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How to Ventilate a Home With Impunity

Zehnder Energy Recovery Ventilator [Photo Credit: Albert Rooks, Small Planet Supply]

You know that old saying, “No good deed goes unpunished”?  It applies to a lot of homes with respect to ventilation.  If you simply bring outdoor air into the house, you pay for it…in more than one way.  The outdoor air you bring in is unconditioned so you have to heat it or cool it.  If you’re in a humid climate, you may have to dehumidify it.  If you’re in a dry climate, you may need to humidify it.

Depending on how you introduce the ventilation air into the house, you may have cool drafts in winter or warm air blowing on you in summer.  Then there’s the issue of ventilation causing moisture to get into building cavities, where it can wet the building materials, cause decay, and grow mold.  And if you want to increase the ventilation rate, you get punished even more.

The most common types of whole-house ventilation systems use the exhaust-only or supply-only strategies.  Exhaust-only ventilation uses fans that are already in the house—bath fans and range hood—with controls to modulate the rate or runtime to provide the required amount of ventilation air.  Supply-only ventilation may use dedicated fans that blow air into the house, the blower in the heating and cooling system, or a ventilating dehumidifier.  Both strategies can create problems with comfort, energy use, or moisture if you try to increase the amount of ventilation air too much.

But there are ways to ventilate a house at higher rates and not have problems.  You just need to use a different strategy.  Let’s look at three ways you can do this.

Balanced ventilation with heat and moisture recovery

The best way to bring outdoor air into a home is to do it with a balanced ventilation system that has heat recovery (everywhere) and moisture recovery (most homes). You probably want an energy recovery ventilator (ERV) that does both heat and moisture recovery rather than a heat recovery ventilator (HRV) that does only heat recovery, but that’s the subject of a future article. Here let’s just focus on the ventilation.

Bringing outdoor air into the house dilutes indoor air pollutants. By doing so continuously, you can keep those pollutants at a low level. Carbon dioxide levels are a good measure of how much air exchange you’re getting. But, as mentioned above, bringing in outdoor air comes at a cost.  With heat and moisture recovery, though, that cost is much lower.  (From here on, I’m going to use ERV to stand for both ERV and HRV.  If you’re one of those weirdos who uses HRVs, you can make a mental substitution when you read ERV.)

An ERV doesn’t let your expensive, conditioned air just leave the house.  Just as you’re required to turn in your shoes before you leave the bowling alley, an ERV makes outgoing air turn in its heat and moisture in winter.  And just as Buster Scruggs had to deposit his guns—including the señorita pistols—when he entered the saloon, the fresh air entering the ERV in summer deposits its heat and moisture into the outgoing air.

Those heat and moisture exchanges eliminate or reduce a couple of the costs you have to pay for ventilation.  By recovering heat and moisture, you have to do less conditioning of the ventilation air once it’s inside.  The heat and moisture exchange also brings the ventilation air close to indoor conditions so comfort problems are less likely.  The equal air flows of exhaust air and outdoor air reduce the likelihood of moisture causing trouble in the building enclosure.

In short, if you want more ventilation air in your house without having to suffer the ventilation penalty, an energy recovery ventilator can do that for you.  Just make sure to get one that has a high efficiency of transferring heat and moisture, electronically commutated fan motors, and a MERV-13 filter on the incoming air side.  (Some of the brands we like are Broan, Fantech, Panasonic, Renewaire,* and Zehnder*.)

With such a ventilation system, you may be able to crank it up to one air change per hour, which is about three times the recommended rate.  Because of the space needed and upfront costs, going higher than that is hard to do, but you can certainly get more equivalent air changes per hour, ACHe, by combining ventilation and filtration.

Balanced ventilation combined with a heat pump for recovery

Another way to get more ventilation air into your house is to use a ventilation system that has a built-in heat pump.  I’ll call it a heat pump ventilator.  Instead of a heat exchanger core as the ERV has, this device uses the heat pump to transfer heat between the air streams.  Currently, you have two options in North America for this kind of ventilator:  the CERV-2 by Build Equinox and the PentaCare V12 by Minotair.

The heat pump gives this device the ability to do heating, cooling, and dehumidification.  They don’t provide much heating and cooling capacity since their main purpose is to provide clean air.  The CERV-2 has a heating and cooling capacity of about 3,000 BTU per hour.  The PentaCare V12 provides about 6,000 BTU per hour of heating capacity and 11,000 BTU per hour of cooling capacity.

The CERV-2 heat pump ventilator by Build Equinox
The CERV-2 heat pump ventilator by Build Equinox

The heat pump in a heat pump ventilator is like an ERV core in that it can introduce outdoor air into the house without that air being at outdoor conditions.  The difference, though, is that the heat pump should be able to heat or cool the air all the way to the indoor temperature and beyond.  In fact, in small apartments and condos, these systems could provide all the heating and cooling you need, but let’s stick to the ventilation and indoor air quality aspects here.

