Furnace Blowing Warm Air Not Hot? [3 Easy Fixes]

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Is your furnace blowing warm air not hot? There could be any number of reasons your furnace is blowing lukewarm air instead of hot air. Luckily, you can fix many of these issues without calling in professional services. Some causes for your furnace blowing but no heat include incorrect thermostat issues, dirty air filters, leaking refrigerant, and others that will be looking at shortly.

furnace not blowing hot

How Does a Heater or Furnace Work?

Furnaces, both electric and gas-powered work by heating up the air and forcing it into the ductwork for distribution throughout your home. Gas furnaces burn natural gas or propane to heat up the air passing through the heat exchanger. A heat exchanger is a series of metal tubes that is heated by the burners and transfers the radiant heat to the air passing through it. At the blower unit, is a furnace filter that screens dirt and unwanted objects from getting into your furnace.

While heat pumps don’t produce heat, they can be an efficient way of heating your home. Heat pumps heat your home by absorbing heat from the outdoor air and transferring it into your home by using the refrigerant. They work pretty much like an air conditioner. Infact, in the cooling season, they work in reverse by cooling your home. So now let’s get a little bit into details on why your furnace won’t blow hot.

Why Is my Furnace blowing warm air not hot?

There are many things that could cause your furnace not to blow hot air but the good part is, you can fix them yourself without calling in HVAC professionals. Where, you need professional help, we’ll advise accordingly. So now, let’s see…

1. Thermostat settings could cause furnace blowing cool air

Once of the commonest and most overlooked cause for your furnace blowing cool air. When your thermostat is not set properly, it won’t give you the heat that you need.  The common mistake many homeowners make is to leave the fan on the ‘on’ mode.

When your fan setting is on the ‘on’ mode, it will continuously blow air even when your furnace is not producing heat. So, when your furnace goes on break, your furnace blower will continue blowing lukewarm or cool air.

The fix? The first thing you should check when you notice your furnace blowing warm air not hot is to check your thermostat settings. This is what you need to check:

  • Make sure your fan setting is not on ‘on’ mode. If, it is, change it to heat or auto. When your fan is on ‘heat’ it will automatically coordinate with your furnace so that, when it is heating, your fan will blow, and if it is on break, your fan will not blow.
  • You need to check also that your thermostat setting is not on the ‘cool’ mode. It is frustratingly easily to accidently put your thermostat to cool mode. If its on the cool mode, it will prevent your thermostat from blowing hot air.

Low battery

Another one more thing that could cause your thermostat to misbehave is a low battery. A low battery could prevent your thermostat from sending correct temperature signals to the circuit control board of your furnace. If the battery has stayed a long time without changing, it might be low and its high time you changed it.

2.Dirty Air Filters Can cause furnace blowing but no heat

After you have checked your thermostat and everything with it looks good, the next thing you want to do is check is the condition of your furnace air filter.

When did you last change your furnace air filters? Air filters are an important part of your heating system and if they are not changed on time, they can get very dirty and eventually get clogged.

Clogged air filters can cause a lot of issues with your furnace. When you have dirty air filters, your furnace will not be working efficiently.  On top of that, air filters clean the air that comesinto our homes. Dirty air filters will block away the amount of air that your furnace needs to heat.

If air is blocked, there won’t be enough air to carry away the heat from the heat exchanger. This will cause the heat exchanger to overheat and eventually shut off the system to prevent other components of your furnace from damage. This happens so fast and the only thing you will notice is a furnace blowing but no heat.

dirty air filters cause furnace not blowing hot

What should you do when you have dirty furnace air filters?

If your furnace air filter is dirty, you need to change it. But here are some of the things you need to know before replacing your air filter.  Before getting a new air filter you need to take note of your air filter such as:

  • Check the size of your furnace filter. It’s a number that looks like this 20*25*1.  This size tells the thickness and the dimensions of your furnace filter.
  • Ensure that you insert your furnace in the correct direction. Put it in the direction of the arrow. Having the air filter in the correct direction will allow the air to flow properly and block away the dirt.

3. Furnace Wont Blow Hot Air Because of Low Refrigerant

Heat pumps use the refrigerant to heat your home. The refrigerant absorbs heat from the outside air into your living spaces of your home. If the refrigerant is low or leaking, your furnace wont blow hot air. In this case, if you think your heat pump is running low on the refrigerant, you need to call an HVAC technician to come and make an assessment on what should be done next.

Sometimes, when your heat pump is in defrost mode, you can notice your furnace blowing cool air. But defrosting doesn’t take longer than 10 minutes so, you should quickly have your furnace blowing hot air again quickly.

Other times, it’s your outdoor unit that maybe blocked by objects or debris such as snow and may not be able to absorb heat from your outdoor air.


What do I do if my heater is blowing cold air?

If your heater is blowing cold air, you should first check the following:

  1. Thermostat settings
  2. If your air filters are dirty
  3. if the ductwork has proper insulation and is not leaking
  4. Check if there is fuel supply (you can call your local gas supply)
  5. If you are using a heat pump, make sure the outdoor unit is not covered or blocked with objects

Why is my furnace fan running but no heat?

If you have a furnace running but no heat, it could mean there is no combustion in your furnace and could be a sign of any of these issues:

  • Pilot light not working: the pilot light is the one that ignites the gas on the burners. If its not there or not working, gas won’t burn and there won’t be heat, to heat up the air passing through the heat exchanger.
  • Dirty flame sensors: A flame sensor closes the gas valve if it can’t detect flames. If it is sooty or covered in dirty, it can not detect flames properly. And If flames are not detected in about 10 seconds, the burners will shut off, causing the blower fan to blow unheated air into your ductwork
  • If furnace filters are dirty, they could be the reason your furnace wont blow hot air.

Final Thought

Your furnace just like any other machine needs regular and proper maintenance to keep it running efficiently and in good condition.  Its best to have maintenance tune up at least twice a year.

Regular maintenance can be done once in 3 months and should involve replacing your furnace filter. If this is not done, it could be the reason for your furnace blowing warm air not hot.




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