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February 24, 2015
Is your furnace only blowing lukewarm air rather than hot air? Here are some reasons this could be happening.
The thermostat is set to “on” rather than “auto”
When your thermostat is set to “on”, the fan in your heating and air conditioning system will run constantly—even when your furnace isn’t actually heating. This can make the air coming out of the vents seem lukewarm.
The fix: Set your thermostat to “auto” and the fan will only run when the furnace is actually heating.
You have a heat pump
Do you actually have a gas furnace? Or is your heater an electric heat pump? New heat pump owners often notice that the air coming out of the vents isn’t as warm as when they had a gas furnace.
Heat pumps typically produce air that is about 90-100°F, whereas furnaces can produce air as high as 120-125°F.
The fix: Nothing. This is normal for heat pumps. As long as the thermostat is reaching the temperature you set it at, it’s working fine.
There’s a problem with your filter
If the airflow to your furnace is restricted, it can cause your furnace burner to shut off while the blower keeps running. This will make the air only lukewarm (or cold).
Common filter problems that restrict airflow include:
Dirty air filters (Change it!)
Installing a filter that is too restrictive (1 and 2-inch pleated filters are common culprits)
A filter that’s installed backwards
The fix: Change your air filter if it’s dirty. And make sure it’s installed correctly.
You have a high-efficiency, two-stage furnace
High-efficiency, two-stage furnaces are designed to run for longer periods of time and turn on and off less frequently. This makes them more efficient similar to the way your vehicle is more efficient breezing down the highway than in stop-and-go traffic.
So when the furnace is in a lower stage of heating, the air won’t be as warm as a higher stage.
The fix: Nothing. This is normal. You can check this by cranking your thermostat up a few more degrees to see if the air gets warmer.
Air ducts are leaking
The lukewarm air coming out of your furnace can also be the result of leaky air ducts. If your ducts are leaking (and ENERGY STAR says that almost all homes leak about 30% of the air in their ducts), cold outside air is making its way into your ductwork.
This can cause the air to feel only lukewarm.
The fix: Duct sealing. Learn more about how to tell if you have leaky ducts and how to fix them.
Gas supply problem
If your furnace isn’t getting gas, or enough of it, the furnace may not be heating anything. Check your gas supply. Was it turned off for any reason? Is there an issue with the gas company?
Go to your furnace and watch it when it turns on. If the burners ignite and heat, the problem isn’t the gas supply. But it could also be a thermocouple or flame sensor problem (see below).
The fix: Make sure the gas valve to your home is turned on. You may need to call in a professional if there’s a gas issue inside the furnace itself.
Thermocouple or flame sensor problem
If there’s a problem with your thermocouple or flame sensor, it won’t let your furnace ignite the burners. Again, you can watch your furnace to see if it ignites correctly.
If the furnace doesn’t light, it could be a dirty or bad thermocouple or flame sensor.
The fix: Contact a professional to figure out the problem and fix it.
Need more help? Contact a professional heating company
These are the most common reasons a furnace will blow lukewarm air. But, of course, there are many other possible causes. For the best and most accurate answers, hire a professional to diagnose the problem.