if:
- You smell gas. Leave, then call your gas utility from outside.
- Your carbon monoxide alarm is sounding. Get everyone out and call 911.
- You see scorch marks, melted wiring or smell burning plastic at the furnace.
- Water is pouring out of the furnace onto electrical parts.
Otherwise, work through the checks below in order. They take about fifteen minutes and require no tools beyond a flashlight.
1. Check the thermostat
Make sure it is set to Heat, not Cool, Off or Auto-changeover stuck in cooling, and that the set temperature is at least five degrees above the room temperature. Replace the batteries if it has them, even if the screen looks fine. Smart thermostats can lose power if the furnace's low-voltage fuse blows or the common wire is loose; if the screen is blank, that is a clue.
A surprising share of the no-heat calls we get in October are thermostats still in cooling mode from September. A thermostat with weak batteries can also keep its display lit while it no longer has enough power to switch the furnace on.
2. Check the furnace power switch and breaker
Most furnaces have a light switch on or near the unit that shuts off power for service. It is easy to bump or for someone to flip it thinking it controls a basement light. Also check the furnace breaker in your electrical panel. If it has tripped, reset it once. If it trips again, leave it off and call.
3. Check the furnace door
The blower compartment door presses a safety switch. If the door is not seated fully, the furnace will not run at all. Take it off and put it back firmly.
4. Check the filter
A badly clogged filter restricts airflow so much that the furnace overheats and shuts off on its high-limit switch, often after running a few minutes. Pull the filter and hold it up to a light. If you cannot see light through it, replace it. Note the arrow on the filter frame, which should point toward the furnace.
5. Look at the status light
Most furnaces built in the last twenty-five years have a small window on the blower door showing a blinking LED on the control board. The number of blinks is a code, and the legend is usually printed on a sticker inside the door. Write the code down before you reset anything. It tells a technician a great deal over the phone.
Tried everything and still no heat?
Call and read us the blink code. We can often tell you what failed before we arrive.
6. Check the vent pipes outside
High-efficiency furnaces vent through two white PVC pipes on the side of the house or the roof. Drifting snow, ice, leaves, a wasp nest or a bird can block them, and the furnace will shut itself off. After heavy snow, clear the area around them carefully.
After a heavy, wind-driven snowfall, our phone lights up with furnaces that shut down because the intake and exhaust pipes on the side of the house got buried in a drift. Brush them clear, turn the furnace off for a minute and back on, and many of them fire right up.
7. Check the condensate drain
High-efficiency furnaces make water and drain it through a small tube to a floor drain or pump. If the trap clogs or the condensate pump fails, a safety switch shuts the furnace down. Look for water in or around the furnace base and check that the condensate pump runs when you pour a little water into its reservoir.
8. Check the gas supply
Make sure other gas appliances such as the water heater and stove are working. If none are, the problem is the gas supply, not the furnace. On propane, check the tank gauge. If a propane tank ran completely empty, the supplier must leak-test the system before relighting.
9. Reset once, then stop
Turn the furnace off at the switch for about a minute, then back on. If it starts, runs a few seconds and shuts down again, do not keep resetting it. That pattern usually means a dirty flame sensor or a pressure switch problem, and repeated resets will not fix it. At that point, call (740) 527-0222 or see our furnace repair page for what happens next.
Blowing cold air?
If the blower runs but the air is cold, check that the thermostat fan setting is on Auto, not On. On "On," the blower runs constantly even between heating cycles, and the air feels cool. If it is on Auto and still blowing cold, the burners are not staying lit, which points to the same ignition and flame sensing problems above.
Heat pumps are different: air that feels lukewarm (around 90 degrees at the register) is normal, and the outdoor unit going into defrost can blow cooler air for a few minutes. Our heat pump page explains what is normal in winter.
No-heat questions
Why is my furnace running but blowing cold air?
Check that the thermostat fan is set to Auto. If it is, the burners are probably lighting and shutting off, usually because of a dirty flame sensor, a failed igniter, or a pressure switch or drain problem on high-efficiency models.
How long can a house go without heat in winter?
In below-freezing weather, a typical house can lose ten to fifteen degrees overnight. Pipes on exterior walls and in crawl spaces are at risk once indoor temperatures drop into the forties. Treat no heat in freezing weather as urgent.
Can I clean the flame sensor myself?
Some handy homeowners do, but it means removing a burner-area part with the power and gas off. If you are not comfortable working inside the furnace, a technician can clean or replace it quickly.
Why does my furnace keep shutting off?
Common causes are a clogged filter tripping the high limit, a dirty flame sensor, a blocked vent pipe, a clogged condensate drain, or a failing pressure switch. The blink code on the control board narrows it down.
Is it safe to use a space heater while I wait?
Yes, if it is on a hard, level surface, plugged directly into a wall outlet, and kept three feet from anything that can burn. Never heat a home with a gas oven, grill or generator.
Next steps
Furnace Repair
No heat, short cycling, cold air or strange noises. Same-day repair on all makes.
Learn more →24/7 Emergency Service
No heat at night or on a weekend. Calls answered around the clock.
Learn more →Tune-Ups & Maintenance
Fall furnace and spring AC tune-ups that catch small problems early.
Learn more →Heat Pumps
Cold-climate and dual-fuel heat pumps, repaired and installed for Ohio winters.
Learn more →Still no heat? Call us.
Calling is the fastest way to get on the schedule. Tell us what the system is doing and we will tell you what it probably is, what it probably costs and when we can be there.
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