Heater Blowing Cold Air [7 Easy Solutions]

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Is your heater blowing cold air instead of heat? If so, there could be any number of reasons your heater is blowing cold air, these include thermostat issues, Dirty air filters, dead blower capacitor, dirty flame sensors. We shall shortly be looking at these causes together many others that can affect how your furnace works.

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Now that wintertime is already here, you are probably turning up your heater all the time to keep you and your family warm.  So, it can be really frustrating to discover that your heater is not blowing hot air at the time when you need it the most to do its job.


7 Reasons Your Heater is Blowing Cold Air

When your heater is blowing cold air, the first thing you want to do is check if your thermostat is set correctly. If your thermostat is set to ’heat’ but your heater keeps blowing cold air, it could be a sign of any of the these issues:

1. Dirty Flame Sensors

A flame sensor is a straight or L-shaped metal rod that is found behind the burners inside your furnace. Flame sensors detect if flames are present. If they cannot detect flames in about 10 seconds the gas valve is closed to prevent any risk of gas leaking into your home.

Flames sensors can stay a long time without cleaning them, typically about once or twice during 15-year lifespan of your furnace. But when dust, dirt and other substances accumulate on the steel rod, they will affect how it works.

When a fame sensor is sooty, it won’t be able to detect flames, so they will lock out burners from igniting. If this happens, there will be no combustion and eventually, not heat generated by your furnace.  Meanwhile the blower fan will continue blowing the cold air. So, only cold air will come out of your vents.

Solution: If you think your flame sensors are dirty and causing the heating issues, try cleaning them. That should sort out the problem.

You can clean the flame sensors by the following steps.

Make sure you turn off your heating system either at your furnace’s power switch or circuit breaker.

Then remove the metal cover from your furnace to locate the flame sensors.

Remove the wire cable connecter and unscrew the flame sensor from its position

 Clean it with sandpaper if there is a lot of build up on it.

When you are done cleaning it, put it back into its original position and close the cover.

2. Heater Blowing Cold Air: Dirty Air Filters

Air filters are among the most common causes for your heater blowing cold air. And they are an important part of your heating system.

Air filters screen dust, dirt, and other debris from air before it goes into the furnace for heating. Dirty air filters will prevent sufficient air from getting into the heat exchanger. When this happens, there won’t be enough air to carry away the heat from the heat exchanger and so overheating will take place.

Overheating will cause the high limit switch to kick into action and shut off your heating system. This is to prevent potential fire and other important parts of your furnace from damage.

When the system shuts off, the blower fan can continue blowing but what you will only get through your home’s vents is cold air.

Solution: Check the condition of your air filters. If they are dirty, change them. Manufacturers recommend changing air filters once in every three months. But if you have issues with air quality or if you keep pets, you will need to change them more often than that.

Air filters can be found behind the return ducts near the centre of your home. This is how you change your furnace air filters.

How to change a furnace air filter

  1. Make sure you turn off your furnace. You can do this at the furnace power switch, circuit breaker or thermostat
  2. Locate the air filter. If it is located right at your furnace, remove the access panel by pulling down the latch.  If your air filter is located behind the return duct the procedure is pretty the same.
  3. Once you have access to the air filter, pull it out and check its condition. If it is dirty, you need to change it.
  4. Check the size of the filter that your heating system uses. It should be in this format 20*25*1. The size tells you the dimensions and the thickness of the air filter. The last number in this case, 1 tells you about the thickness of your filter.  The 1- 2inch filter need replacement every 1- 3 month. The 3–4-inch type need replacement every 6-9 months.
  5. Once you have a new filter place it into position making sure you take note of the direction of air flow. The arrow on the filter should be away from the door.
  6. After you are done, close the access panel or vent cover before turning on your heater.

3.Leaking Ductwork

The ductwork of your heating system moves the warm air throughout your living spaces. The ducts are usually installed outside the area that is not heated. Because of that, they may be exposed to extreme weather conditions and at times they may have leaks on them or disconnections in their joints especially if they were not installed properly.

Another cause for leaky air ducts is rodents. If these find their way into your attic, they can bite and cause damage to your ductwork.

When you have leaky ducts, the hot air that your furnace generates will be escaping from the air ducts before reaching your home and the cooler air will get in, (there will be a mix.) Because of that, the air that will make it into your home will not be as hot.

