Heater Blowing Cold Air?  Furnace Troubleshooting from ASM.

Heater Blowing Cold Air? Furnace Troubleshooting from ASM.

You might think that a heater blowing cold air is not a common problem for a Santa Clarita heater repair company, but this one was emailed to us by Jeff over in Redlands where it can get quite chilly in the evenings, especially in the winter.  As I’ve said before, most of the topics we discuss come from questions we are often asked but I like this one in particular because we got an email about it.  Regardless of whether you are in the Santa Clarita area or in Michigan, knowing how your heater works and what problems it may have is the key to figuring out why you have a heater blowing cold air.  I know that heat is not usually a priority in Southern California, but if you live in the San Bernardino mountains then a heater blowing cold air can be pretty concerning when the temperature starts to drop.  Those of you who read our articles regularly, know that we are a small, U.S. Veteran-Owned HVAC company in Southern California, and pride ourselves in giving people honest, straight answers to their questions.  This will be no different.  This article will address some of the most common causes of oil, gas, and electric heaters blowing cold air instead of heating properly.  We will tackle the subject, in order, by working our way from the most likely causes to the least probable.

1. Occam’s Razor: Is Your Heater Blowing Cold Air Right After You Turn it On?

oil heater blowing cold air from radiatorI have found that Occam’s Razor works more often than you might think, even in the HVAC business (What does HVAC stand for?).  Occam’s Razor states that all things being equal, the least complex answer will most often tend to be true.  With a heater blowing cold air, the first question I’d ask someone is if cold air is blowing from their furnace right after they turn their heat on.  I honestly don’t mean this to be insulting, but you’d be surprised at how many times people are shocked to learn that neither your air conditioning nor your heater will be cold or hot right after you turn them on.  Just like the hot water doesn’t run in your faucet immediately after you turn it on, it takes time for the recently heated air to travel through your ducts to your registers and into your house.  An electric heater and a gas furnace blowing cold air will be exactly the same, so give it a few minutes after your turn it on.  It will also depend on how much insulation you have around your ducts and how old your furnace is.  For more on Occam’s Razor, visit Wiki: Occam’s Razor.

2. Thermostat: A Furnace Blowing Cold Air Could be Due to Your Thermostat.

gas heater blowing cold air because the pilot light is outAgain using Occam’s Razor, we can assume that the next likely culprit for your heater blowing cold air is your thermostat.  Ensure that your thermostat is set higher than the temperature in your home.  Otherwise your electric heater or gas furnace will not turn on.  Again this seems obvious, but don’t just assume that this is written for the stupid – you wouldn’t believe how many times I’ve been called in for a heater blowing cold air only to find out that it was actually a cold war between the husband who wants the heat lower than his wife!  I’m not a marriage counselor, so I will leave you with this: before you call us in, make sure you ask your better-half.

Another possible culprit for your heater blowing cold air is the batteries in your thermostat.  Some thermostats are connected to central power and some have central power with a battery backup, but what some people don’t realize is that there are some thermostats that work on batteries only, so check your thermostat and replace your batteries accordingly.  It is also important to know that the batteries don’t have to be completely dead for your thermostat to act up.  This can cause your heater to blow cold air due to improper temperature inputs from the thermostat.

3. Ductwork: A Not So Obvious Cause for a Heater Blowing Cold Air.

gas furnace blowing cold air into ductingIf you are not familiar with walking in your attic, then do a little research first.  Remember that you can’t just go walking around or you risk putting your leg through the roof.  Make sure that you only walk on the joists, the wooden planks running across your attic (usually 2X4s).  A heater blowing cold air could be a result of a leak in your ducting.  Over time, the changing temperatures wear on all sorts of things and your ductwork is no exception.  Your ducts are usually in the attic and outside of your insulation, exposing them to the bitter cold.  Ensure the that you check these before proceeding to the next step because these can be an easy fix but a serious problem.  If your heater is blowing warm air but that air is venting into the attic or mixing with colder air from your attic, then the net result will be that your heater is blowing cold air.

Another possible problem is the accumulation of dirt and debris inside of ductwork which can slow down airflow.  This problem is rarely (very rarely) severe enough to fully block airflow, but it is worth looking into.  If airflow is blocked too much, it can cause your heater to overheat which can cause it to kick off as a safety precaution.  This would not result in a heater blowing cold air, however, and would be an entirely different problem.  If you think you might have this problem, instead try: Furnace Not Blowing Air.  I don’t recommend paying to have your air ducts cleaned unless they are severe and prevention goes a long way in avoiding this problem (proper use of filters, etc).

For more on this, try: Should I Have My Air Ducts Cleaned?

4. Check Your Pilot Light: Gas Furnace Blowing Cold Air.

If you have an electric heater then skip this section.  However, if cold air continues to blow through your heating unit then it is more likely to be a serious issue.  The next step I’d take is to check your pilot light.  A pilot light is a small flame that is burning constantly to ensure that your heater can ignite easily (rather than relying on an ignitor that can fail).  These can be blown out sometimes though, usually as a result of the gas company shutting down lines for maintenance or emergencies.  For information on how to check your pilot light, try the following video:

If you are unable to determine if your gas furnace is receiving the proper gas supply, then try another gas appliance in your house.  Often times if you have a gas furnace then you will also have a gas stove and oven, so start there.

