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What Does Rust On My Furnace Mean?

Furnace rust massachusetts

An oil-fired furnace is a reliable way to heat your Massachusetts home, and if it’s properly cared for, it should last a long time. But eventually, wear and tear will take its toll, and you might start noticing some problem signs.

Like rust, for example.

The question is what would cause a furnace to rust, considering it’s not in obvious regular contact with water? Let’s take a look at the causes of furnace rust and what you can do to prevent them.

What are the risks of a rusty furnace?

You probably know that rust occurs when metal, oxygen, and water mix, and that too much rust will eventually corrode that metal.

What you might not know is how serious that problem can become in a furnace, especially if the rust occurs in the ventilation or heat exchanger areas. Corrosion in these places could lead to a release of dangerous combustion gases (carbon monoxide, sulfur dioxide, and nitrous dioxide) into your living area – something you definitely want to avoid.

What causes rust on a furnace?

Rust and corrosion in a furnace can be caused by a number of conditions – here are three common reasons for rust formation on your furnace, and what you can do to prevent each.

1. Water leaks or high humidity – Water from nearby pipe or rainwater leaks can cause moisture buildup near your furnace, and an over-humid home can force moisture to accumulate in the return ducts of your heating system.


  • Invest in waterproofing measures such as better drains, downspouts, caulking and weatherstripping.
  • Address any plumbing issues promptly (call a pro; don’t do it yourself!).
  • Consider installing a whole house humidifier/dehumidifier to keep humidity under control.

2. A leaky air conditioner – In many HVAC systems, your furnace and air conditioning units are installed very close to one another – in some cases, one is directly on top of the other. If your AC leaks, that moisture can get in the furnace. A cracked drain pan or clogged condensate line can also leak into your furnace, eventually causing rust.


  • Get professional air conditioning maintenance every year.
  • Check your drain pan to see if it is damaged or overflowing. If it is, call a professional to have the faulty part replaced.
  • Pour about a cup of vinegar down the condensate drain line every 3 months or so to keep it clear. If you’re not sure how to do this, contact us.

3. Combustion gas / exhaust problems – As hot gases and vapors move away from the heat of the burner and toward the flue through the heat exchanger, they cool and can form water vapor. Inadequate ventilation for the gases and water vapor could cause the water vapor to pool and corrode the heat exchanger and flue.


If your furnace is more than 15 years old and you notice signs of corrosion, it’s time to think about a furnace upgrade.

Need expert A/C or heating service, or a furnace upgrade for your Middlesex or Norfolk County home? Needham Oil Complete Heating & Cooling is there for you. Contact us today to learn more – or to get a FREE, no obligation estimate on a new, high efficiency furnace or boiler.