Your furnace turns on, but instead of feeling warm, you’re feeling chillier than before. You check the vent, and now you’re wondering, “Why is my furnace blowing cold air?”
If your furnace is running but the air from your vents is cold, there are a few common reasons why.
At Hamilton Home Comfort, this is an HVAC service call we see regularly throughout Hamilton, Stoney Creek, Ancaster, Dundas and the surrounding area. Sometimes the fix is simple. Other times, cold air is a sign that the furnace is shutting itself down because something is not operating properly.
Let’s look at the top causes of a furnace blowing cold air.
Check Your Thermostat First
Start with the easy one. Your furnace is blowing cold air because it may be set to COOL. Make sure your thermostat is set to HEAT and that the fan is set to AUTO.
If the fan is set to ON, the blower can continue circulating air even when the furnace burners are not running. That air will eventually feel cool coming out of the vents.
It is also normal to feel slightly cooler air for a short period at the beginning or end of a heating cycle. If the furnace keeps blowing cold air for several minutes while the house is calling for heat, there is likely another issue.
A Dirty Furnace Filter Can Cause the Furnace to Overheat
This is one of the first things we check when your furnace is blowing cold air.
A badly clogged furnace filter restricts airflow through the furnace. When the furnace cannot move enough air, the temperature inside the unit can become too high.
The furnace has safety controls designed to shut the burners off if this happens.
The blower may continue running, which is why you can suddenly feel cold air coming from the vents even though the thermostat is calling for heat.
Check your furnace filter and replace it if it is dirty. Also make sure your return air vents and supply registers are not blocked.
An Improperly Sized Furnace Can Also Overheat
This is something homeowners do not always realize.
A furnace can overheat because it is not sized properly for the home or, just as importantly, for the ductwork.
If a furnace is too large for the duct system, it may produce more heat than the ductwork can properly move away from the furnace. The temperature inside the furnace rises quickly and the high limit safety switch can shut the burners down.
You may then feel the blower continue running with cooler air coming from the vents.
This is one reason furnace sizing matters. Bigger is not always better.
A furnace should be properly matched to the heating requirements of the home and the airflow capacity of the duct system.
The Furnace Is Blowing Cold Air Because It May Not Be Staying Lit
Another common issue that leads to a furnace blowing cold air is a dirty flame sensor or an ignition problem.
You may hear the furnace start. The burners light for a few seconds and then shut back off.
The furnace may try several times again before eventually stopping.
To the homeowner, it can simply seem like the furnace is running but blowing cold air.
There are several possible causes, including the flame sensor, igniter, pressure switch, gas valve, venting or another safety control. This is where proper diagnosis matters.
Changing parts based only on a fault code or symptom is not the same as finding the actual problem.
Check the Furnace Venting Outside
During a Hamilton winter, snow and ice can sometimes block the intake or exhaust pipes of a high-efficiency furnace.
Take a look outside and make sure the furnace venting has not been buried by snow or blocked by debris.
Do not disconnect or modify the venting yourself. Furnace venting is part of the combustion system and should be serviced by a qualified technician.
It May Be an Airflow Problem
Sometimes the furnace itself is working properly, but the home still feels cold because the heat is not moving properly through the ductwork.
Some of the key reasons your furnace is blowing cold air are dirty filters, blocked returns, closed vents, dirty blower wheels, incorrect airflow settings or ductwork that simply cannot move enough air.
This is why diagnosing a heating problem involves looking at the whole system, not just the furnace.
What Can You Check Yourself?
If your furnace is blowing cold air, before calling for service, check the thermostat, inspect the furnace filter, make sure your vents and returns are open and look outside to ensure the furnace intake and exhaust are clear.
If you have checked those things and your furnace is still not blowing warm air, that is when it is time to call a professional like the Hamilton Home Comfort team.
One of our licensed HVAC gas technicians can come to your home, properly diagnose the furnace and determine why it is not completing a normal heating cycle.
Repeatedly resetting the furnace or changing parts without knowing the cause can sometimes hide the real problem. Our goal is to find out what is actually happening before recommending a repair.
To look at why your furnace is blowing cold air, call Hamilton Home Comfort, and one of our licensed HVAC technicians can inspect the system, diagnose the problem and explain what is causing it before recommending a repair.
Call Hamilton Home Comfort at 905-643-098
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