Why is my furnace blowing cold air?

If your furnace blowing cold air, then it’s mean that something you did wrong. It might be possible that your furnace become overheated or there might be some problem with your furnace thermostat, sensor of flames, pilot light and with air filters or with ducts, supply of gas, condensate line.

furnace blowing cold air

Basically furnace is designed in a way that its provide heating to the house. If your furnace is not able to provide heat and instead of heat your furnace blowing cold air, then there is something wrong with the furnace.

HVAC contractor for proper inspection and for schedule service, but for all of this you need to adjust your busy working schedule and pay various service fees and many other things. Well instead of calling your professional HVAC contractor, you have to solve your problem by yourself. Is it great or not?

Well, furnaces are not a complicated appliance. With very basic information of How does a Furnace Works? and by understanding your furnace issue that why your furnace blowing cold air, you can easily solve this problem by yourself. Your furnace starts providing heat again to your house in very less time instead of calling your HVAC contractor and spending a lot of time and money.

Working of furnace to understand why furnace blowing cold air:

Forced air heating system are one of the most types of furnaces. Basically, furnace consume fuel in order to provide heat to your house. Then such warm air is transferred with the help of fan through ducts and with heating vents to your house. Cold air returns towards furnace to make such air re-heated.

Different furnaces consume different fuels to provide heat to your house. Furnaces consume natural gas, oil, electric, propane and wood as a fuel. Natural gas furnace and liquid propane gas furnace work in a similar way like just connect the pipe with your natural gas main line or with the tank of liquid propane gas.

working of furnace blowing cold air

Working of thermostat:

Basically, when your house temperature decreases below your furnace thermostat settings then your thermostat turn your furnace on. The valves of gas open and your furnace burners expels the gas through the small outlet. So, the ignitor builds a spark and some hot surface to light up the gas. Such burning gas warms your heat exchanger. The production of exhaust through the process of combustion which is vented out of the house through your house exhaust pipe.

Warm air which is blowing from vents and increase the temperature of house. After that thermostat turn your furnace off once your house temperature is levelled with the temperature of thermostat.

Why Your Furnace blowing cold air? Here’s 8 Reasons:

It’s not a big reason that your air coming from your furnace is frigid. It’s happens when your furnace is not being active since last season. It takes time, but after 15 minutes still if your furnace blowing cold air then there’s might be some furnace issues.

Furnace blowing cold air issue easily solved by yourself. It’s as simple as you flip the switch easily or filters swapping. Some complex issues require expert. Regardless of complexity issues. You have to hire a professional HVAC contractor if you are not comfortable to check all by yourself.

Here are 8 reasons why your furnace blowing cold air:

  • Thermostat issue.
  • Pilot light goes out.
  • Dirty air filters.
  • Faulty of flame sensors.
  • Overheated furnace.
  • Leakage in ducts.
  • Clogged or blocked condensate- line.
  • Low supply of gas.
reasons furnace blowing cold air

Let’s discussed one by one to understand completely that why your furnace blowing cold air.

Thermostat issue:

Basically, thermostat send signals to the furnace to turn your furnace on and off. If your thermostat settings have incorrect settings or any wiring issue, then you will face this issue that your furnace blowing cold air.

Incorrect settings:

First of all, you have to make sure that in your thermostat settings you select the option of heat not the option cooling. Always, check your thermostat on first priority because in your house it might be possible that someone change your thermostat settings.

Fan settings:

Also check your fan setting because sometimes fan frequently runs when you set it on. So that’s why unheated flow of air is transferred from vents to your house between heating cycle. You have to change your fan settings to “auto”. So, in this way it will run only during heating cycles.

Inaccurate wiring:

A thermostat which is installed recently in your house then there are many chances that your thermostat wiring is not in a correct way or thermostat is not compatible with the furnace. Just because of these issues thermostat send incorrect signals to furnace and you will face issue like furnace blowing cold air. So, call your professional technician to check that thermostat is compatible or not and wiring is correct or incorrect.

Also Read: Why Does my Thermostat say Heat On but No Heat?

Dirty air filters:

Clogged or dirty air filters obstructs the flow of air through which insufficient supply of cold air to get heated. Restricted flow of air increases the inside temperature of furnace which forcing the furnace limit switch in order to stop the heating cycle.

What is the right time when you have to change your air filters? It depends on some important factors. You have to note that how many pets and how many members in house, size of the house, on which location you are living and which types of air filter you are using, allergy sensitivity and time and date when you change your air filters last time.

Replace such dirty air filters with the new one. After that restart your furnace after the furnace cooled down.

Pilot light goes out:

Furnaces which is manufactured before 2010 having a pilot light run with gas which can be blown out easily. But modern furnaces come with electric ignition. Basically, pilot light looks like a small flame which burns continuously and ignites the burners to start the heating cycle. If your furnace pilot light goes out than your furnace is not able to provide heat.

You have to read all the necessary details which is mentioned on the owner’s manual and find the location of pilot light by following the owner’s manual instructions. But remember one thing in your mind that gas is flammable. Again, I mentioned that you have to contact with professional HVAC contractor if you feel uncomfortable in performing all these task by yourself or your pilot light goes out repeatedly.

pilot light reason of furnace blowing cold air

Relighting a pilot light involve such steps:

  • Turn of the switch or breaker circuit box.
  • Find assembly of pilot light and turn on the valve of gas. Wait 15 minutes until all the gas is escaped because there’s a risk of explosion and after that light up the pilot light.
  • Turn valve of gas to pilot. Now, press to hold rest button which is available near the valve of gas. While depressing rest button, also press ignitor to light the up pilot light. After that release the ignitor once the pilot lights. You can use a match or barbecue lighter in order to light your pilot light. (Remember hold the rest button for at least 20 to 30 seconds).
  • Turn the power of furnace and the valve of gas on.

