Demystifying Ventilation and Energy Recovery Ventilators (ERVs)

Green Building Service Provider - SawHorse, Inc.

SawHorse, Inc.

Jun 10, 2020
Demystifying Ventilation and Energy Recovery Ventilators (ERVs)

I was talking to a friend about ERV’s recently and they had a puzzled look on their face when I kept mentioning that I wanted to put an E.R.V. in my house.  I realized that they did not know what ERV stood for and they thought I was referring to an RV.  

An ERV is an “Energy Recovery Ventilator” (which is much different from an RV which is a “Recreational Vehicle”...).

To be fair, I barely knew what one was several years ago and I am a green builder.

Before we get into what an Energy Recovery Ventilator even does, let’s discuss fresh air and why it is so important..

According to the EPA, the air inside a house is more often more polluted than the air outside of the house.  In many cases it is not a small percentage like 20% but worse it is a factor like 2 to 5x or more, which is 200% to 500% worse!!  This issue is that most of us spend 90% of our time indoors so are subject to all of this bad indoor air quality for longer periods of time.  Even if you have fresh air in your home, you may have poor air quality at your office which is where you are likely to spend ⅓ of your time.

Many of us suffer from allergies as well so this polluted air may be worse for them.

When you are outside- no one ever says “I’m going inside to get some fresh air..”   We always go OUTSIDE to get fresh air even if we live in a city.  Instinctively we know that indoor air is not ideal and are programmed to want to get fresh air from sources that are cleaner.  Even cave men knew this (pretty sure there are cave drawings to prove this - or not).

The current COVID19 crisis is making people more aware of the air that they breathe and also aware of how BAD indoor air quality (IAQ) actually is right now.  The good news is that green building programs and high performance buildings have had ventilation strategies as part of their programs for decades, so we do not have to reinvent building science.

One of the recommendations from the C.D.C. and W.H.O. to combat COVID19 was to ventilate.  Sounds good- “Let’s just ventilate!”   Soooooo… OK…. And… Crickets...  

Reminds me of the final scene in Samuel Beckett’s “Waiting for Godot”


“VLADIMIR: Well? Shall we go?

ESTRAGON: Yes, let's go.

They do not move.”

I don’t think people intentionally ignore ventilation, it is just very confusing and much more complicated than opening a window or door.  What if it is cold or humid outside?  No one wants to lose their precious (costly), comfortable conditioned air.

As a green builder, I assumed that most other designers and builders knew about ventilation and different strategies for getting the bad air out and the fresh air in.  After reading blogs, listening to webinars and looking at forums on the topics, I realized that there is some talk in a small building performance bubble about fresh air, however the general public has no idea what to do therefore just like Vladimir and Estragon - THEY DO NOT MOVE.

To be clear, I am not blaming the consumer.  The consumer wants solutions, however there is so much noise and partial solutions that I can understand why consumers don’t know what to do.  Also, why should consumers be the experts?  The design and building community should know about fresh air and be educated about it so they can offer solutions to the consumer.  Unfortunately, I know this is not the case since I see ads for webinars from the manufacturers that make equipment that helps with fresh air constantly advertising about these strategies.  I am also a renovation contractor and I never see designed fresh air solutions in existing homes.  If this was common knowledge amongst HVAC technicians and builders, then why are there so many classes that have Building Science 101 solutions?  I still have to specify fresh air with my HVAC contractor and make sure the architects specify proper vent fans.

My industry is not very user friendly and when you look at technical manuals, manufacturers’ websites and green building checklists, it is easy to get confused and just ignore the problem.

My goal is to demystify, myth bust, and give you useful tips regarding ventilation and discuss some common sense, easy to understand strategies.

Let’s first get rid of the notion that “Buildings Need to Breathe.”  No they don’t - the occupants of the buildings need to breathe.  We DO NOT want UNCONTROLLED air coming into our artificial environments.  We DO need air for us to breathe, however this needs to be CONTROLLED so we can make sure the source of the air coming in is clean.  For example: An old house is typically leaky and the “fresh air” could be coming in from the attic, walls, crawl space etc.  This means that the air is bringing pollution from those areas into the house like mold, lead dust just to name a few.

Sounds Good “Mr. Hoots” BUT…  How do we control the air?

There are 3 main strategies for getting fresh air and they are NOT mutually exclusive:

  1. Exhaust Vents.  You simply have exhaust fans that take the BAD air out.  This sounds good and can work if designed properly.  If you just have vent fans in bathrooms and above your cooktop, then you will need “Make Up” air to replace the air from these vents.  If you don’t account for make up air, then it will just find its way back into the building through gaps and cracks such as doors and windows which can bring in unfiltered, and unconditioned air.  A “vent only” strategy can create additional problems and make the air worse if not designed properly. 

