Gas Furnace Flames BurnersWhy Is My Furnace Leaking Water? 

Paul, Last year my husband and I purchased a brand new gas furnace and air conditioner split system. We followed your advice and want to thank you. It is much improved over the old system and the utility bills are lower. Everything went smoothly with the contractor and installation.

It’s been a little cold at night and we’ve been running the gas furnace at night. This morning we found water all over the floor near the furnace. I never thought furnaces could make so much water. Can you please advise? Why is my furnace leaking water, Paul?

Gas furnace repairWhy Is My Furnace Leaking Water

Hello Pam, thanks for the great comments and I’m glad I helped you. Good advice goes a long way, especially with HVAC systems. First, the furnace you bought is a high-efficiency gas furnace. Secondly, they can also be referred to as condensing gas furnaces. They also make condensing gas boilers also. These systems are ultra-efficient compared to the older gas furnaces.

Believe it or not, gas has a small amount of moisture to it. When your furnace is running the heat from the fire turns this moisture into steam. In older furnace systems, the steam and byproducts of combustion would be exhausted up the chimney or flue. That’s all changed with newer modern furnaces.

Modern High-Efficiency Condensing Gas Furnaces

Why Is My Furnace Leaking Water?

All that heat in older furnaces wasted going up the chimney or flue. That was a huge waste and it was very inefficient. Now, modern condensing gas furnaces have two separate heat exchangers that absorb as much heat as possible. The “heat that” is now used in your home rather than vented. When you cool a gas down, including steam, it will change to liquid. That is what is happening inside your furnace heat exchanger. Especially the secondary heat exchanger.

Furthermore, because a condensing gas furnace creates water it also has a drain. It’s typically the same drain your air conditioner uses to drain condensate from the evaporator coil. Sometimes this drain will become plugged or clogged. In that case, you need to unplug it or unclog it so it will drain. There are a few ways to do this:

  • Find the end of the drain and use a shop vac to suck the pipe clean.
  • Go to the furnace and open the drain line. Then apply pressure to the line to blow it out. Some shop vacs have a reverse feature of a blow instead of suck.

Preventing Furnace Water Leaks

Why Is My Furnace Leaking Water?

First of all, after you unplug the pipe, you’ll want to avoid another water leak in the future. Most leaks such as this are a result of algae plugging up the drain. Algae love cool dark places and the condensation drain line makes a great home for them to thrive. However, they create problems such as clogging up your drain line. When you cleared the line you evicted them. Now you want to keep them from returning. Here are a few ways to help keep it clean:

  1. Regular maintenance. Any maintenance schedule would include checking and treating the condensation drain lines. Most contractors offer this service through a maintenance agreement. Many HVAC contractors also have specials in the Spring and the Fall for maintenance or check and clean specials.
  2. If you are a DIYer then you can purchase some special tablets. You can find some here. Place the tablets in the pan of the evaporator coil. As they absorb water they will slowly dissolve and go down the drainpipe. This will kill the algae so it’s no longer a problem.

Other Problems that could Cause Water Leaks

Why Is My Furnace Leaking Water? Leaks can occur because of:

  • Cracks in the drain piping, pan, or other drain components
  • A bad seal in the heat exchanger cover
  • On older gas furnaces, corrosion causing pits or pinholes where water can leak.
Component and Piping Cracks

condensing gas furnace

90 Plus Condensing Gas Furnace

There are other reasons why your condensing gas furnace can leak water. Much of the drain system and some of the parts are made of plastic. This includes the drain pan and the drain piping. In some circumstances, the plastic can be cracked. This will cause your gas furnace to leak water. If the pan is cracked it needs to be repaired. If you have a new gas furnace, such as yours, this can be covered under warranty. The warranty should cover the part only unless you have an extended part and labor warranty. Note: most HVAC equipment warranties only cover parts and not labor.

Bad Seals

If the crack is in the piping it can be repaired fairly easily. A cracked PVC is easily repaired for minimal cost and labor. If you are handy and into DIY you could likely repair it yourself. However, if the system has a bad seal on the heat exchanger cover, I recommend calling an HVAC service company for repair. A bad seal could mean other underlying problems with the furnace. These problems can be serious and need to be addressed.


The condensation from the furnace gases is corrosive to metals. This is the reason why the components are stainless steel or plastic. These materials resist corrosion over a long period of time. However, older furnaces can develop corrosion on some parts and begin leaking water. The holes start off very small and get bigger over time. Eventually, if not corrected, this will cause pressure problems in addition to water leaks and the furnace will fail to run. This problem needs to be addressed by an HVAC service company.


There you have it, Pam! A comprehensive list of why your gas furnace is leaking water. I think your problem is easily corrected since you have a new furnace. I’m guessing it’s simply the drain piping plugged up. Once that gets rectified you’ll be happily heating again with no water leaking. Good luck and happy heating!!

High Performance HVAC Heating & cooling

Why Is My Furnace Leaking


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