Solution: If you think there are leaks in your ductwork try look for gaps between joining sections. Seal any gaps with metal tape. If you are not too sure about what to do. Call in a professional HVAC technician to come and give a hand.

4. Dead or Dying Furnace Limit Switch

When your air filters are left without replacement for a long time, they may continuously cause your heating system to overheat and sometimes short cycle. Repeated overheating of your furnace can kill the furnace limit switch.

The limit switch is there to shut off your system when overheating takes place. When it is not working or dead, your HVAC system will think that your system is overheating. To prevent further trouble, your HVAC system will turn on the fan to cool everything down.

That’s why you will only receive cold air from your vents.

Solution: A furnace limit switch can be hard to locate. So, if you think you might be having problems with your high limit switch, it is wise to call in professional service.

5. Heater Blows Cold Air: Burner Issues

Burners are another component of your furnace that’s vulnerable to dirt and grime related problems. When dirt and grime accumulate on the burners, gas won’t be able to come through and so there won’t be combustion in your furnace. No combustion means, no heat been generated but the fan will keep blowing only cold air.

Solution: Some motivated DIY homeowners can clean the burners themselves. But its best to call in professionals to come and give them a thorough clean up.

6. Thermostat Settings

  • Make sure that you check the thermostat settings. Sometimes, you can accidently change your thermostat settings.  When your thermostat is on ‘on’ your heater won’t blow hot air but cold.

Solution: Make sure that you change your settings to auto or heat.

  • Another common mistake among homeowners is the setting on the fan. If your fan is set to on, it will continue blowing cold air when your furnace is on break.

Solutions: If fan setting is set to ‘on’ change it to auto or heat. Your furnace should stop blowing cold air

  • And if your thermostat uses batteries to operate, check if your battery is low or dead. A dying battery can affect how your thermostat works. It can cause your thermostat to send false temperature signals to your furnace.
  • Solution:  If the battery is low or dead, change the battery to bring your heater on.

7. Heat Pump Blowing Cold Air: Leaking or Low Refrigerant

Are you using the heat pump system to heat your home? Remember a heat pump system uses the refrigerant to transfer heat between your indoor space and the outdoor air. If your heating system has a leaking or low refrigerant, you won’t be able to get the heating that you want in your home.

Solution: If you think your system might be running low on the refrigerant, it is a good idea to call in a professional to come and check the refrigerant charge. They should be able to advise what to do in no time.


Why is my heat not coming through the vents?

If heat is not coming out through the vents in 1 or two rooms, you might have closed or blocked registers. Sometimes registers can be closed accidently. You need to make sure you open them all. Move away any objects placed near them such as furniture, rugs. These might be blocking the hot air from freely circulating into your room.

What is the limit switch on a furnace?

The limit switch detects any overheating in your furnace. If the temperature exceeds the normal working temperatures, the limit switch will shut off your system to prevent possible fire and other parts of your furnace from complete damage.

A furnace limit switch does not only keep your system safe when there is overheating, it also regulates the cycling of your furnace. When the desired heat is reached, it will turn on the blower fan.

Repeated overheating of your furnace can damage the high limit switch. If this happens, combustion cannot take place, but the system will continue blowing cold air instead of heat.

How long should a furnace run?

How long a furnace should run during a heating cycle depends on many factors.  But on average, a heating cycle can take anywhere between 10 to 15 minutes.    

How do I know if my furnace flame sensor is bad?

You can only know if your furnace flame sensor is bad if you inspect it. If the sensor insulation is missing, you might need a replacement. If the sensor insulation is intact but has lots of soot build up, you might just need to clean it up.

To clean your flame sensor, you’ll need to use a sandpaper or steel wool. Gently rub the rod with it until it looks clean enough. When you are done cleaning it, use a piece of cloth to wipe out the rod.

How often should you change flame sensor?

You can expect to change a newly installed flame sensors in about 5 to 10 years.

Final Thought on Heater Blowing Cold Air

There are many reasons your heater is blowing cold air. We have discussed the most common ones and how to resolve them. The bottom line is your HVAC system needs regular maintenance just like any other system so that it does not stop working at the time you need it the most.

We recommend you have HVAC maintenance tune up twice a year. In the spring and fall. Change filters, let the HVAC technician examine all other parts of the furnace.

Regular maintenance of your heater will not only keep it running efficiently, but it will also prevent unforeseen breakdowns which can be more costly in the future.

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