Signs a Gas Furnace Isn't Venting Properly | Peak Home Performance
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A gas furnace combusts natural gas, and this generates the heat that warms the air. However, the hot combustion gases cannot come into direct contact with the air that moves through the furnace and into the home because the gases are harmful to breathe. The way a furnace trasnfers the heat from the combustion gases to the air is through a component called the heat exchanger, a chamber that collects the gases so that its metal walls heat up and warm the air moving around the exchanger.

Once the heating process is complete, the leftover combustion gas byproducts safely exits the furnace through a vent. This ventilation stage is critical for the furnace’s safe operation. Problems with the venting can lead to the toxic gas byproducts (principally carbon monoxide) to go the wrong direction and end up in the house and also speed up the effects of corrosion on the furnace.

If a furnace receives regular maintenance, its ventilation should work safely. But problems can still occur with home  heating in Castle Rock, CO, and poor gas furnace venting is a serious problem that we want to help you identify. Below are the most common warnings for gas furnace venting malfunctions.

Water condensation on the furnace

A furnace wouldn’t normally have water condensing on its surface. But if the exhaust flue is clogged, it will lead to poor air circulation that the build-up of the vapor from the cooled combustion gases. Since you should never see water dripping or leaking down your furnace, call for our technicians right away when you’ve got a “sweating” furnace.

The color of the gas jet flames changes

You can observe the color of the flames from the burners by either looking through a glass pane (for a sealed combustion furnace) or simply looking into the furnace itself (older atmospheric combustion furnaces). The flames should burn blue. If they change to red, yello, or green, it indicates a problem with the combustion process. The burners might be burning carbon monoxide that’s moving the wrong direction. Turn the furnace off at the furnace switch and leave it off until our HVAC pros have had a chance to look at it.


Gas byproducts that don’t effectively escape from the furnace through the venting pipe will turn into soot inside the pipe. Not only is this a warning of poor venting, it will further block the flue and worsen the problem. As with other warning signs, when you see this, shut off the furnace and call for technicians. Don’t use the furnace until you have an “all clear.”

Carbon monoxide detectors go off

Carbon monoxide detectors are necessary for any home that uses natural gas. They offer a major line of defense against poisoning from CO, which is a colorless, odorless, and tasteless gas. If you’ve had to vacate your home because your CO detectors went off, we strongly recommend you have technicians examine the furnace after the gas company has given your family clearance to return. (Please call the gas company first; call us later.)

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