Why Is My Furnace Blowing Cold Air? Quick Guide for Recurring Furnace Issues:

With winter around the corner, we understand how worrisome and uncomfortable it can be to cope with a malfunctioning furnace. Read on for potential solutions for anything a furnace blowing cold air intermittently or constantly.


A Smart Home Works professional HVAC technician will be able to pinpoint the cause of your furnace’s defect. The root cause may lie anywhere, in the thermostat, or within your furnace unit. We do not recommend trying to fix it yourself, as unreliable repairs can ultimately be hazardous to your home and your family’s well-being. Please contact us for any questions. Your solution may require some basic maintenance, replacement parts or furnace upgrade.

The most likely cause for your furnace blowing out cold air lies in your thermostat

Even if you call someone for repairs, they will logically start their investigation there.

1)Is your furnace blower blowing cold air only some of the time? Is the furnace blower also constantly running?

If your thermostat's fan is set to "ON"

  • Check your thermostat's settings - If your thermostat's fan is on, then simply change your setting to "Auto". Checking your thermostat to ensure the "Fan" setting is set to "Auto" instead of "On" will ensure the fan is only in motion when the furnace is actively heating your home.
  • If this solves the problem, then great!

If you have set your thermostat's fan to "Auto" and your furnace blows hot air and sometimes cold air.

  • If switching the thermostat's fan setting to "Auto" only solves the problem temporarily, and your furnace then starts blowing cold air for notable increments of time, the problem is likely due to a dirty filter, which we recommend replacing as soon as possible. Click here for information about maintaining your furnace.
  • Many furnaces come equipped with a limit switch, which is a safety component that prevents your furnace from overheating. If your furnace's blower still blows cold air after switching your thermostat's fan setting to "Auto", then the fan is likely blowing air into an obstructed filter, causing the furnace to quickly overheat. In cases like this, the fan will continue to run as a safety measure that is intended to cool down the furnace, which has become overheated from being overworked, due to blockages caused by its dirty filter.
  • In most cases, replacing your furnace's filter will help to "unclog" your furnace, thus allowing the blower to blow warm air without obstructions. Allowing your furnace to overheat due to a dirty filter can wear out the furnace's heat exchanger, which can end up costing you a pretty penny. The reason for this is simply because the furnace will need to run hotter, and for longer periods of time, until your home is warm enough - Eventually, this will degrade the heat exchanger until it is rendered unusable.

Smart Home Works offers protection and maintenance plans on all of our equipment. We also provide plans for your existing equipment.  

2) My furnace is blowing cold air at all times, the thermostat is set to "heat", and its fan is set to "Auto"

  • First off all, check your furnace for a lit pilot light.
  • Is the pilot light on?
  • If yes - then we recommend replacing your filter right away.
  • If this solution does not work then we urge you to call a technician as a precaution against carbon monoxide poisoning.
  • If the pilot light is off:
  • Turn your gas off immediately. The lever should be located in close proximity to your furnace.
  • You can light the pilot light by first locating the instructional label on your furnace, and then following its instructions.
  • If you have trouble with this, then you will likely find a switch on the bottom half of your furnace with the words "Pilot", "Off" and "On" inscribed on, or around it.
  • Once you have found the switch, turn the furnace's switch to the "Off" position for a good 4 minutes. Skipping this step can be dangerous or fatal, as explosive gas may be spreading throughout your home. Take this time to locate the "Reset" button around the same area on your furnace.
  • Once 4 minutes passes, turn the switch to the "Pilot" position, with a lit barbecue lighter or long match in hand.
  • Follow the gas line, from the pipe / source, through to the switch, and continue following it into the chamber's opening, where the actual pilot can be lit (See below videos for a more visual representation.)
  • Insert the lit flame into the opening while keeping the "Pilot" light pressed for approx. 1 minute.
  • Please note that many furnaces use an electrical element to ignite the gas. It can be difficult to repair a damaged element and we recommend calling an experienced technician for repairs or replacements.

Once lit, turn the switch to the "On" position.

Keep an eye on the pilot light to ensure that it stays on.
 

If the pilot lights don't burn blue then call a technician for cleaning and service.

Further, if the pilot light won't stay lit, there may be a malfunction in your central heater's thermocouple, and it simply needs to be replaced or shifted back into proper positioning.


We are available 24/7 in Ontario, Alberta and British Columbia - Please Don't Hesitate to Contact Smart Home Works for any questions or concerns.