How to Prevent Furnace Rust – Causes & Solutions

Prevent Furnace Rust Causes & Solutions
A furnace is a reliable and energy-efficient way to provide heating for Pacific Northwest homes. Proper maintenance (annual fall furnace tune-ups) can help a furnace last a long time, but wear and tear eventually takes its toll. As your furnace ages, you may start to notice problems such as rust and corrosion.

You may be wondering “why would a furnace rust?”. Unless you have a boiler or hydronic system, there really isn’t any water involved, so where does the rust come from? If you don’t have any water leaks nearby and your furnace is located in a dry area, why might rust occur?

Rust is an iron oxide that results when metal, oxygen, and water/moisture mix together. When rust accumulates, it can eat away at your metal furnace and cause serious issues, especially with regard to safe ventilation. This is especially concerning when it comes to your heat exchanger. Rust can cause heat exchangers to weaken and develop cracks.

By ignoring rust around your furnace, you could end up with dangerous combustion gases (carbon monoxide, sulfur dioxide, and nitrous dioxide) entering your living spaces.

If you have a rusting furnace, it may be time to consider a furnace replacementContact Pacific Heating & Cooling to assist you with all your furnace maintenance, repair, and replacement needs.


Unfortunately, rust and corrosion can and often do crop up over the course of your furnace’s lifetime. Water vapor can get into the system very easily, produced as a by-produce of burning gas. Similar trace elements such as carbon dioxide produce very similar results. If they aren’t dealt with, they can foster corrosion on the sides of your furnace, which means rust.

Rust tends to be a self-perpetrating cycle, because it can harbor more moisture, causing it to spread. Eventually, it may interfere with your furnace’s ability to function.

You can detect the presence of rust flakes by checking the bottom of the furnace housing; waiting until it shows up on the surface of the furnace may be too late. A service technician can stem the damage caused by rusting, usually by cleaning the burners and ensuring that they’re operating the way they should. Eventually, however, unchecked rust will render your furnace useless, forcing you to purchase another one.




Water leaks near your furnace, from pipes or outside rain and weather, can cause unwanted moisture to build up around your furnace. Also, if your home is humid, moisture can enter your heating system through the return ducts.


If you have water coming into the home, invest in waterproofing solutions, such as improved drainage and downspouts, caulking and weatherstripping. Contact a plumber to fix any plumbing leaks you may have.

To keep humidity levels under control, speak with an HVAC professional about whole-home humidification solutions. These systems maintain ideal humidity levels to improve indoor air quality and prevent moisture-related problems such as mold and rust.


Often your furnace and air conditioner sit right on top of each other. So, when your AC leaks, it affects the furnace as well. Water condensate from your AC should drop into the drain pan and get diverted outdoors via the condensate drain line. If your drain pan is cracked or the condensate drain line is clogged, water can begin to leak into your furnace and cause the rusting chemical process to commence.


If you have a leaking air conditioner, check your drain pan to see if it is damaged or overflowing. If the drain pan is cracked or leaking, contact a professional for an inexpensive replacement. Sometimes the crack is so small that you can fix it yourself with epoxy glue.

To keep your condensate drain line clean and clear of mold, algae, and other blockages, pour about a cup of vinegar down the condensate drain line every 3 months or so. Learn more about cleaning and preventing clogged condensate drain lines.


When the heat exchanger in your furnace converts the heat energy from your gas, liquid vapor is created. If you have proper ventilation and a working heat exchanger, there shouldn’t be any rust risk. However, if your furnace has ventilation problems, this water vapor can accumulate and cause rust and corrosion around the furnace.


Rust is often a sign of a ventilation and exhaust problem. Sometimes, all you need is some professional TLC to get the problem fixed. Your technician may be able to improve your ventilation and solve the issue. Other times, however, rust can simply mean that your furnace has been in operation for a long time and needs a replacement.

Over years of consistent use, combustion gas exposure and water vapor can cause irreparable damage to the system. If your furnace is over 15 years old and you begin to notice rust and corrosion, speak with a professional about replacement options.

The best way to avoid furnace rust is to set up a regular furnace maintenance schedule with a qualified HVAC technician. Additionally, make sure you have working CO detectors on every level of the home and outside of each sleeping area.

Our expert technicians will examine the HVAC unit for signs of rust, keep the burners clean, and provide solutions to keep rust from developing in the future.

We know that furnaces can rust and our trained staff of experts can find and fix the problem for you.

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