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Why Is My Furnace Blowing Cold Air

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Old 11-10-2018, 09:14 AM   #1
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Why Is My Furnace Blowing Cold Air

If your furnace is blowing cold air, it’s not mocking you. It’s a cry for help. The good news is that the problem is usually simple to diagnose and repair. The heating repair experts at HVAC Philly will list the five most common reasons why your furnace is blowing cold air.

The Thermostat Fan is Set to ON
The most common reason that you’re getting cold air instead of the expected warm air is that the fan is set to ON.

Why would that cause a problem?

Typically, a thermostat has only two fan settings — ON and AUTO. When the fan is set to AUTO the fan goes into operation only when the furnace burner is on. This results in the fan blowing warm air into your home. When the burner turns off so does the fan.

However, if the fan is set to ON, the fan blows nonstop, regardless of whether the burner is operating. That means that if the burner is not operating, the fan blows air that has not been warmed. In other words, you get cold air at the time of the year where you couldn’t imagine anything that you’d want less.

This problem can easily occur when there’s a warm afternoon and you decide to circulate the air in your home by turning the fan setting to ON. Later, after sundown, the temperature drops, but your furnace is still circulating cool air.

The Thermostat Isn’t Working Correctly
Like your favorite laptop, a computerized thermostat may have encountered a problem that requires the system to be restarted. Restarting it may correct the cold air problem.

If the thermostat uses a battery, the battery may have become too weak to correct power the system. Replace with the suggested make and model of battery.

The Filters Are Dirty
Your furnace needs to breathe good, clean air. It does so thanks to a hardworking air filter.

If the filter is neglected and becomes clogged, not enough fresh air reaches the furnace. As a result, the furnace’s heat exchanger can overheat. To prevent damage to your furnace the burner shuts down.

The fan, however, may continue to blow for a short time in an attempt to bring the temperature of the burner down to an acceptable level.

That would explain why you feel cold air.

Furnaces that burn oil also have an oil filter. If this filter becomes clogged, the burner may have trouble firing.

The Burner Is Dirty
The filters help keep the burner clean. But if the filters have been neglected, there’s a good chance that the burner has had time to get covered in grime. Dirt on the burner could prevent it from firing properly.

The Pilot Light Is Out
Do you have an older furnace with a pilot light?

If the pilot light has gone out, the furnace can only produce cold air because the burner won’t light. You can easily understand this principle if you also have an older gas water heater that uses a pilot light. You know from experience that when the pilot light goes out on the water heater you can anticipate a cold shower.

If you have trouble keeping the pilot light on the furnace lit, the furnace’s thermo-coupler could need replacing. Replacing a thermo-coupler is not terribly difficult, but the project is not within every homeowner’s comfort zone.

That’s why it’s good to know that our technicians are ready and equipped to tackle any of your furnace repairs. So, if your furnace is blowing cold air for any reason, and you feel that the repair is beyond your ability, contact HVAC Philly.

Last edited by HVACPhilly; 11-10-2018 at 09:17 AM.
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Old 03-29-2019, 02:54 AM   #2
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Re: Why Is My Furnace Blowing Cold Air

Your furnace may be blowing cold air because the filter is too dirty. A dirty air filter blocks airflow over the furnace's heat exchanger, causing it to overheat.
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