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Why is your furnace leaking water?

Condensate leak in this furnace has led to permanent damage

As a licensed home inspector based in Buffalo’s Southtowns (in the Orchard Park area), I inspect several types of heating systems.  Searching for water leaks is something I do in every home I’m in, but my clients can be surprised when I report that the furnace is leaking water.

But how can this happen?

When residential furnaces took a leap into the realm of higher fuel efficiency, big changes were made to their design that created the conditions needed for leaks to sometimes form.

Let’s first cover a few basics.

What is a high-efficiency furnace?

“High-efficiency” is generally understood to mean that a furnace is rated at 90% AFUE or better. AFUE stands for “annual fuel utilization efficiency”, basically meaning that if a furnace is rated at 90% AFUE that only 10% of the energy going into it is used to operate the furnace.  

Amana high efficiency furnace
A high-efficiency furnace (condensate drain on the left)

Up until several years ago, 80% furnaces were common.  Today, furnaces that exceed 98% AFUE are available.  Homes in Western NY can still be found with 80% furnaces in them, believe it or not. 

The Spruce has a good article that explains the basics of high-efficiency furnaces very well.

How is a high-efficiency furnace mechanically different?

To attain 90% AFUE, several changes had to be made to the furnace design:

  1. A second heat exchanger was added in order to pull more heat out
  2. Combustion air is pulled in from the outside
  3. Exhaust air is cooler
  4. PVC pipe is used for fresh-air intake and exhaust – no more metal flues (note that upgrading to a high-efficiency furnace can indirectly cause issues with venting tank-style water heaters).
  5. Condensation occurs within the furnace and in its exhaust.
  6. Better models have a variable-speed fan to reduce air flow when possible
  7. Some models have a regulating gas valve, which reduces the flow of natural gas, helping to further improve efficiency
  8. The heat exchanger is mechanically sealed, meaning that the home inspection can’t include a detailed inspection of this part of the furnace.
Internal view of a high-efficiency furnace in a Hamburg, NY home

Why do high-efficiency furnaces produce condensation?

90+ percent AFUE furnaces are designed to extract more heat than older systems. To accomplish this, a secondary heat exchanger is added.  Since more heat is pulled from the air as it moves through the furnace, the exhaust air naturally is cooler. 

Cooler exhaust (and a chemical reaction involving natural gas) invites condensation, which the design of the furnace needs to properly account for.  This condensation is typically referred to as “condensate” when dealing with furnaces.  

Condensate is acidic, and can actually cause permanent damage to metals and plastics that are used in the furnace.

Older, less efficient furnaces have very hot exhaust that left the furnace and vent pipes.  If the vent pipe runs for these furnaces are properly configured, only small amounts of condensation might form in the exhaust piping.

Where’s condensate go when it leaves the furnace?

That’s a very good question and it seems that some furnace installers don’t think about it.  Furnace manufacturers provide a discharge on the side of the furnace.  This discharge should have a “trap”, which prevents gas from passing from inside of the furnace to the ambient air. 

In Erie county and the greater Buffalo area, I often find that the condensate discharge is connected to a pipe that’s been inserted into a hole in the basement concrete slab.  This hole should pass all the way through the slab, allowing the condensate to drip to the earth, where it should dissipate.  

Some basements, usually in homes from the very early 1900’s or earlier, have dirt or gravel floors.  In these homes, it’s best to plumb the condensate discharge to a condensate pump.  These pumps should contain a neutralizer material to reduce the acidity of the water, and should pump the condensate to a sump pit or floor drain. 

What are the consequences of leaking condensate?

Condensate leaks that go unchecked can cause permanent damage to the structure of the furnace, damage to the compressor and in certain conditions mold may begin to grow.

The cost of furnace replacement alone can be several thousand dollars.

What do we look for as home inspectors?

I’m very much in favor of improving the energy efficiency of everything we use, and furnaces are no exception.  But it’s important that a home inspection includes an assessment of a furnace in order to determine if condensate might be causing an issue. 

I always remove the covers on furnaces in order to perform a physical inspection of the internal components.  There are few common problems that I see.

  1. The condensate drain line is clogged – The drain line (usually PVC pipe) should be cleaned annually with a brush.  It’s common to find these pipes clogged with bugs and other nasty stuff.  If the line becomes clogged, a condensate backup can form, leading to rust and corrosion within the furnace.
  2.  Internal leaks – Leaks near the bottom of the exhaust pipe, inside of the furnace, are very common.  These leaks seem to occur when bolts and other connectors rust out, and if unchecked then they can cause massive damage to the furnace body.  
  3. Check the condensate pump – If a condensate pump is installed, physically examine it for damage, and make sure that the final destination for condensate makes sense.  
A burned-out condensate pump. It was found unplugged, with condesate collecting inside of the furnace.

The best way to get the longest life out of a high efficiency furnace is to have a qualified HVAC technician inspect it annually, including performing routine checks of its operation (including the air conditioner).  

We recommend annual checks to all of our home inspection clients in Western New York, simply because an unchecked leak can essentially destroy a furnace within just a couple years.  

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About the Author:

Bradley Beck

Bradley is owner of Alto Home Inspection, LLC.  He lives just south of Orchard Park, in Western New York’s Southtowns, and inspects homes throughout the Buffalo area, and is NY Licensed Home Inspector #16000086029, NY Certified Mold Assessor #MA1606.  He also serves as Technical Director of Alto Home Inspection’s Radon Laboratory.

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