One of the most important parts of HVAC maintenance is ensuring the heat exchanger is in good condition. A heat exchanger is an essential part of the furnace that funnels harmful gases outside through the flue while collecting heat to distribute through the house via the ductwork.

Heat exchangers don't last forever, however. Once they start to rust or crack, they can allow carbon monoxide and other gases to leak into the air distribution system, creating a dangerous and potentially deadly health hazard for your family. Read on to learn more about what causes rust on your heat exchanger and how to address the damage.

What causes your heat exchanger to rust

Rust is another common danger to heat exchangers, weakening their structural integrity and allowing dangerous gases to seep out. Rusty heat exchangers can be caused by:

• A furnace that's too big: If your furnace is larger than it needs to be, it can heat your home too quickly. This can be a problem because the water vapor from the combustion process will accumulate inside the heat exchanger. If the furnace only runs briefly, the water won't evaporate, causing the exchanger to rust.
• Outside water sources: Water dripping from air conditioner evaporators is a frequent culprit for rusty heat exchangers, and water can also accumulate from a plumbing leak or if the furnace is located in a damp area of the home. You can help extend the life of your furnace by ensuring there are no water sources nearby that are liable to drip onto the heat exchanger.
• Ineffective venting: Certain combustion fumes can cause metal to rust if they aren't appropriately vented from the furnace. Poor venting poses other health and safety risks, so it's doubly critical to address if that turns out to be the cause of the rust.

A diagnosis that can save your life

Carbon monoxide is odorless, colorless and tasteless. It can cause death before you even realize it has contaminated your environment. Therefore, it is critical to identify broken heat exchangers before they cause a problem.

If your furnace shuts down at random times or if the carbon monoxide detector goes off in your home, these may be warning signs. In either event, you need an HVAC technician to inspect your furnace immediately. There are several ways that the inspector can test the integrity of your heat exchanger:

• A visual inspection to look for any cracks or rusted areas
• A flue gas test that can discover high levels of carbon monoxide or other gases
• Special heat exchanger leak tests using dye, smoke or a camera

How to remove rust from a heat exchanger

If you are comfortable accessing the rusty parts of the furnace and the damage isn't too extensive, you can attempt to clean the unit using an appliance brush with steel bristles. You can also use a liquid descaler to try to remove some of the rust. However, carefully follow the manufacturer's instructions for the furnace and the cleaning solution and ensure the cleaner is appropriate for whatever material the heat exchanger is made of.

Not all damaged heat exchangers are salvageable. Once a heat exchanger has a crack or a hole, it must be replaced. This can be expensive and labor-intensive, so it may be a good opportunity (and more cost-effective) to replace your furnace, particularly if the problem was caused by a furnace that was too big for your home. Look for models with stainless steel heat exchangers to help prevent a repeat appearance of the rust.

Consult with a qualified HVAC technician to verify the condition of your heat exchanger and advise you on the best path forward if yours is already damaged. Scheduling a yearly maintenance inspection for your HVAC equipment can also help prevent rust from building up on the heat exchanger in the first place.