Whether you have a conventional or high-efficiency furnace, prepare yourself for mishaps. For furnaces, leaks are relatively common. If you have a leaking furnace, you may wonder: why is my furnace leaking water? There are many possible reasons.

Before Calling an HVAC Contractor, Do This:

If you notice a puddle of water at the base of your furnace, here is what you do:

  • Turn the heating system OFF right away.
  • Turn the thermostat OFF.
  • Shut OFF the valve to the gas line and the electrical breakers.
  • Call an HVAC professional as the problem is likely severe.
  • Mop up the puddle of water.

Furnace Type

One of the first questions the HVAC profession will ask you is the type of furnace. There are various reasons why your furnace could be leaking, depending on the type of furnace. For instance, condensation leaks are more likely with high-efficiency furnaces.  

High-Efficiency Furnace

If your furnace’s AFUE (Annual Fuel Utilization Efficiency) rating is 90% or greater, you have a high-efficiency furnace. Look for the yellow energy guide sticker on the heating system.  You can recognize a high-efficiency furnace by its white PVC vent pipe.

High-efficiency furnaces extract heat from the combustion gases for a more extended period than a standard-efficiency furnace does, causing the gases to cool, then condense. The condensation flows to a floor drain. 

Conventional Furnace

A standard-efficiency (conventional) furnace has an AFUE of less than 90%.  A metal exhaust vent pipe carries away the gases produced during the combustion process. It releases them outside while they’re still hot.

Each type of furnace causes leaks in different ways.

High-Efficiency: Leak or a Clog in the Condensate Lines

As discussed above, a high-efficiency furnace creates condensation through the heat exchange process. The condensation drains away from your furnace through the condensate line and the drain trap. If the condensate lines are leaking or clogged, a puddle of water will form around your furnace.

High-Efficiency: Condensate Pump Not Working

A condensate pump pushes the water through the lines and away from your furnace. If the pump is not working correctly, it can cause leaks.

Conventional Furnace: The Vent Pipe Not Working

The vent pipe carries away gases from your home while they are still hot. However, in an improperly sized venting pipe, some of the hot gas gets trapped. Over time, these hot gases cool down, causing condensation. The condensation will then leak outside your furnace.

For Any Furnace: The Humidifier is Leaking

Many homeowners choose to add a humidifier to their heating system to add moisture to the air. Water is constantly moving through humidifiers. A leak in the humidifier would cause a puddle of water around the furnace.

For Any Furnace: The Air Conditioner is Leaking

Air-conditioners absorb moisture from the air inside your home, so they also have condensate lines and a drain pan.  A clogged condensate line in the air conditioning system will cause leakage..

Don’t Forget About the Air Filter

If your furnace is leaking water, you should swap out the furnace filter after the repairs and before you turn your furnace back on. Dirty filters can cause a furnace to blow cold air or continually shut on and off.

A wet, dirty, or clogged filter will restrict airflow and put extra strain on your furnace.

Why Is My Furnace Leaking Water – Final Thoughts

Get your furnace inspected regularly, and you will not have to deal with unpleasant surprises. It is common for furnaces to leak water. The key is to figure out why it is happening. Once a technician has diagnosed the problem, fixing it is not that hard. An experienced technician and do that and ensure the smooth operation of your furnace.

If they rule out a condensation leak, there are other possibilities, all still requiring a professional. One option may be an issue with your whole-house humidifier if you have one connected to the furnace. It could be leaking inside your furnace, though having an annual maintenance appointment on your HVAC system should prevent this from happening. Another possibility is an internal drain system clog. If your air conditioning is running and shares an internal drain with your furnace, the drain could be blocked, sending the water to your furnace.