5. Check the Filter: If Your Oil Furnace is Blowing Cold Air.

If you have an oil heater blowing cold air then the problem is similar to your gas furnace; it may be starved of a fuel source.  Oil furnaces have a filter that ensures contaminants don’t get into the heating system which ensures that everything is running smoothly.  These become clogged sometimes, so you need to ensure that you are changing them regularly.  Fortunately, they are pretty inexpensive and easy to change.  A furnace blowing cold air is not going to happen for no reason, so keep working your way up from most obvious to least obvious.  If you have an oil furnace blowing cold air then try this quick video on how to change your oil filter:

6. If your Oil Furnace is Still Blowing Cold Air, Check Your Flame Sensor.

If your heater starts by throwing warm air out of your registers, but you are soon left with a heater blowing cold air, then I would suspect that your flame sensor needs to be cleaned.  If your flame sensor is encrusted with residue and carbon, then it will initially turn on but will quickly lower the temperature due to inaccurate readings.  Start by taking a look at it to ensure that it is in good shape, then clean it by using the following method:

7. Still Have a Heater that is Blowing Cold Air? Check It’s Size.

Size is not a matter of preference.  There is a properly sized heater for your home, and there is also a heater that is too big or too small.  It is not uncommon for people to think that bigger is better, but in the age of energy efficiency this wisdom is no longer the case.  Modern furnaces are designed to blow air for a specific amount of time that is customized for your house size.  If it is too big, then it will heat your house too quickly and will never allow this warm air to penetrate the insulation, causing your heater to kick on frequently throughout the day, increasing your energy costs.  If it is too small, then it will not have a sufficient amount of power to heat your home.  In Southern California and the Santa Clarita area, you likely have a package system that runs both your air conditioning and your heating.  For information on how to properly size your heater, try:

heater blowing cold air from gas furnace

If You Still Have a Heater Blowing Cold Air, Now You Call in the Pro’s.

At some point, no matter how much you know it is just simpler and cheaper to call in the pro’s.  A licensed HVAC contractor will have a far better idea of how to fix a heater that is blowing cold air than your average joe.  You’d change the oil in your car, but would you attempt to replace the transmission?  Probably not, and in the end you’d end up paying far more than you would have if you had gone to a heating professional in the first place.  For information and tips on how to pick a licensed HVAC contractor in your area, try:

At the end of the day, most heating units last about eight to ten years.  The biggest culprit may just be that your electric heater, oil furnace or gas furnace needs to be replaced.  There are signs and symptoms though, so don’t go off half-cocked.  Do your research.  Here’s a place you can start:

Save Money on Your New Furnace & Air Conditioner by Watching This Short Video

If the troubleshooting isn’t going well, and you think you might need a new furnace, then before you do anything, you should think about trying this online service.  It will save you money, tell you what you should be paying, and keep you from falling for the tricks and traps of predatory contractors:


Prevention: How to Keep Your Heater from Blowing Cold Air

I am honestly not throwing a plug in for our company because we don’t do furnace inspections, but you should put consideration into having your furnace or heating system inspected every few years.  Several components can develop cracks over time from the constant heating and cooling of parts over the years.  This is especially prevalent in your heat exchanger if you have a gas or propane furnace.  This can be dangerous because it can leak carbon monoxide into your house, causing carbon monoxide poisoning.  A proper inspection will detect this before it becomes a problem.  For more information on what your heat exchanger is, how it works and what you need to do to keep it in good shape, take a look at: Furnace Heat Exchanger.

If you live in the northern states and have harsh winters, I’d recommend you have your furnace inspected from a reputable contractor every two years.  Make sure that they use an endoscopic camera and other specialized equipment, not just their eyes!  Modern equipment is key to doing this right and that is what you are paying them for.  If your furnace is the only means of heating your house in the winter, then a heater blowing cold air can be dangerous, so I’d recommend you have your furnace or heater inspected annually, usually in the fall is best.

Ok, so I won’t beat a dead horse here, but if you’re still in trouble, and don’t know how much a new unit might cost, it’s worth spending 15-minutes on our HVAC Design & Consultation Program.  It’ll give you tons of useful information, including: a fair installation price for your specific project, proper sizing information for your AC and furnace, AFUE recommendations, price breakdowns, and much more.  If you decide to use it, here’s the information:

Click Here to Learn More About the HVAC D & C Program

Final Thoughts on a Heater Blowing Cold Air

There are several things that you can do about a heater blowing cold air, but if you keep running into problems then it might just be time to call in a heating repair company.  If need be, here is a useful article on: HVAC installation cost, and how to get a fair price.  How serious of an issue this is depends greatly on where you live.  If you live in the north (Bozeman, MT, etc), then I’d recommend you have your furnace inspected annually to ensure that it is working properly.  I’d also recommend that you install a backup means of heat…just in case. ASM has over 25 years of heating and air conditioning experience.  If you have questions about a heater blowing cold air or any other HVAC related topic, try the ASM Air Conditioning and Heating Blog.  For those of you who live in Santa Clarita, San Diego or general Southern California areas, we’d be happy to help you, just click below:


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Timothy Kautz
About the author
Tim K.

The University of Virginia - 2005 / The Wharton School of Finance - 2016 / U.S. Naval Aviator 2005-2015. At All Systems Mechanical air conditioning and heating, we believe that the experience our clients have is every bit as important as the products they receive. Simply put, our results speak for themselves, and we'd be happy to help. If you're in the market for a new AC or furnace, make sure that you get a fair price! Try our online calculator; click the tab on the top of this page for more information.

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