Just like oil and water, rust and furnaces are two things that simply don't mix. Whenever you or your HVAC technician spots rust forming anywhere on the furnace, it's a clear sign that there's a problem somewhere. You can't afford to simply ignore rust – even the tiniest speck will eventually grow into a serious problem that eventually compromises your furnace's performance and longevity.
There are a few ways that rust can take hold within your furnace. The following explains what causes rust, how it can spread, and what you can do to stop rust in its tracks and prevent it from spreading further.
Air ducts are a common source of trouble for most heating systems. Whether the underlying cause involves improper installation, physical damage during inspection, damage caused by pests, or adhesive failures, these issues can cause one or more sections of your ductwork to come undone.
The resulting crack or gap in your furnace's ductwork can give moisture-laden air a way into the heating system. The excess moisture can even condense into liquid form and run down to the furnace, paving the way for rust formation. Rust is relatively uncommon on ducts since most ductwork is made from galvanized or stainless steel.
Leaking condensate from another, external source can also cause rust to form on your furnace. This commonly happens on combination HVAC systems with the air conditioning unit situated above the furnace. A clogged or broken drain line or a leaking condensate tray can allow water to drip onto the furnace below.
You can stop an A/C condensate leak in its tracks simply by keeping the drain line intact and clear of debris. If your A/C system uses a metal condensate tray, check the pan for any signs of rust or corrosion. Otherwise, watch for cracks or other types of damage that could create a condensate leak.
Improper ventilation not only poses performance issues for your furnace, as well as a potential health hazard for those inside your home, but it can also lead to rust formation. Without proper ventilation, cooling exhaust gases carried from the heat exchanger to the flue can cause condensate to form on both components, eventually leading to rust and corrosion.
A missing flue cap or leaks around the flue or chimney can also allow rainwater to leak into the flue and heat exchanger, resulting in rust formation. Replacing a rusty flue is easy, but the same can't be said about replacing a heat exchanger. Rust formation can eventually lead to harmful and dangerous cracks in the heat exchanger itself.
For more information on furnace repair, contact local professionals.