HRVs, Part 1 of 3: Why Houses Need Them & What They Do

Many months ago I wrote a blog about how houses can often havemoisture 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.

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.

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.

 

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 aradon 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.

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 - Email - Eden Prairie Home Inspections

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 everythings, and it was all working fine.  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 why 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 any more than vapor barriers in walls create moisture problems.

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 - Email - Home Inspector Minneapolis

RELATED POSTS:

• HRVs: Why Houses Need Them and What They Do
• HRVs: Maintenance and Operation
• HRVs: Installation Defects

Whole House Humidifiers Harm Houses

Whole house humidifiers can cause serious problems, especially in Minnesota.  Sometimes, they even destroy houses.  This topic gets a lot of people whipped up, and I always hear the same arguments for having a humidifier: they prevent your hardwood floors from cracking and splitting, they make the house more comfortable, you’ll get nosebleeds if you don’t… etc.  I admit, those are all valid arguments.  Nevertheless, the damage caused by an improperly used humidifier can far outweigh the benefits, and humidifiers are almost never used properly.  From all the houses that I inspect in Minneapolis and the surrounding areas, I know this.  


A whole house humidifier gets installed in the furnace ductwork, adding moisture to the air that gets distributed throughout the house.  The amount of moisture that's added is manually controlled with a dial that is usually located next to the thermostat, or next to the furnace.  To maintain the proper level of humidity, you need to know what the temperature is going to be, and then adjust the settings on the humidifier accordingly.  This is why I have such a problem with whole house humidifiers!  Here’s an excerpt from an owner’s manual:  


"it is important to anticipate a drop in outdoor temperature and reduce the setting  accordingly to avoid excessive condensation. For example, with an outdoor temperature of 20°F the correct setting will be 35% RH. If the temperature is expected to fall to 0°F that evening, then merely reduce the setting to 25% several hours prior to the temperature change."  


Get out!  Nobody does this!  Ok, one person does.  A buyer’s father attending an inspection got defensive about humidifiers when I told his son that I don’t recommend them.  When I explained that most people don’t use them properly, he said he adjusts his humidifier twice a day.  My jaw dropped and I told him I was impressed.  This is definitely the exception, and if you’re one of the few that use a humidifier properly, give yourself a pat on the back.  


The biggest problem with whole-house humidifiers is that too much moisture gets added to the air, and the moisture condenses on the windows, walls, and especially in the attic.  Excessive moisture in the home causes mold growth.  Excessive moisture on the windows stains the finish and damages the wood.  Excessive moisture in the attic causes condensation, which can rot the roof boards.  I’ve inspected many attics with rotted or delaminating roof boards, and in every case there was a whole-house humidifier present.  


If you've read my previous blog about attic bypasses, you know how moisture gets in to the attic. My recommendation is usually to not use whole house humidifiers unless you’re an extremely ‘type A’ homeowner, which means you’ll check the weather forecast and adjust your humidifier all the time.  I do have a bit of good news about humidifiers though- the newest ones come with automatic humidity controls, so you don’t need to adjust the settings all the time!  I learned about these at Aprilaire’s web site, and I think they’re a great idea that will probably save a lot of homes from damage.  If you must use use a whole-house humidifier, use one of these.  

Reuben Saltzman, Structure Tech Home Inspections - Email - Minneapolis Home Inspections