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Heater blowing cold air: step by step DIY repair guide

Last updated: Nov. 3, 2022, 3:07 p.m.

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Is your furnace or house heater not working? You're not alone, which is why we wrote this guide and filled it with DIY heater troubleshooting steps. Do your best to methodically work through each step and there's a decent chance you can get your heater working again.

Before we start though, I want to make sure you know which kind of heater you have. There are two common types of house heaters: gas or propane furnaces and electric heat. As you might guess, furnaces burn gas to create heat and electric heaters use electricity to generate heat. As a general trend, the US is moving towards electric heat, but as of today, the vast majority of homes are heated by gas burning furnaces.

This matters because some of the troubleshooting steps differ depending on what kind of system you have. If you don't already know what you have, an easy way to check is via your utility bills. If you don't have a gas bill and you don't have a propane tank, you have electric heat. If you do have gas or propane, you will need to go to your actual heating unit inside and Google the model number (typically written on the inside of the panel that covers the system). If your model number pulls up something called an air handler, you've got electric heat.

Figured out what kind of heater you have? Good, let's get started.

NOTE: The following heater & furnace troubleshooting steps are not super technical or difficult, but please don’t do them unless you feel comfortable working around your HVAC system. It is a serious piece of machinery and you can get hurt if you aren’t careful.

Your heater or furnace is blowing cold air

Step 1. Check your thermostat’s fan settings

If your heater blows cold air sometimes and warm air other times, your thermostat’s fan setting may be set to “On” rather than “Auto”. Quickly confirm that’s not the case. If it is, your cold air issues should go away after switching it.

Step 2. Change your filter

Dirty filters can cause all sorts of issues for HVAC systems. As time goes by and debris builds up, your system has to work harder and harder to push air into your home. Eventually, it throws in the towel and quits.

So to be safe, go ahead and change out your filter if you haven’t done so in the last few months. If you’ve never changed it, just look for a grill like the one below:

return air vent

Once you find it, open it up and pull the filter out. Make a note of the measurements written on the side and then go to your local Walmart or home improvement store and buy a replacement.

If you don’t have a big metal vent anywhere, your filter may be located next to the actual heating system. To replace it, go to your furnace or air handler (the technical name for an electric heater) and look for a door or opening where a filter might hide. Once found, replace it with a clean one.

Note: When replacing your filter, be careful to put the filter in the correct way. Most filters have an arrow on them that points in the direction air should flow. That arrow should point up if your filter is behind a metal vent in the ceiling or towards the heater if next to the actual unit. Sounds backwards, but that’s how your system works.

Step 3. Confirm your gas valve is on and that you have gas/propane flowing to the furnace (furnace only)

It’s hard for your furnace to work if it doesn’t have any fuel flowing to it. To confirm yours does, you can check a couple things. One, make sure your gas is turned on at the meter. If your furnace runs on propane, confirm you have propane in your propane tank.

Once you do that, make sure the gas valve next to the furnace is set to “on”. To do that, you have to know where your furnace is. A good rule of thumb is that if you live in a home built in the last 20-30 years, it’s probably in the attic. If it’s not in the attic (or you have an older home), then it’s most likely in one of the following places: utility closet in your garage, hallway closet, or crawlspace underneath your house. If you’ve never seen yours, they look something like this:

furnace.original.jpg

Once you find it, look for a gas valve like the below and make sure it’s in the “on” position (the handle should be aligned with the piping).

furnace gas valve

Note: If a technician or utility has turned your system off at the gas valve for safety reasons, do NOT turn it back on.

Step 4. Confirm your pilot light is lit (furnace only)

If you have a very old furnace, your pilot light may need to be relit. To do that, find your pilot light assembly (usually near the bottom of the furnace, with a dial that says “on”, “off”, and “pilot”) and turn the switch to off. After waiting 10-15 minutes for any lingering gas to clear, turn the switch to “pilot” and use a lighter or match to relight the pilot flame. Once you’ve got a strong flame, turn the switch to on.

Note: Old furnaces are dangerous. Be careful while lighting your pilot and always read the manufacturer manual first.

Step 5. Reset your system

Sometimes a quick reset is all your heating system needs to get working again, especially if you just removed a really dirty filter. To reset yours, find the breaker panel and look for the switch that controls the heater. It could be labeled a few different things depending on what kind of heater you have, but usually it’s labeled “furnace”, “air handler”, “AC”, “heat pump”, or “HVAC”. Go ahead and turn all of the above switches off. Wait five minutes, then turn them back on.

Note: If you have a really old furnace (pre-1990), you may have to manually re-light the pilot light after resetting. See step #4.

Other common heater or furnace not working issues

Furnace short cycling

If your furnace is repeatedly turning on then off, there’s a very good chance you have a dirty flame sensor. If you feel comfortable cleaning it yourself, you can follow this Youtube video to do it.

Furnace not igniting

If your furnace will come on but won’t light, you may have a broken or cracked furnace ignitor. If you feel comfortable doing it yourself, this is a good video on how to replace it.

Heat won't come on

If your thermostat says the heat is on but you're not getting any heat, you may have a bad wiring connection between the thermostat and the heater. To check, first take your thermostat off the wall and confirm all of your wires are connected securely to the thermostat base. Once you do that, put your thermostat back on and set it to heat. Then follow the first five minutes or so of this Youtube video to check if your heater is receiving the heating call from the thermostat. You'll need a multimeter to do this. If you're not getting 24 volts at the heater, you either have a faulty thermostat or a break in the thermostat wire.

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