Most household furnaces are durable and dependable as long as they receive regular maintenance. But even with the best care, a furnace can fail to blow heat when you expect it to. If your furnace won't blow heat, you need to identify the source of the problem before you can determine whether you can repair it yourself or you need to call in a professional.
Duct and Blower Issues
Your furnace can function normally and still not blow heat if there's a problem with your ducts or blower. The blower is an electric fan, so it can stop operating if there is a power outage or mechanical failure. Also, your furnace may seem like it is blowing cold air when you first turn it on because there is cold air in your ducts; it can take even longer for hot air to make it to rooms that are farther from your furnace. Finally, obstructions or leaks in your heating ducts can allow heat to escape or prevent it from reaching a vent.
The thermostat that you use to control the temperature in your home can cause the furnace to stop blowing heat. The furnace probably won't turn on or blow heat if the wiring that connects the thermostat to the furnace's control module is severed or corroded. A faulty thermostat may cause the furnace not to produce a flame or to stop blowing even though the temperature of your home is below the threshold you set.
Fuel and Fire
Each type of home furnace creates heat by burning a fuel such as wood, natural gas or oil. Your furnace might not be blowing heat because something has disrupted the delivery of fuel, including a closed gas valve or an empty oil reservoir. Gas furnaces won't produce heat unless their pilot lights are burning. Flipping a switch or lighting the pilot may be all you need to do to get your furnace running.
Your furnace can fail to produce heat even if the blower continues to run, filling your home with recirculated air that is the same temperature as the rest of the home. This occurs when the furnace's heating element fails completely or the furnace shuts itself off automatically. Automatic shutoffs can point to a problem with the furnace's exhaust system, which may present a risk of carbon monoxide buildup. If your furnace has a restart button, you can press it to see if the furnace is capable of producing and blowing heat or if it shuts down again due to a more severe problem.
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