With just a few short weeks until the official start of Spring, you might be thinking this is a strange time to worry about your furnace breaking down. But as close as Tulip season may seem, it can still get down into the 40’s at night here in Ventura County. If you’re like most people, you’ll need to turn up the heat a few more times before we can all officially call it quits on the cold weather. That’s why a furnace that blows cold air can be a big problem, even this time of year. If you’ve turned on the heat only to find there’s a chilly wind inside your home, here are ten reasons that could be happening. And, because we’d hate to leave you without any solutions, we’ll also set you up with a few possible DIY fixes for a furnace that’s just not doing its job.

10 Reasons Your Furnace Could be Blowing Cold Air


1. Thermostat Issues

Sometimes, when your furnace is sending an Arctic chill instead of the heat you crave, it’s not your furnace’s fault at all. There’s a good chance it’s the thermostat that’s causing the problem. Focus your initial troubleshooting here, since you have the greatest chance of fixing the problem yourself without having to call in a professional. Thermostat issues come in many flavors with just as many solutions. These range from the face-palm obvious to the slightly complex and technical. In that order, here they are:

  • The fan is set to “on”. If your fan is set to continuously blow, it will at some point be blowing cold air. The reasoning here is simple: furnaces don’t pump out hot air nonstop. Warm air is sent through your ducts when the thermostat says it’s needed. When the air temperature reaches the setting you’ve chosen — let’s say 70 degrees  — the furnace can take a break until the air drops below 70 again.
    • But if you’ve set your fan to be “on” at all times, it’ll blow cold air during the furnace downtime. Therefore, set your fan to “auto” to coordinate it with the furnace and you’ll just have hot air blowing, not cold.
  • Somebody else is adjusting the thermostat. If you’ve ever worked in an office where someone has cranked the AC way up high to the discomfort of others, then you know where this is going. Just like we all have varying degrees of tolerance for cold, we all have different ideas of how warm the house should be, too.
    • Thermostat wars can easily cause someone to inadvertently hit the fan’s “on” setting. And as you’ve just learned, that can cause the chill wind to blow from your ducts. Time for a family conversation about compromise.
  • The battery is low. In addition to the other silly mistakes people make with their thermostats, a low battery is easily remedied. Some thermostats work in conjunction with the home’s central power system but others rely on a self-contained battery unit. This latter type is the kind that will occasionally run low and must therefore, be checked from time to time. Even a low battery that’s not quite dead can cause problems. It definitely does not need to be totally dead in order to cause your thermostat to misbehave.
    • Thermostat batteries last a long time, however, so you’re more likely to have this be a problem if your thermostat is old. Either way, check the batteries before calling in an HVAC professional — you could save yourself some embarrassment.
    • From here on out, we’ll be moving into a slightly more complex set of furnace issues. Unlike the problems you’ve seen so far, they’re not usually caused by user error and are therefore more likely to require outside help.
  • You missed something during your DIY thermostat installation. Some choose to call in a professional but others go the do-it-yourself route when they get a new thermostat. With the explosive growth in popularity of programmable thermostats we’ve seen in the past couple of years, this problem is actually quite common.
    • We’ve seen countless homeowners who choose a great new thermostat but it’s simply not compatible with the existing heating system in their home. If you suspect that’s what has happened in your case, your next step would be to call an HVAC expert who can easily tell what’s what. Even you’ve really made a mess of things with a cobbled-together system of DIY components, a professional will be able to sort it all out and identify what’s causing your thermostat issues.


2. Filter Issues

There is actually one more user error that could be the source of your cold air issues. After you’ve checked the batteries and after you’ve surveyed the other household members for sneaky thermostat-adjusting activities, there’s one more “easy” fix and that’s to change the filter in your heating system. A dirty old filter is a clogged filter and that can restrict the air flow to your HVAC system. This, in turn, could limit the ability of your furnace to produce and distribute warm air throughout your home. Some furnaces even contain a built-in shutoff that kicks into action when the filter gets clogged, in order to prevent an overheated burner. When that happens, the furnace will blow cold air. Filters are cheap and easy to change. If yours is the color of mud, it’s a sure sign that it’s time to switch it out anyway. After doing so, restart your furnace. Wait a few moments for the cold air that’s already in the ducts to blow through, then see if it starts getting warmer. If so, congratulate yourself on finding a solution without having to call in the experts. If changing the filter and restarting doesn’t do the trick, here are some more possible solutions.


