Reuben's Home Inspection Blog

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HRVs, Part 1 of 3: Why We Need Them

Many months ago I wrote a blog about how houses can often have moisture problems when old furnaces are replaced with high efficiency furnaces. The fix that I mentioned at the end of the blog was to install a Heat Recovery Ventilator, or HRV.  The character in that blog finally got around to installing an HRV in his house, and solved all his moisture problems! For the first time since he installed his high efficiency furnace, he no longer has condensation on his windows during the winter, and he couldn't be happier about it.

Today I'll share some basic information about how HRVs operate and why they're needed in today's newer, tighter houses.

New Houses Don't Breathe As most people know, new houses are constructed much tighter than they used to be - they don't leak air all over the place.  I've heard a lot of old-school home inspectors and building contractors complain about this, and you probably have too.  The rant goes something like this: "We build houses so damn tight that they don't breathe, and they end up rotting from the inside out!  Things were a lot better when we didn't have all these stupid house wraps."

These cranky doom sayers are only partially right - yes, we build houses tighter today, but we've also figured out how to prevent mold and moisture problems, and how to improve indoor air quality.  This is where HRVs come in.

HRVs Provide Fresh Air An HRV works by constantly bringing fresh air in to a house and exhausting stale air.    The air that gets brought in to the house gets passed through a screen at the exterior, then through a filter inside the unit, then through the HRV core, which is actually a heat exchanger.  The heat exchanger allows the fresh outdoor air to get warmed by stale indoor air right before the indoor air gets exhausted to the exterior.  This allows about 60 - 80% of the heat in the air to be re-captured.  The diagram below illustrates this principal.

HRV Heat Exchanger

To understand how an HRV works, interlock your fingers together and picture warm air flowing through fingers in one hand, and cold air flowing through the fingers in the other hand.

Interlocked Fingers2


HRVs Remove Moisture 
Besides providing fresh air, HRVs also remove a lot of moisture from the air.  Old, drafty houses get dry in the winter because they're leaky, and the moist indoor air is always getting replaced with dry outdoor air.  Not so with newer houses - they stay humid during the winter, and HRVs are often needed just to get rid of all the excess humidity.  As the warm, moist air passes by the cold air, the moisture will condense.  This is why HRVs have a drain running out the bottom.

HRV Basics SuperVentor HRV

HRVs Lower Radon Levels Because HRVs constantly change out the air in a house, an HRV will reduce radon levels when working properly.  During a recent Eden Prairie home inspection that I also performed a radon test at, I had the HRV running during the majority of the radon test, but I tripped the GFCI outlet for the last hour of the radon test during my inspection.  Look at the jump in radon levels at the house from NOT having the HRV running!  Any time a radon test is performed, if there is an HRV present at the house, it should be up and running throughout the duration of the radon test.

Radon Graph

HRVs Have Many Names If you hear any of these terms, someone is probably talking about an HRV:

  • Air-to-air heat exchanger
  • Air exchanger
  • Whole house ventilator
  • Big square thingy in the furnace room
  • VanEE system (brand name)
  • ERV

The last one, ERV, stands for Energy Recovery Ventilator.  These are similar to HRVs, but ERVs are pretty rare here in Minnesota - I think I've seen two of them, ever.  They're designed for more humid, southern climates.

If you don't have an HRV at your house and you think you need one, you could always just turn on an exhaust fan and leave it running.  This will be very inefficient, but it will change out the air in your house.  I call this the Poor Man's HRV.

Next week I'll talk about the maintenance needs of HRVs, and the week after that I'll discuss installation defects.

Reuben Saltzman, Structure Tech Home Inspections

        

Comment balloon 14 commentsReuben Saltzman • December 15 2009 06:00AM

Comments

Great information that will be helpful in future. Thanks for sharing.

Posted by Retired Notworking almost 9 years ago

I've started seeing a lot more HRV's lately in the higher end homes wehn I'm expecting them

Posted by Al Wright (Affordable Canadian Home Inspections) almost 9 years ago

Good job presenting info on HRVs. I have rarely seen them, but have recommended them quite often.

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) almost 9 years ago

Colleen - thanks for reading!

Al and James - I actually see quite a few of these.  I'll have good information to share about how to inspect them and what to tell your clients about them in my next two blogs.   I started writing this blog and split it off in to two blogs, then three.  

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) almost 9 years ago

"Big square thingy in the furnace room" That's too funny.

 

Posted by Vince Santos, Southeast Michigan Home Inspector (StepByStep Home Services LC) almost 9 years ago

I have seen 2 count em' 2 in 5 years of inspecting homes.  Seems like they would be more popular.  I guess common sense solutions just don't get much attention.

Posted by Jim Allhiser, Salem, Oregon Home Inspector (Perfection Inspection, Inc.) almost 9 years ago

Reuben---hard to imagine why these things are required in new construction----leaves one wondering how "real" concerns for indoor air quality really are:)

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) almost 9 years ago

Jim-If there is one thing I learned from this blog, it's that these things aren't too common in other parts of the country!  We have a ton of them here in Minnesota.  

Charles and Vince- do you guys have many of these in your area?

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) almost 9 years ago

Pretty rare Reuben.  1 a year maybe.

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) almost 9 years ago

Never.

Posted by Vince Santos, Southeast Michigan Home Inspector (StepByStep Home Services LC) almost 9 years ago

Great blog, Reuben. I have never seen an HRV on an inspection in my area, but there is always a first!

Posted by Jason Aldrich, Sequim, WA Home Inspector (Aldrich's Home Inspections, Inc.) almost 9 years ago

Thanks Jason.  I'm guessing they'll be more popular very soon.  Most new houses in Minnesota have them, and it's been that way for about the last ten years or so.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) almost 9 years ago

Probably cost is a factor and I am thinking they wear out quickly with all the moisture they are around.

Posted by Gene Allen, Realty Consultant for Cary Real Estate (Fathom Realty) almost 9 years ago

They're actually quite simple devices, with a life expectancy of around 20 years.  I'm sure cost is a factor... but when homes are required to be built with one, it's a lot easier to hide the cost.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) almost 9 years ago

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