Common problem when upgrading to a new high efficiency furnace

Home Inspector with Sweet Home Chicago Inspector LLC

Last year I saw an amazing number of new  high efficiency furnaces being installed in homes due to the U.S. Government energy tax rebates of up to $1500. Although this was a good thing, many homeowners chose not to upgrade their water heaters to high efficiency direct venting units. This in turn has caused a large number of "Orphaned Water Heaters", meaning that water heater vents into the chimney alone.

Back in the day, homes were designed with chimneys to vent both the furnace and water heater. The coal and oil fired furnaces provided plenty of heat needed to create positive pressure inside the chimney which allowed the byproducts of combustion (carbon monoxide and water vapor) to safely draft up and out the chimney. Even when natural gas came along there was still enough heat being expelled to allow positive pressure.

When high efficiency furnaces (90% and above) arrived they no longer needed to be vented out of the chimney and use PVC pipe to vent to the outside. By removing the furnace from the chimney it is now over sized to vent the water heater. Heavier cold air will settle down inside and since a water heater does not run nearly as much as a furnace, the chimney cools rapidly. The water heater now vents into a cold chimney which causes condensation on the inside from the water vapor produced by combustion. It can literally be"raining" inside the chimney now. This excessive moisture can cause mortar to fail, spalling of bricks, plaster degradation, paint bubbles, and mold/mildew problems. Another issue is safety. There also may not be enough positive draft pressure to expel all of the carbon monoxide gas produced by combustion which could spill back into the house. This situation is worsened when you create negative pressure conditions inside the house by running bath and kitchen vent fans and your clothes dryer.

As part of my inspections I always test for carbon monoxide in the house, but if there is an orphaned water heater I strongly recommend having a "worst case scenario" combustion analysis be done to ensure that there is no carbon monoxide spilling back into the house even if the worst possible conditions occur. This test is performed by running your combustion appliances and by turning on all exhaust fans, dryer and furnace air handler to make sure that your combustion appliances do not back draft from the negative air pressure these devices are causing.

If this test fails, you should either have a properly sized stainless steel chimney liner installed for the existing water heater or upgrade to a new direct venting unit (PVC vent pipe to outside).

Here are some of my inspection photos showing a perfect example of this problem.

Typical Orphaned water heater - The mortar around the connection into the chimney had an extremely high moisture reading of 27% from the buildup of condensation due to insufficient draft.

The condensation buildup was causing moisture to permeate the chimney mortar and seep down the wall allowing mold and mildew to grow.


The increased condensation inside the chimney has caused the mortar to deteriorate resulting in loose brickwork.


The moisture trapped inside the brick has caused the brick face to split (spall) due to the pressure exerted during the freeze thaw cycle.


The condensation caused by the orphaned water heater has also deteriorated the firebox and rusted out the damper making the fireplace unsafe to use.


My testing found the carbon monoxide level and draft pressure were just at the threshold of failing, so the owner decided to have a new stainless steel chimney liner installed as the water heater was not that old and in excellent condition. They also need extensive work on the chimney to repair the moisture damage caused by this situation.

If you have a home with an orphaned water heater be sure to have it tested today, as it could save you a lot of money in the long run as well as ensure that your home is safe from possible carbon monoxide exposure.

I am certified by the Building Performance Institute (BPI) to perform this testing.

Give me a call today if you have questions (773) 559-3360




Comments (9)

Raiza Schwartz
West USA Realty - Ahwatukee, AZ

So, what causes the insufficient draft?  I'm kind of confused.

Feb 15, 2011 06:05 AM
Randy Ostrander
Lake and Lodge Realty LLC - Big Rapids, MI
Real Estate Broker, Serving Big Rapids and West Central MI

Good call Dave. I have an old school house converted with the chimney in the rear of the house. Never noticed until pieces of chimney started popping and falling down. Time to notice is BEFORE you need to reset chimney bricks.

Feb 15, 2011 06:10 AM
Li Read
Sea to Sky Premier Properties (Salt Spring) - Salt Spring Island, BC
Caring expertise...knowledge for you!

Thank you for this key and informative post!

Feb 15, 2011 07:01 AM
David Ward
Sweet Home Chicago Inspector LLC - Chicago, IL

Hi Raiza,

An insufficient draft can be caused by a cold and over sized chimney. As cold air is heavier than warm air, this cold air sinks and pools at the bottom. As warm air rises there was never a venting problem for both the furnace and water heater combined as the furnace pushed out so much hot air the chimney stayed relatively warm. When you take the main heat source away (the furnace) there is a significant drop in temperature inside the chimney, plus the chimney is now far larger than it needs to be for just a water heater. It is these two factors that reduces the ability of a water heater on its own to draft properly.

The water heater runs far less frequently than a furnace so it is not capable of warming the chimney, therefore it's pretty much cold all the time. When the water heater vents into the cold chimney the water vapor condenses (like warm air on a cold window and you see the moisture) and that moisture stays inside.

For carbon monoxide to draft back into the house BPI has designed a "worst case scenario test". This sets up the home for the worst possible negative pressure inside. (All doors closed, all the vent fans and the clothes dryer running) All this air being sucked out of the house through these fans creates negative pressure inside as opposed to outside.  This pressure difference can cause the water heater to backdraft into the house due to the cold over sized chimney.



Feb 15, 2011 07:07 AM
Matt Grohe
RE/MAX Concepts - Des Moines, IA
Serving the metro since 2003

David: This is really helpful information and a phenomenon that I was not aware of. I have a high efficiency furnace as well but our water heater is a an on demand which has a strong power vent so I think I'm probably ok.

Feb 15, 2011 03:59 PM
Howard and Susan Meyers
The Hudson Company Winnetka and North Shore - Winnetka, IL

Very informative post Dave.  Carbon monoxide issues are such an important concern and so many consumers believe that this is not something they have to think about when they have a newly installed high efficiency system. 

Feb 16, 2011 01:08 AM
Morgan Evans
Douglas Elliman Real Estate - Manhattan, NY

I agree, very informative and helpful.  Something like this is not going to come easily to the average home owner until its nicely explained.  

Feb 18, 2014 04:22 AM
Jack Moore

I didn't realize there was such a huge tax rebate on those. I'll have to keep that in mind, along with the orphan water heater issue. I'll need to take a look at my heaters before I make any big decisions. I'll be sure to remember that this can be an issue.

Sep 21, 2015 07:51 AM

but in the summer, my furnace isn't running anyway. So, my water heater is the only thing exhausting. so, shouldn't I be good?

May 14, 2016 11:38 AM