3. Issues with the High Limit Switch

A neglected dirty filter can even cause certain parts of your HVAC system to fail. Your system can weather a few rounds of overheating, but that can’t go on forever. Repeated overheating can lead to failure of your high limit switch. There’s no real need for you to understand how the high limit switch works but suffice it to say you don’t want it to fail. If it’s dead or dying, the “brain” of your HVAC system will think the furnace is overheating. To remedy that problem, it will turn on the fan to cool everything down. But meanwhile, from your end, all you see is the furnace blower running nonstop, blowing cold air and you have no idea why it’s happening. Not realizing your high limit switch is dead (or even that such a part exists), you’ll be scratching your head on this one. The switch might also be hard to find, another reason you may need the help of an HVAC technician.


4. Condensate Line Issues

Another component of your HVAC system that requires regular maintenance is the condensate. When your HVAC system is operating in hot weather, the condensing unit is continually dripping water into a drip pan. That water is removed via piping that eventually makes its way outside your home, where the water drains off. Those lines have a tendency to get plugged with dirt and grime, especially if you’re not performing regular maintenance on them. Of course the more you use your unit, the more slime you’ll have in the pipes. Once they’re clogged, your float switch will get tripped, causing your system to shut down. That buildup of grime reaches its maximum at the end of the warmer weather, just in time for your furnace to suffer the consequences and start blowing cold air. The moral of the story here is to keep those condensate lines clean and open. HVAC companies have all kinds of specialized tools for clearing out slimy condensate lines.


5. Problems at the Source

Here’s an obvious question that too many homeowners forget to ask: are all systems “go” at the fuel source? We’ll sneak in a few more face-palm moments here — there’s one for every type of furnace out there, and they’re dead-simple to fix:

  • Oil-powered furnaces. Is there any oil in the tank?
  • Electric furnaces. Is the furnace switched on at the breaker panel? Is the furnace itself turned on?
  • Gas-powered furnaces. Is the gas supply valve open? If it’s not, there’s little chance the fuel will ever reach the burners.


6. Outside Air in the Mix

The ducts that carry the warm air throughout your house can be subject to wear and tear. They’re often placed outside the heated area of our home, meaning they are exposed to extreme temperature changes all year ’round. Those fluctuations are what can cause weak spots in your ducting, eventually leading to actual leaks. Actually, there are all sorts of perils that can lead to leaky ducts. For example:

  • Ducts that have come unhooked can also develop leaks
  • Previous fixes involving duct tape can come undone when the tape peels
  • Ductwork can fracture when it’s not properly screwed together

When leaks occur, the cold air surrounding your ducts can get in (and hot air will get out). That outside air in the mix lowers the temperature of the air that’s just come from your furnace. The cooler air is what ends up blowing into your living areas.


7. Pilot Light Issues

Has your gas company had to perform maintenance lately or has there been some emergency work done on the lines? If so, they might have caused your pilot light to go out. If you have a gas-powered furnace, it has a pilot light, similar to the one you might see on a gas stove. These can get blown out, too. Older furnaces are more likely to let the pilot light go out. Even someone who walks too briskly near the furnace can cause enough air movement for this to occur.


8. Gas Valve Issues

Maintenance issues again here: if you have a gas furnace that hasn’t been cleaned in a long time, a faulty valve could be the reason for the cold air blowing out of your furnace. Dirt and debris can collect on the valve, causing it to shut down. Or, the valve could simply be worn out. This is another reason to have your system checked regularly.


9. Burner Issues

Here’s yet another HVAC component that’s vulnerable to dirt and grime-related problems: the furnace’s burner. When it’s clogged, there won’t be any fuel getting through. Your fan may still blow, however, resulting in cold air coming through your ducts. The solution is to have your burner cleaned. Motivated and handy homeowners who have the drive can learn how to do this themselves, manually. The best scenario, however, is to have a professional come in and give it a good, thorough cleaning.


10. Flame Sensor Issues

An old or dirty flame sensor will cause your furnace to blow cold air, too. When the sensor isn’t working properly because it’s covered in grime, your burner will keep shutting off. The fan, however, may keep on rockin’, causing that cold air to blow. Regular maintenance…


A Final Word on Your Heating System

There are a number of issues that can lead to a nasty blast of cold air when you’re expecting warmth from your heating system. Some can be self-diagnosed and fixed. Others will require outside help from an HVAC professional. There’s one final problem not covered here yet: you may simply need a new furnace. If that’s the case, you’ll definitely want an expert to weigh in before you start acting on the final solution. They’ll help you assess the right type and the correct size heater for your home. But in the meantime, try these fixes we’ve covered here, starting with the simple solutions that you can apply yourself. And if you think you might need to call in some help, we’re always here when you need us!

(805) 988-1800