Reuben's Home Inspection Blog


Moisture Problem Caused By High Efficiency Furnace


Nelly has lived in the same split entry house since it was built in 1980, and has never had any moisture problems with his home until recenty.  Shortly after replacing his old mid-efficiency natural draft furnace with a high-efficiency furnace, Nelly started noticing a host of moisture problems with his house.  It started with condensation on the windows that never used to be there, and developed in to water spots on the ceiling around the skylights, which were caused by excessive condensation in the attic.

Nelly called the HVAC company that installed his furnace and complained about the moisture problems he was having.   A badly cracked heat exchanger could lead to moisture problems in a home, and a vent that is not properly venting to the exterior could also cause serious damage to the home. The installers came out and checked everything; no problems.  Why is Nelly having moisture problems now?
The answer has to do with combustion air and dilution air.  On a standard furnace, combustion air and dilution air are taken from inside the house.  Combustion air provides the oxygen that is required for combustion, and dilution air helps to lower the temperature of the exhaust gases.   When you add up the combustion air and diluation air, it equals quite a large volume of air that is constantly rising up and out of the house during the heating season.


Combustion air and dilution air get replaced with cold, dry outside air.  This is part of the reason that older houses get so dry in the winter.  Is this starting to make sense?
High efficiency furnaces save energy by taking combustion air directly from the exterior, rather than wasting the heated air in your home for combustion.  When Nelly replaced his natural draft furnace with a high efficiency furnace, he stopped wasting all that warm, moist air.  In reality, the high efficiency furnace didn’t ‘create’ the moisture problem; it just stopped fixing another problem.
In order to address the moisture problems in his home, Nelly has a few options.  He could install a continuous exhaust fan to constantly remove air from the home, but this obviously wouldn’t be a very Green thing to do, because all of that warm air would always be replaced with cold air.  Nelly could run dehumidifiers all winter, but again, this would be expensive.  Nelly’s best option would be to install a heat recovery ventilator (HRV).  An HRV will constantly change out the air in the house while at the same time removing humidity from the house.   More on HRVs another day.


Reuben Saltzman, Structure Tech Home Inspections


Comment balloon 5 commentsReuben Saltzman • June 24 2009 09:02PM


Hey Reuben great post! A situation commonly overlooked in the exact scenario described. Always love seeing other fellow MN inspectors providing good information that the general public can benefit from . Keep it up!

Posted by Randy King (Prokore Inspections) about 11 years ago

I agree with Randy, a great post.  I also want to acknowledge the great information from the MN Department of Commerce.   Lately I have been referencing one of the paragraphs from the file

"Always test for combustion air during and after any remodeling project. Additions and remodeling change the air leakage characteristics of your home."

Posted by Jim Mushinsky (Centsable Inspection) about 11 years ago

Randy - thanks.  This is one of those situations that's tough to identify during a home inspection, but it's something to think about.

Jim - agreed about the Minnesota Department of Commerce.  We used to order those handouts from them by the thousands up until about 2003, when we changed to all digital reports.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) about 11 years ago

Great post Reuben!  That could be very helpful during the week of cold weather we have here in Central Texas!  LOL    You keep putting good info out there!

Posted by Glen Kotulek, Austin Home Inspector, schedule online (Home Critique Property Inspections LLC) almost 11 years ago

Ha!  Yeah, I heard it almost went down to freezing there once... ;)

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) almost 11 years ago