Why Is My Furnace Blowing Cool Air? A Common HVAC Contradiction.

why is my furnace blowing cool airI have a friend who hates running but does a marathon every single year. He tries to tell me why: that it ultimately feels good to prove to himself he can do it. But, all through his training, he’s seems to me to be pretty unhappy. My friend’s feelings are a living contradiction.

I understand him and I don’t. I have no idea why he runs that marathon when it seems to be his least favorite thing on the planet. Surely there are other ways to stay healthy and go after a lofty goal. I do, however, understand the nature of contradictions. In the HVAC repair business, we see our fair share of them, like air conditioners that sometimes blow hot air and furnaces that, in the dead of winter, start to blow cold. Like my friend’s dislike of running, but annual decision to enter a marathon, these things can be a bit frustrating. So, let’s dig into why that furnace contradiction happens—and how to get it to stop giving you the cold shoulder.

So Hot the Air Turns Cold

What turns so hot that its air becomes cold? I know this sounds like an ancient riddle, but the answer is your furnace when it’s overheated. This is almost always the case when it starts off blowing hot air, but then begins to blow cold—and it’s usually a habitual problem that doesn’t just go away.

All furnaces have something called a limit switch. It’s a safety device that turns off the burners inside your HVAC system so it no longer creates heat. The furnace does, however, continue to blow air, attempting to cool itself until its temperature has fallen to a safer level.

One of the potential causes for this is a dirty air filter, which blocks air from flowing freely within. When this happens, the furnace must run longer to heat the scant air that’s reaching it, trying in vain to warm your house. While a dirty air filter is quick and simple to replace, the other potential cause for this is more complex: a malfunctioning heat exchanger. This is a far more serious problem you shouldn’t ignore or attempt to cure on your own. It’s a job for a trained HVAC technician.

What’s Behind the Pilot Light

One of the most common causes for a furnace blowing cold air is a pilot light going out. The reason it went out, however, tends to vary. And, this is a problem that pertains strictly to combustible furnaces.

Usually, a pilot light repeatedly going out means there’s a problem with the thermocouple. This is the copper rod you see the pilot light hitting, and it controls the gas valve and the ignition. Basically, if a thermocouple is bad, it will prevent the pilot light from staying lit. It usually means the thermocouple needs to be replaced, but it could also mean that it’s just dirty. You should have an experienced tech look at this tricky piece of equipment.

Trickier Issues Behind the Cool Breeze

It’s frustrating when a furnace blows cold air—anyone who’s experienced it would agree with that. But, it’s also a frustrating problem to fix because of how many different potential causes there are. While the pilot light and overheating are the chief culprits, you’ll need a trained HVAC technician to also evaluate:

  • An inadequate gas supply: The gas valve that releases fuel to your furnace could be faulty or, if you haven’t cleaned it in a while, it could be clogged by debris. Either way, when the gas valve isn’t functioning, the furnace doesn’t have anything to convert into heat and so the air it omits remains cold.
  • Duct issues: Leaks or holes inside ductwork can cause many different problems, and one of these is preventing hot air from reaching your home. Many houses have ducts built into their basements or attics. If these ducts get holes, the heated air from your furnace may be escaping into rooms you never use, preventing it from reaching you in ones you do.
  • A condensate line clog: In newer, more efficient furnaces, it’s likely there are condensate drain lines which remove the water that results from the heating process. If these lines happen to get clogged, the furnace usually has a safety precaution that prevents the burners from lighting, making it so the air that is blown out of your HVAC system stays cold.

All of these problems are somewhat complex, and, in most cases, labor-intensive to fix. Leaky ductwork is nearly impossible to correct on your own, given how large duct systems tend to be. Messing with the gas supply is always a dangerous idea because of the flammable nature of the fuel. And, clogged condensate lines are tricky, too, because the problem is just as much a plumbing issue as it is HVAC. I’d suggest calling a trained professional to handle any of these scenarios.

One last thing I’d like to note is that before considering any of the problems we’ve talked about here, check your thermostat. It’s not unusual for a busy homeowner to have accidentally set it to ON rather than AUTO. When in the ON position, furnace fans blow constantly, even if the temperature is set low and not calling for more heat.

But, if that’s not the case, I’d recommend calling a professional. A furnace that blows cold air is such a frustrating contradiction, especially during the frigid winter months when you really need that heat. The causes are so varied that it’s often really tricky to pinpoint any one of them—it’s best to let a pro take a look. They stand a much better chance at making sure your furnace is doing what you want it to, rather than the exact opposite.

Stop getting frustrated at the cold air coming out of your heater. Call the professionals at Bell Brothers today to get this common problem cleared up.