I must say if your furnace pilot light won’t stay for longer time then contact with professional to solve this problem.

Faulty of flame sensors:

Flame sensors of the furnace is look like a thin metal rod which is present on burners assembly in the front of your furnace pilot light. When your furnace turns on, the gas extract from valve of the gas in the combustion chamber and pilot or the ignitor light up the burners.

Basically flame sensor ensures that flames are burning or not when your valve of gas is open. If your furnace flame sensor does not detect the flames even when gas on. So, its immediately shut off your system. This process save your house from the buildup of gas because gas is flammable and there’s a risk of explosion.

Normally, flame sensors are coated in the soot and with carbon deposits from your furnace flames. So, for dirty and faulty of flame sensor are unable to recognize the flames. It’s immediately shut off your furnace and in result you will face this issue furnace blowing cold air.

Now, test your flame sensors by just turning off your furnace at least for the 5 minutes. If your air is warm at first and then becomes cold slowly when you again turn your furnace on. Air changes from hot to cold because heat exchanger starts heating up until your flame sensor cut your furnace burners off.

Replace or you can easily clean your furnace flame sensor by following such steps:

  • Turn your furnace power off by using breaker of circuit box or switch.
  • Remove your furnace access panel and then find furnace flame sensor its look like thin metal rod bent and cover with the white porcelain around mounting end.
  • Remove sensor and take out this sensor. Carefully detach your furnace flame sensor from wire.
  • If your flame sensor tip is full black, then you have to clean it. Rub it with steal to remove all the dirt. If porcelain casing damaged and cracked than you have to replace this sensor. Got a new one and move to step 5, 6.
  • Smoothly attach the wire with the new sensor and put the sensor back to its position. Tight all the screws and after than replace your furnace access panel.
  • Turn the power of your furnace on.

Overheated system:

If the furnace overheats, there’s a limit switch which shut down your furnace immediately when the furnace overheats just to prevent from fire and build-up of carbon monoxide because there’s a risk of explosion. So, cold air temporarily runs from vents until your whole furnace shuts down. These are factor which cause your furnace to overheat like mechanical failure, old age and build-up of dirt.

Mechanical failure:

It’s possible that one of your furnace components might be failed or malfunctioned. Fault may be in the system of limit switch which turn your furnace off. Risk of carbon monoxide which is coming from the damaged or cracked heat exchanger is also the cause to overheat your furnace.

Dirty build-up:

Dirty restrict the flow of air and clogged filters are also the cause to overheats your furnace. So just replacing your old filters with the new one won’t solve the whole issue. For this purpose, contact your professional HVAC contractor to clean your whole furnace professionally.

Old Age:

You will see a major decrease in the performance of furnace as it becomes older and this is also a major reason of furnace overheats again and again. Furnace run for at least 15 years but it’s necessary to do its maintenance time to time. If you leave your furnace unchecked, then its efficiency decreases sooner. Furnace replacement is one of the best option here.

Leak Air Ducts:

There might be some cracks and holes in the ducts through which all the warm air escapes. So, when air comes towards the duct it feels less warm. Leakage is very difficult to find. So, contact your professional HVAC contractor to find and seal all the leaks.

Clogged condensate-lines:

Gas furnace with the high efficiency creates water and its known as condensate which normally drain in the PVC pipe which is called condensate line. You will see that water is accumulated around your furnace if condensate line become clogged with the dirt, debris and dust. So, furnace will shut down for the safety measures and your fan frequently circulate the unheated air.

Also check dampness or check if your furnace is leaking water. Unclog the condensate line but for your convenience, I want to tell you that cleaning of condensate line is a tricky process. Contact with professional if are not feeling comfortable with this task. If you want solve this issue by yourself than here are some steps through which you can easily unclog your condensate line.

Unclog your condensate line:

  • Turn your furnace off by using switch or circuit-breaker box.
  • Find your condensate drain pan. Dry the pan with the vacuum. Remove the pan dirt with the water and dish soap. Replace your condensate pan.
  • Follow your condensate line from pan that where the condensate line end. Normally, line end to the outside of your house. Use vacuum and remove clog from end of your condensate line.
  • Turn your furnace back on.

Gas supply is low:

Basically, gas furnaces cannot operate with the supply of gas. If there’s any issue with your gas supply line or with the valve of gas. Immediately call your professional to detect the gas supply.

If you are using propane, then make sure that propane tank is empty or not. If its empty, then make a call to company and refill the tank. If you are using natural gas. So, make sure that you cleared all the payments on time. If not than company will cut your gas supply.

Troubleshooting why furnace blowing cold air:

When you are facing issue like your furnace blowing cold air in the winters. First of all, try to solve your problems by yourself but if you are not comfortable with the tasks then call professional HVAC contact for all these tasks. In this way, you will save your so much time and your house will rapidly come back to its cozy temperature.

Do simple task by yourself like check the air filters, gas supply is fine and re-light your furnace pilot light, clean your flame sensors and unclog your furnace condensate line. All these tasks are easily done by even any non-technical person.

Always, contact with HVAC professional if you are not comfortable with the tasks or if by applying such troubleshooting measures, issues are still there. You have to leave such tasks on professionals in which electricity, gas and major repairs are involved.

Just do a proper schedule maintenance because it increases the life span of your furnace and increase efficiency.

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