If your home is equipped with a gas furnace, you would expect it to blow warm air on demand. If it’s blowing cold air instead, there are a number of factors that could be causing the problem. As a savvy homeowner, you can take care of some of them yourself, without the immediate need of a heating repair contractor. Let’s dive into how to troubleshoot a gas furnace blowing cold air.
We like to start with the simplest/easiest source of problems for your furnace and progress to more complicated issues. If you work your way through this list and your gas furnace doesn’t start working, it’s time to call a local furnace repair expert like Dor-Mar Heating & Air Conditioning.
Tools and Supplies
Before we begin, gather up the following:
- New furnace filter (to fit your furnace)
Furnace Troubleshooting Steps
We recommend you take your time, and step through these possible problem sources one at a time. There is a fairly good chance you will be able to solve the reason why your furnace is failing to produce warm air, without having to call in a heating repair expert.
Open the Heat Registers
Believe it or not, opening the heat registers is the quickest way to restore the flow of warm air into your home. Those families with children know that sometimes, those heat registers can somehow be mysteriously closed after a play session. No matter the reason the registers were closed, if too many get shut, the furnace could start to overheat and shut down automatically.
There are other issues that can rear their ugly heads when the heat registers are closed. Blocking heater vents causes your furnace to work longer and expend more energy than it would normally have to do, which can shorten it’s useful life. When you close a vent, blocking air from flowing, that increases the pressure inside your ductwork. That can cause airflow leaks, or make existing leaks worse, especially at seams and joints. We recommend keeping, at a minimum, 80% of your heat vents open to prevent unnecessary pressure from building up in your furnace and ductwork.
Check the Thermostat
I know it sounds obvious, but check to see if your thermostat is not set to COOL, when it should be set to HEAT (especially early in the heating season). Your thermostat fan switch should be set to AUTO. Check to see if the thermostat is set to a temperature higher than the current room temperature, high enough to kick the heat on and begin to heat the room.
If the thermostat is set properly, you can also check batteries, and circuit breakers/fuses, and be sure to check the interior of the thermostat cover. Believe it or not, dust and debris inside the thermostat can cause it to malfunction. In some cases, dust can conduct static electricity. Just run your vacuum cleaner with a soft upholstery brush or use a can of compressed air (i.e. Dust-Off). Failure of the transformer that provides power to the thermostat can also prevent it from communicating with the furnace.
If nothing works so far, your thermostat may need to be replaced.
Does the Furnace Have Electric Power?
After you verify the thermostat is set correctly and is getting power, you need to make sure the furnace itself is receiving electric power. Check the circuit breaker (or fuse if you’re in an older house) to make sure it’s not tripped.
In addition, most furnaces have an electrical switch installed next to the furnace. Check to be sure it’s turned on. It’s always possible it has been inadvertently switched off.
Reset the Furnace
Many furnaces have a reset button, often red in color on the front, side or back of the furnace. Try depressing the button until it clicks. It’s a sort of a breaker. If the furnace powers up, you will have heated air running through your home again. If you’re lucky, resetting the button takes care of the issue. The reset button could have been tripped by a power surge in the house electric, or even a wind gust blowing down the exhaust, or it would shut the furnace down if it gets overheated or other electrical problems. If resetting it turns the furnace back on, but it trips again, the furnace may have other issues. It’s possible there is a clogged air filter or blocked ducts that could cause the furnace to overheat.
Service Door Cover Check
Furnaces have a cover door that includes a safety switch. It’s possible that leaving the door open after doing maintenance, like changing the filter. Check to ensure the door is shut securely, and that may fix the problem.
Dirty Furnace Filter
Believe it or not, dirty furnace filters can be one of the most common causes of a malfunctioning furnace. A severely dirty furnace filter can limit the airflow through the furnace, causing it to overheat, then shut off automatically.
Another possible factor could be the MERV rating on the filter. The higher the number, the more restrictive the filter, possibly causing it to clog easier.
Replace the filter, press the reset button (if your furnace has one), and see if it begins to blow warm air.
We recommend you change your filter monthly, to ensure your furnace works properly. Leaving a dirty filter in your furnace can cause all kinds of problems, and it’s a relatively inexpensive fix.
If you’re unsure what MERV rating is right for your furnace, contact our customer service team and we’ll advise you.
Standing Pilot is Out
Older furnaces are equipped with a “standing pilot,” essentially a small open flame that sits there, waiting for the furnace to fire up. Sometimes a stray gust of wind can blow out that pilot light, and a safety valve cuts off the gas supply (to ensure there isn’t an unsafe build-up of natural gas that could lead to an explosion).
To re-light the pilot, turn the red valve to the “pilot” position, depress and hold the button down while simultaneously applying a flame to the pilot. After the flame engages, hold the pilot button for 60 seconds. When you release the button, the pilot should stay lit. If that’s the case, rotate the button to the “on” position and step through the thermostat procedure above until the furnace actually fires up.
It might take several tries to get the pilot lit. If the pilot won’t remain lighted, there might be an issue with the thermocouple or ignition system.
Bigger Service Issues?
If none of these simple procedures works to get your furnace blowing warm air again, then you may be dealing with a more complicated issue or issues. This is when it’s time to call a professional furnace contractor.
How We Can Help
Our team offers a wide array of routine maintenance and emergency services for your heating and cooling system, allowing your family to breathe fresh, safe, clean air and be comfortable year-round. We also offer a number of add-ons to your HVAC equipment that can improve your home’s IAQ, such as electrostatic air filters, whole-house humidifiers, duct cleaning, and more.
Schedule an appointment for HVAC system troubleshooting or maintenance with us today by using our online contact form, or call one of our seven neighborhood offices listed below. We pride ourselves on our customer-focused service, and our reviews show it.
Dor-Mar…Your Climate Hero!