Most central furnaces don't use or produce water during normal operation, so it can be quite surprising to see water leaking around your furnace. There are a few uncommon problems that can cause liquid to pool on the floor around your furnace. Here are three possible causes for your furnace leaking water.

1. Flue Condensation

In a conventional furnace, condensation may collect in the furnace flue if exhaust gasses cool too quickly. This can happen if the combustion temperature of the furnace is too low. A leak in the flue can also cause condensation to form when cold air enters the flue.

Flue condensation may seep down the inside of the furnace housing and pool around your furnace. Insufficient furnace combustion temperature may be caused by dirty furnace burners. A punctured flue must be replaced to avoid the risk of a carbon monoxide leak, so call an HVAC professional for an inspection.

2. High-Efficiency Furnace

High-efficiency furnaces make use of condensation within a secondary heat exchanger called the condensing heat exchanger. The secondary heat exchanger recaptures heat from the exhaust gas. By reusing exhaust heat, high-efficiency furnaces can achieve an AFUE rating of 95 percent.

If the condensing heat exchanger cracks, you may see water leaking around your high-efficiency furnace. A cracked heat exchanger will severely impact the energy efficiency of your furnace and may leak harmful exhaust gasses into your home. A furnace inspection will confirm that your high-efficiency furnace is safe to operate.

3. Secondary Sources

Sometimes, a water leak around your furnace isn't caused by a problem with the furnace itself. Other components near your furnace can leak water under certain conditions.

The AC condensate drain pan is one of the first culprits to consider. The drain pan captures condensation from the AC evaporator coils so it can drain outside. Water leaks around a furnace are often caused by cracks or overflow in the drain pan.

Some furnaces have built-in humidifiers that add moisture to the air before it reaches your ductwork. Furnace humidifier leaks are possible when the humidifier cracks due to age and wear. In other cases, a loose connection between the humidifier and your plumbing may cause a leak. Your HVAC technician will inspect any other appliances near your furnace that could cause leaks.

A water leak from a central furnace is a rare issue, but it can sometimes indicate a serious problem. Shut off your furnace and call an HVAC contractor if you see an unexplained water leak near your furnace.