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Nothing is more frustrating than a furnace that is blowing cold air while it is freezing outside. Everyone expects warmth when they get home, and the last thing they would like to know upon returning from a long day of work is that their heater is malfunctioning, specifically their heater blowing cold air. It is a double black eye, so to speak. Everyone is feeling cold, and the utility meter is still up and charging the homeowner for every minute the furnace is failing to heat the house.

At this point, the best thing to do is to call in for a professional HVAC diagnosis and servicing. Getting a furnace heating repair company to check on the furnace is the best and fastest way to get the heater up and running once again.

In the meantime, while waiting for the licensed HVAC technician to arrive, know more about why the heater is blowing cold air, and what can be done to prevent this from happening, moving forward.

How the furnace works

To fully understand how and why a furnace would suddenly blow cold air, it is best to know how it normally functions. The thermostat nudges the heater to start working. Once the thermostat detects that the air inside the house is now below the desired temperature set by the homeowner, it automatically turns on the furnace to keep the house warm.

The thermostat sends a signal to the furnace’s control board to turn on the heating mechanism after which the whole heating process will start. Once the combustion commences, it will warm up the heat exchanger. Once combustion occurs, flue gasses are released by both the heat exchanger and the exhaust pipe.

Once the gasses are released, the heater’s blower fan spreads the air past the furnace’s heat exchanger. The cold air will then absorb the warm air and will get blown through the home vents. Now that this process is finally explained, it will now be easier to understand the possible reasons why a furnace would suddenly blow cold air.     

When it comes to furnace failures, we at Reliable Standard Heating and Air have seen everything, so it is easier for us to diagnose, and troubleshoot! Here are the top reasons why a furnace or home heating system would suddenly start blowing cold air into the house:

The furnace is blowing cold air because there is no fire within

Ignition failure is the most common reason why there is no fire within a furnace. Without that flame, a furnace cannot produce heated air that warms up the house. Ignition is crucial, and once it fails, here are the possible reasons why it will suddenly malfunction:

A failing thermostat can prompt the heater to blow cold air through the vents even when the heat is on. Find out what you should check first if your heater isn’t working at anytime.

The thermostat is a crucial component in heating the home. It sends a signal to the furnace’s circuit board to start heating the home upon detecting that the desired temperature no longer matches the ambient temperature inside the house.

The heat cycle however will only be properly initiated if the thermostat is working in optimal condition. Sometimes, it only prompts the blower motor to start but not the furnace’s burner. If the burner fails to start, the blower will only blow cold air towards the house since the burner is not heating the air.

A failing circuit control board may be at fault when the furnace is blowing cold air

The circuit control board is responsible for prompting the furnace’s blower motor, and burner to turn on and start the heating cycle. As the heart of the furnace or home heating system, it immediately processes the thermostat’s message to start heating the home.

However, there are instances when the circuit control board would suddenly fails. Sometimes it triggers the blower motor to work, but fails to make the furnace’s burner start working. Once this happens, the air is blown into the vents but the air will be cold. The air blown will not be hot since the burner is not working.

But what causes the circuit control board to fail? There are several possible reasons, but the most common is a lack of proper maintenance and cleaning. If the homeowner is very diligent in seeking regular furnace maintenance, yet the circuit control board still fails, then it is best to inspect if there are any leaks within the system.

When water leaks out of the furnace, it is possible that it dripped unto the circuit control board, and caused damage. In this case, seek professional repair right away before the situation turns for the worse and causes permanent furnace damage.  

Faulty electric furnace ignition or broken pilot light

One of the possible reasons why a furnace will blow cold air is when the pilot light keeps going out or is broken, or for newer furnaces – when the electric furnace ignition is failing. Both the pilot light and the electric furnace ignition are responsible for igniting the flame that warms the air that is blown into the house. When it fails, there will be no flame that will heat the air.

The gas furnace blows cold air because then it runs out of fuel

A gas furnace will start blowing cold air once it runs out of fuel or if there is something wrong with its gas valve. Lack of gas is a surefire reason why a gas furnace would fail to ignite. Once this occurs, try checking the gas supply or the valve. For homes that utilize propane instead of natural gas, regular refilling is needed to keep the heater from blowing cold air.

Furnace blows cold instead of hot air due to possible triggering of safety features

Furnaces have a safety feature that automatically turns on once it detects a failure or abnormal heating circumstances. Once the furnace starts blowing cold air, and the above-mentioned reasons are not the culprit, then a safety mechanism may have been triggered. For instance, when it overheats, safety mode is activated, and the heater will stop blowing warm air. Once this occurs, get in touch with a licensed HVAC technician who can correctly diagnose and safely troubleshoot the issue.

Overheating the furnace could prompt it to blow cold air

It may be ironic, but an overheating furnace does not blow hot air. On the contrary, a furnace that is overheating will only blow cold air because of a safety mechanism that is triggered once overheating is detected.

The main limit, which is a safety feature of the furnace, turns off the furnace’s ignition mechanism, paving the way for cold air to get blown through the vents. When the safety mode is triggered, the furnace will stop blowing warm air to keep everyone safe. If this mechanism is frequently triggered then it means that the furnace keeps on overheating. You can read more about Why Auxiliary heating is showing on thermostat here. It is wise to get it professionally checked as soon as possible.

