Furnace Turns Off Before Reaching Desired Temperature [DIY Fix]

If your furnace turns on, heats a little bit, then turns off before the temperature reaches your desired thermostat set point, try these simple solutions; the fix might be easier than you think.

Check the furnace filter, duct work, and vents — anything blocking the free-flow of air will obstruct airflow to the heat exchanger in the furnace. And if the heat exchanger gets too hot from lack of airflow and overheats, a safety mechanism will shut off the furnace to protect it.

Problem: Furnace fan was set to ON, but no air was being circulated or pushed through the vents. Also, the furnace would turn ON to the heat setting, but no heat was coming out of the vents long enough to reach the desired thermostat setting of 70 – the furnace would shut off shortly after firing up. What could be causing the problem? Consider these possible culprits:

  1. Dirty filter. Have you checked your furnace air filter lately? Maybe it needs to be changed. Change it each month or 6 months or according to the furnace manufacturer directions.
  2. Blocked airflow. Do you have any intake air vents OR other vents completely covered or blocked off? If the registers (vents) are covered, that will inhibit the flow of air from the furnace and create an increase in air pressure backing up into the heat exchanger, and can trigger the furnace to turn off prematurely.
  3. Thermostat wiring. Did you install a new thermostat lately? Is the wiring correct?
  4. Thermostat setting. Is the thermostat set to “Heat”?
  5. Duct work is wonky or not attached properly. Foil seal the connections to prevent air leaks. In my mom’s house, turned out that the duct work had not been put back together after some construction. Ridiculous. For most people, this will not be your problem.
  6. Furnace powered On? Is the furnace switched “ON” at the furnace and/or breaker? This is something to check when you switch between heating and cooling seasons.

Thermostat Wiring

My friend had just installed an ecobee thermostat (at my recommendation), he assumed that the formerly working furnace was now malfunctioning because of the new ecobee. Fair hypothesis.

He sent me a photo of the wiring. All looked fine at both the furnace and the thermostat.

I asked if he had made any other changes or if her furnace filter had been changed lately. Since the furnace was relatively new, I doubted the filter was the problem. Turned out that the register closest to the furnace had been completely sealed off, and the air being pushed out from the furnace had no where to go, so the furnace was overheating. He removed the sealing foil tape and the furnace could breathe again.

Dirty Filter

Remember to change your dirty old furnace filter. A clogged filter can restrict the air flow to your HVAC system.

A clogged filter limits the ability of your furnace to produce and distribute warm air throughout your home.

Some furnaces even contain a built-in shutoff that kicks into action when the filter gets clogged, in order to prevent an overheated burner. When that happens, the furnace will blow cold air. Filters are cheap and easy to change.

After replacing your furnace filter, restart your furnace. Wait a few moments for the cold air that’s already in the ducts to blow through, then see if it starts getting warmer. If so, congratulate yourself on finding a solution without having to call in the experts. If changing the filter and restarting doesn’t do the trick, here are some more possible solutions.

If you hear your furnace kicking on and off quickly, this can be a symptom of a clogged furnace filter or a blocked heat vent.

Closed Heating & Cooling Vents, Dirty Ducts

Just as a clogged filter can obstruct the air flow and decrease pressure at the blower fan, a blocked vent has the same effect.

Ducts get dirty

Most HVAC systems use a forced-air system, which means the furnace heats and cools air that is circulated through the ducts in your home. The air is pushed into your rooms while the existing room air is pulled in through a separate set of return ducts (cold air return ducts), where the air is heated by the furnace.

  • Dirty Ducts: Clean off your air return duct panel with a rag or remove any dust or dirt with your vacuum’s extension attachment. Periodically vacuum your duct work as far as you can.
  • A blocked heat vent or clogged duct work can prevent air from circulating through the furnace unit, which can overheat the heat exchanger, tripping the limit switch and shutting off your furnace. Obviously, this will prevent your home from heating properly. If this overheating happens often, the electronic “high limit switch” safeguard can fail, at which point the furnace won’t come on to heat your home at all.

Leave a Reply

Your email address will not be published. Required fields are marked *