If you notice a damp spot under your furnace or a drip coming from the unit, you should call an HVAC contractor to diagnose the problem. There are a few potential sources of water--some more common and easier to fix than others. Your HVAC contractor will probably identify one of these three issues as the culprit.

Why Is My Furnace Leaking?

1. Condensation

As air flows through your furnace, it leaves behind droplets of water. A drain line extends from the side of your furnace and may connect with lines from other appliances before draining either outdoors or through your sewer.

It can clog with dust and debris that are washed into it along with your furnace condensate. If this happens, the water can overflow, leaving visible dampness. In this case, the drain line usually just needs to be cleaned. You can do this yourself with a wet/dry vac or leave it to a professional.

2. Faulty Humidifier

Because the furnace dries out the air so much, many systems have a humidifier attached to put water back into the air. This is connected to your home’s water supply line, which can leak for several reasons, including damage to the pipe, corrosion over time, or shifting in the home’s foundation.

The humidifier might also be turned up too high, causing excess condensation, and you can reduce it by adjusting the settings. The right setting for your unit depends on your furnace and your home’s natural humidity; aim for about 40% to 50% in the home.

3. Secondary Heat Exchanger

If you have a high-efficiency furnace, it has two heat exchangers. The first one absorbs the initial heat from the fuel being burned, and the second one absorbs even more heat from the furnace exhaust before it escapes.

This cools the air down enough to cause even more condensation in the secondary heat exchanger. When it’s broken or rusted, this water can leak out. This is less common than the other problems but more difficult for your HVAC contractor to repair. The secondary heat exchanger is an internal part of the furnace itself. You may have to replace the furnace in this case.

If you aren't sure where your furnace leak is coming from, call an HVAC contractor from Albright Heating & Air Conditioning. Based in Columbia, MO, and serving Boone County, they have over 25 years of experience and offer 24/7 emergency service. Learn more about their furnace and heat pump repairs and installations online or call them at (573) 875-7888 for a free estimate.