A cracked heat exchanger can cause a furnace to blow cold air even if the heat is turned on

A cracked heat exchanger can cause multiple problems in a household. Apart from endangering the whole home with possible carbon monoxide leaks and consequent poisoning, it can also cause cold air to get blown from the vents.

If the homeowner suspects a problem with the furnace’s heat exchanger, then he should immediately get in touch with a licensed HVAC technician who can inspect and fix the problem right away.

A malfunctioning main limit is causing the furnace to blow cold air

The main limit, which is the safety mechanism that turns off the ignition when overheating is detected could be at fault when the furnace blows cold air even if it is not overheating. When the main limit gets worn out or gets too dirty, it can sometimes fail and incorrectly activate the safety mechanism. When this happens, the furnace could blow cold air for days on end.

Restricted airflow can cause the furnace to blow cold air 

When airflow is impeded, the furnace can suddenly blow cold air instead of hot. Since the furnace relies on airflow, ventilation, and ducts, any restriction in these areas can cause the blowing of cold air throughout the vents inside the home. Besides, when filters are obstructed with so much dust build-up and filth, there will be less warm air that could get into the house, and the whole home will be a lot colder. You may also be wondering to yourself, why is my furnace leaking water and how to fix it, you can read more here. Below are some of the possible scenarios:

Cold air is blown through the vents even when the heat is on because the filters are dirty

If filters are not replaced or cleaned regularly, dust and debris can build-up can impede airflow to and from the house. When the debris accumulates, the whole heating system is bound to get less efficient in heating the home, causing the whole house to feel a lot colder as compared to when the filters are newly replaced. When air is obstructed, cold air will not be replaced with heated air, and since there is less air to heat and redistribute throughout the house, the whole property will seem colder than normal.

HVAC experts recommended that homeowners change their air filters every quarter. If there is a household member who has dust, pollen, or mold allergies then the filters must be cleaned or replaced before it reaches three months. Professional cleaning can also help in ensuring the HVAC’s optimal operations.

The furnace blows cold air because of obstructed supply airflow

When furniture or upholstery blocks the supply vents, then expect the furnace to blow cold air. The furnace fails to distribute warm air throughout the house since there is not enough airflow coming in. Moving forward, homeowners should make sure that nothing is blocking the airflow vents throughout the home so as not to hinder the efficiency of the furnace system.

The furnace is not blowing warm hair because of blocked return vents

The furnace needs air to heat, but when the return vents are obstructed, there will not be enough air to heat up. This will then lead to cold air inside the house. When this happens, the furnace will blow cold air since there is not enough air to warm and blow throughout the house.

Incorrectly sized ductwork can also make the furnace blow cold air into the house. If the ductwork is way too small, it is unable to take in the recommended amount of air to heat up and redistribute throughout the house.  And when there is not enough warm air being blown, the furnace will not have a choice but to blow cold air throughout the house.

Clogged coils trigger the furnace to blow cold air instead of hot

Dirty and ill-maintained evaporator coils can cause furnaces or central home heating systems to blow cold air. When the HVAC’s evaporator coil is too filthy to process the air passing through it, then the home heating system will surely start supplying the home with cold air instead of hot. This is because the air that is supplied to the system will be obstructed as it tries to get past the evaporator. The key here is to ensure that the evaporator coils remain in good condition, and is not filled with dirt and debris.

Guide in troubleshooting a furnace that is blowing cold air

  1. Check the thermostat setting. The first thing to do is to check whether the thermostat is set into heating mode. Sometimes, it is mistakenly set to cold, that could be one of the reasons why your hvac is not blowing warm air homeowners expect. It is also possible that the Fan mode is set to ON mode instead of auto. If this is the setting, the furnace will continue blowing cold air since the fan will continuously work even if the heating cycle has yet to start.
  2. Inspect the vents throughout the house. The second thing to check when the furnace seems to be blowing cold air is the vents or air registers. Make sure that nothing is obstructing these vents, as restricted airflow can cause colder air to linger at home and the furnace to consequently blow cold air.
  3. Check the furnace filters and replace them if necessary. The homeowner should ask himself about the last time he cleaner or replace the furnace filters. If the answer is more than three months ago, then it is high time to change the filters right away. Clogged filters impede airflow to and from the furnace, prompting it to blow cold instead of hot air.
  4. Call a licensed HVAC professional for help. When all of the above seems fine, then the culprit could be something else. Only a licensed HVAC technician could correctly diagnose the problem, and perform a safe and long-term fix. Moreover, only a licensed professional could give you the peace of mind that you deserve, since DIY repairs could lead to technical issues that could cause permanent damage to the furnace and the home structure. Fires, carbon monoxide poisoning, and damaged furnaces are just some of the risks. Call a pro and get the problem fixed ASAP.

How to prevent the furnace from blowing cold air

The best way to keep the furnace from blowing cold air is to keep the furnace well-maintained. Routine professional maintenance will keep home heating troubles at bay. And besides, it will ensure that the furnace will last longer, and operate safely. During a routine annual inspection, the licensed technician will professionally clean all heating components, tighten mechanical connections, and replace parts that are about to get worn out.

Stay warm and toasty this winter

With this, it is best to keep in touch with a trustworthy and efficient licensed HVAC service company. At Reliable Standard Heating and Air we can make sure that you stay warm and toasty every winter. We will be more than happy to perform furnace inspections, tune-ups, and maintenance, as well as troubleshooting, upgrades, and replacement. Schedule your HVAC inspection today through our hotline or our online appointment system.

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