  2. Fresh Intake Air.  Another strategy is bringing controlled “fresh air” into the building through the HVAC system.  This is code in many places already.  This does the opposite of removing the air from spaces like vents and it creates a positive pressure inside the house by forcing fresh air into the house with power vents versus sucking the air out of the house.  This is typically brought into the central HVAC system so the air can be filtered and conditioned if needed.  In humid climates, I recommend additional dehumidification, or this “fresh air” will have tons of moisture in it during certain times of the year.

  3. ERV/ HRV The last strategy is using an ERV or HRV, and  Energy Recovery Ventilator or Heat Recovery Ventilator.

Let’s first discuss how an ERV works and the climate zones that it works well in and then discuss the HRV.


While Solutions 1 and 2 do have some merits for bringing in fresh air you may also have an “energy penalty” for this unconditioned air.  In the Northern climate zone during the winter, fresh air is freezing and brought directly into the house or building will definitely wake you up.  If you don’t heat this air up you will be uncomfortable.  In the South, and you will hear “It’s NOT the HEAT it’s the HUMIDITY!”  This fresh air could have lots of humidity with it making the house uncomfortable...

An ERV is a device that sucks out the “Bad Air” while bringing in “Fresh Air.”  Sound like a vent fan right?  Kind of.  The ERV is like a vent fan and fresh air intake, however it does not have the energy penalty because of its unique technology that transfers energy from the air and humidity from the air as it leaves and enters.

An ERV is a good choice for a fresh air solution without having to pay the energy penalty to condition this extreme air coming into the house.  You could avoid any energy penalty at all by shutting off the fresh air supply, however you could then be creating a “Sick Building” which will make you, your family or coworkers sick as well.

OK- now we get to debunk another myth:  ERV’s don’t work up North cause they freeze up.  This used to be true, however the technology has greatly improved so the ERV is better designed for cold weather climates now.  

Confession.  For years I thought that ERVs were not good for Southern climate zones because they brought in humidity.  I was even told this by a manufacturing rep who did not really understand the technology.  Being a building science geek I wanted to understand exactly how these systems worked and now I am enlightened!!

Now that I did my research and fully understand ERVs, I realize now that what I was told was not entirely true.  The whole point of an ERV is to deal with humidity as  well as heat transfer.


So this is how it works...  

It’s MAGIC!  Honestly that is easier than explaining the technology behind the Enthalpic “Magic” Core.


In the humid Summer:

When the colder conditioned air is sucked out of the house it passes by the warm air from the exterior, however these 2 air streams do not touch or mix!!  The enthalpic core allows for the air to transfer heat from the outside air to the cool air being sucked out therefore allowing cooler air to be pumped into the house.  This same core transfers the humidity as well.  The humidity from the air transfers across this membrane within the core as well so the humidity level is less than it is outside.  It does not equalize the temperature and humidity 100%, however it does make enough of a difference so that the humidity levels inside the house are not greatly affected by pumping in the outside air.


In the dry Winter:

The cold dry air passes by the warm wetter air and it is warmed and moistened before it enters the house.  Same as the Summer- just in reverse.


Physics 101- “Heat moves to cold” and “Wet moves to Dry.”  Once you understand that, the principles of enthalpy make more sense thus demystifying how the ERV works.

If you are a visual person, here is an animation on how the ERV works.  


Here is a complete video showing the unboxing of a Panasonic ERV and inside look at the parts of the ERV that help bring fresh air into the building.  

All of these technologies are great and work even better when used together.

In some climate zones, HRV’s (Heat Recovery Ventilator) would be the ideal choice instead of an ERV.  This helps with the heat transfer only and does not balance the humidity.   Check with your building performance specialist or heating and air company to properly size and specify a solution for your home.

For my house, I am going to use each of them in the following manner:

  1. Ventilation- Bath fans in each bathroom with an additional moisture sensor for bathrooms with showers.  Watch here for details on bath fan design. (Should we do a thumbnail and video here?)

  2. ERV- This will be used on each level (since I already have a couple spot ERV’s) and run continuously since I have 3 boys and a dog helping to create pollutants.  You could get a whole house system as well that is designed to move a larger volume of air.

  3. Fresh Air intake- As I am designing my new HVAC system, I am including fresh air that will run through a dehumidifier.  This can also help remove some of the humidity from the inside of the house if needed.  This is also helpful to bring in “make up air” for the vents in the house like the range hood and the bath fans.  When my range hood runs I can smell some of the soot from my fireplace so adding this make up air instead of cracking a windows will help keep my utility costs down.


Matt Hoots is a green building contractor and owner of SawHorse, Inc a design + build firm in Atlanta, GA.  In response to COVID19, SawHorse created a playlist on youtube called FRESH AIR FRIDAYS with new Indoor Air Quality videos released each Friday on YouTube.  In addition to IAQ, Matt has created videos on High Performance Building Products and Design Grade Products.  On the weekends Matt either works on HIS HONEY DO LIST or Makes Things with his 3 Boys.

Fresh Air Fridays:

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01 - of - 01 Comments

Thanks for hosting this content- I enjoyed putting it together.

By Matt Hoots I Jun 11, 2020