Heater Blowing Cold Air? Find Out Why This Happens

There is nothing more frustrating than coming home after a long day, stepping into what should be your warm, cozy home, and instead finding it chilly and uncomfortable. First, you check the thermostat and find that it is set properly but then you discover that the vents are blowing out cold air.

 So is your air conditioner fan blowing but not cooling? There can be numerous reasons why your HVAC system’s furnace seems to be working but is blowing cold air through the system. We’ll take a look at some of the situations that may cause this problem, but let’s first look at how the system works as a whole

Potential Causes of Why Your HVAC Heater is Blowing Cold Air

In simple terms, central heating is a heating system that enables your HVAC unit to send warm air blowing out of vents into your home to keep it at a comfortable temperature when it is cold outside. When this system has issues, it can malfunction and not provide the warm airflow that you rely on to heat your home. So, the question comes back to what can cause this situation.

Thermostat Problems

A thermostat issue can cause your central heating to blow cool air. One simple reason is that you have simply set your thermostat incorrectly. Check the thermostat and make sure it is switched to the heat position. Also, make sure it is set to “auto” rather than “on.”

If your unit was set to cool, you’ve likely located and corrected the problem by switching it to heat instead of cool. Setting it to “auto” will let the thermostat do its job to maintain the temperature you’ve set instead of putting the fan unit in a constantly running mode. This will save energy and unnecessary wear and tear on your system.

Furnace Slow to Heat

Just like other home appliances such as your oven or a space heater, it takes a bit for your furnace to warm up and begin putting heat back out into your home. If your unit has been off or set to only come on at much cooler temperatures, turning the heat up or turning it on may not provide immediate results.

You may find the air coming from vents is cool until the system has had a chance to things up. This is not abnormal. However, if you still don’t have warm air flowing after about fifteen minutes, you should investigate further.

Dirty Filter

If you haven’t kept up your air filter maintenance, it may have gotten clogged with dirt and debris. When this happens, airflow can be blocked, preventing the system from operating properly. If the obstruction of airflow is bad enough, it can trigger the emergency shut-off to your heat exchanger to prevent overheating. You’ll be left with a fan blowing out only cool air.

Pilot Light Outage

If you have a gas furnace, your system relies on a pilot light to ignite the gas that provides heating. If the pilot light goes out, the gas is useless and you’ll be left with nothing but cool air. While many newer models don’t require a pilot light, most older systems do. They can get damaged, obstructed, or fail.

Overheated Systems

The safety switch built in protects your system from damage due to overheating but it also prevents fires, explosions, and exposure to carbon monoxide. The shutdown can be triggered by several problems:

  • Dirty air filters – Airflow can become obstructed
  • Age – As your system ages, parts can become worn and not operate properly
  • Failures – Components have worn out to the point of inoperation, creating a malfunction that affects the overall system

Preventative Maintenance

Between seasons is a great time for routine preventative maintenance. Whether you choose to do your DIY service or hire a professional to service your system, an inspection, and tune-up before that first operation of heat in the fall and air conditioning in the spring can save you a lot of mid-season angst.

You will want to make sure you clean around the external and internal units, replace your air filters, and clean the condensation lines. It’s a good time to adjust the thermostat to suit the current season, as well, allowing you to adjust to warmer or colder months gradually as the temperature changes. Check for puddles, low refrigerant, damaged drip pans, and other damage.

Beyond routine checks, be sure to act on any recommended maintenance. Your system should give you 15-20 years of satisfactory operation, but failure to keep it maintained can reduce its lifespan dramatically.

When to Call the Professionals

There is never a bad time to call a professional HVAC technician if you are having issues. However, if you can address minor problems yourself, you’ll save money and time. Take the time to know your system and how it operates.

This will help you diagnose minor issues that you can repair and limit intervention from a qualified technician to those repairs that are beyond your abilities. Anytime you are experiencing issues with your HVAC system, it is best to call on a professional.

You might also want to have periodic home energy audits or home inspections performed to better understand not only your HVAC system but the other factors in your home that might affect how well it does its job for you.

These types of reviews by a professional can alert you to things like air leaks, poor insulation, potential failures, and other such situations that can affect your system’s operation and energy consumption, and knowing is half the battle.

Armed with a list of improvements that can be sorted according to budget and importance, you can be better prepared to absorb the costs of upgrades or repairs and have the time to shop around for the best solution.


We’ve provided several tips and recommendations to determine why your heater might be blowing cold air throughout the system, but in most cases, seeking professional help is the way to go. If you’ve tried everything and can’t resolve the issue, contact your local HVAC company to get the issue resolved quickly and effectively.

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