June 30, 2020

My furnace is starting to rust – what’s causing it?

When you think of rust, you might think that a rusting furnace is kind of odd because it’s is usually associated with water. A furnace doesn’t use water to produce heat, it uses air, so why would it rusting?

A furnace, when properly maintained, can last a long time, but wear and tear takes its toll. So as your furnace ages, you may start to notice some rust and corrosion forming. Rust is a type of corrosion and occurs as a result of a chemical reaction between water and metal in the presence of oxygen. It’s certainly not something you want to occur to one of your most valuable and expensive home appliances.

So how does it start in the first place?

If you have a gas furnace, it could be a result of the reaction between the combustion gases and metal. The combustion gas turns into a vapor after heat transfer and then is vented out the furnace flue. Over time, the vapor that comes into contact with metal will lead to corrosion over time. So a furnace at the end of its lifecycle will normally show signs of rust.

If your furnace is still well within its life expectancy, then it could be a result of not being vented properly. You’ll want to schedule a service call ASAP to get the exhaust system looked at and repaired. If your heat exchanger is showing signs of rust, it can lead to cracks which could result in carbon monoxide entering your home – a significant hazard.

Other signs of rust can be trigger by:

  • Damaged ductwork that is allowing an increase in humidity inside the ventilation system can cause rust.
  • An air conditioning unit installed above the furnace that is leaking condensation. 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. The water and chemicals will create corrosion on the furnace as it drips down onto it.
  • Water leaks near your furnace caused by pipes, cracks in the foundation, etc. could cause unwanted moisture near your furnace and contribute to rust. If the unit is in a high humidity area, it will also be more susceptible to rust.

You probably won’t see any rust on the outside of your furnace unless you have a water leak that’s coming into contact with the furnace. It’s the rust you don’t see that can be dangerous: rust on the heat exchanger.  You can detect the presence of rust flakes that are internal by checking the bottom of the furnace housing. 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.

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.

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