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Kilowatt Heating, Air Conditioning and Electrical Blog

Can My Gas Furnace Rust?


When you think about parts of your home that might suffer from rust, you’ll first think of exterior metal parts that may rust from rain, or perhaps the water heater. The gas furnace isn’t likely to be on the top of your list of items in danger of rusting. After all, a gas furnace doesn’t use water to provide heat the way a boiler does. It never uses water at all.

But, to answer the question in the title, yes, your furnace can rust. It’s more common than you might think and can become a major problem requiring professional furnace service in Encino, CA

In this post, we’ll provide you with information on how rust can start on your furnace. You can use this for prevention and to help you know when to call us to examine the furnace. 

Rust from combustion gases

This is the rust problem a furnace is most likely to encounter, and it’s also the most concerning since it can lead to a gas furnace becoming hazardous. A furnace doesn’t use water to heat the air, it uses the hot combustion gases from the burners to do it. However, these gases contain vapor that reacts with the metal of the heat exchanger as the gas cools down. This vapor is vented out of the furnace through a flue, so there isn’t much time for it to react with the metal. However, over time this can have an effect and cause corrosion in the heat exchanger. If the furnace isn’t venting properly, the process will move faster.

Corrosion in the heat exchanger is a major problem since it weakens the metal of the exchanger and allows cracks to start. If the combustion gas escapes through the cracks, it will get blown into the house and become a health hazard. The best way to stop this problem is with an annual furnace inspection when technicians carefully examine the exchanger for corrosion and test for proper venting. 

Rust from humidity

Moisture in the air can affect the furnace and create corrosion. This usually comes from humid air getting into the ventilation system and circulating through the furnace. Our dry local conditions make this a less likely possibility, but moisture can still get into the ducts after rainy weather or if the ducts have air leaks.

Rust from the air conditioner

The standard HVAC set-up, which your home probably has, locates the air conditioner over the heat exchanger of the furnace so they can use the same blower. As the air conditioner runs, it draws moisture from the air and collects it in a condensate pan, then pumps it outside. But the condensate pan can crack or overflow, or the drain may become detached from the pan bottom. This will allow water to drip down onto the furnace’s heat exchanger and create the danger of rust. When you call for AC repair because of a condensate problem, our technicians will also check to see if the furnace was affected. 

At the first sign of rust on your furnace, give us a call!

Call our certified service technicians 24/7 for exceptional customer service! Kilowatt Heating, Air Conditioning and Electrical serves Los Angeles and the San Fernando Valley.

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