Another difference between an ERV and a heat pump ventilator is that heat pump ventilators also recirculate air in the house to provide more filtration, heating, cooling, or dehumidification of the indoor air.  This combination of ventilation and filtration with high-MERV filters can provide another level of cleaning the air.  Both are important.

The heat pump ventilator eliminates some of the penalties associated with ventilating at higher rates: comfort and durability.  But unlike the passive heat exchanger in an ERV, the heat pump does use energy.  The heat pump ventilator, as a result, will cost a bit more to run than an efficient ERV but you should see lower ventilation costs than with exhaust-only or supply-only ventilation systems.

Supply-only ventilation with a ventilating dehumidifier

In hot, humid climates, your best bet is probably a dehumidifier.  When outdoor dew points are 75 to 80 degrees Fahrenheit for many months and you even use the air conditioner in January, this method will allow you to ventilate at higher rates without the humidity.  This requires a ducted dehumidifier that pulls some air from the conditioned space and some air from outdoors.  Ultra-Aire** is the brand we like.

Ventilating dehumidifier Ultra-Aire
The Ultra-Aire* ventilating dehumidifier in the Energy Vanguard office

With the proper controls, a ventilating dehumidifier can run with or without the compressor on for dehumidification.  With the compressor off, you can ventilate without dehumidifying.  You could also put an electronic damper and controls on the outdoor air duct to dehumidify without ventilating.

All three of the methods described above allow you to crank up the ventilation with impunity.  OK, maybe you don’t get complete impunity but you’ll certainly get a lot less punity.


Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.


* Disclosures:  Renewaire and Zehnder have provided one each of their ERVs to Energy Vanguard.

** This is an affiliate link.  You pay the same price you would pay normally, but Energy Vanguard makes a small commission if you buy after using the link.  Also, Therma-Stor, the manufacturer of Ultra-Aire dehumidifiers, has been a sponsor of the Energy Vanguard Blog in the past and has provided complimentary equipment to Energy Vanguard.


Related Articles

3 Ways to Get Cleaner Indoor Air With Filtration

Balanced Ventilation for a Not-So-Old House

An Update on the Residential Ventilation Debate


Photo credits: Lead photo of Zehnder HRV by Albert Rooks of Small Planet Supply, used with permission.  CERV-2 photo from Build Equinox, used with permission.  Ultra-Aire dehumidifier photo by Energy Vanguard, used with impunity.


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This Post Has 9 Comments

  1. I have learned much from you & admire & respect your expertise. I have several major comments based on my experience in the mid-Atlantic region. I’d appreciate your response and explaining where I am wrong. (1) ERVs create numerous indoor air quality problems in humid climates: (a) they remove insignificant amounts of moisture & unless their incoming air is ducted directly into a large capacity dehumidifier, ventilating during the humid months will increase indoor moisture levels far above 50%; (b) ERV paper cores will develop mold ; (c) “balancing” airflows as a practical matter is virtually impossible with modern variable speed HVAC air handlers & separate, unmatched fan speeds in the ERV & a dehumidifier. (2) Ventilation with outdoor air can help dilute indoor air CHEMICAL contaminants, but does not eliminate them, & outdoor air is often contaminated with hazardous chemicals & can make indoor air worse. MERV 13 or even higher particulate filtration on ventilation equipment does NOTHING to ELIMINATE health-hazardous CHEMICALs [VOCs & non-VOC] in indoor & outdoor air. A complex mix of toxic & health-hazardous chemicals are contained in & emitted by many building materials & furnishings & occupant activities–& in ‘tight’ buildings, those emissions get concentrated. Diluting them is certainly better than not diluting them, but that only reduces health hazards, not eliminates them. Outdoor air is increasingly contaminated with a complex mix of toxic or health hazardous chemical constituents generated locally [gas powered lawn eqpmt; pesticides; vehicle exhausts; neighbor’s emission of toxic ‘scented’ laundry products from dryer vents; etc] and regionally [rural farming chemicals; urban/suburban air pollutants; climate ‘wild’ fire smoke & toxic airborne chemicals]. Fact: climate change is bringing not only more pandemics but significant increase of air pollutants. So I see several major & serious deficiencies: (1) ‘energy efficient’ ‘tight’ buildings create MORE indoor air health risks with increasing use of building ‘envelope’ materials that raise significant health /IAQ issues; (2) FILTRATION of CHEMICALS in indoor & outdoor air is NOT receiving priority attention in development of ventilation strategies & technology; (3) virtually NO significant effort is expended to prevent TOXIC & health-hazardous materials from being incorporated into building materials & furnishings, ‘green’ or ‘recycled’ materials. Builders & consumers are usually left with only a choice between ‘more toxic’ & ‘less toxic’ materials.

  2. Terry,
    Looks like you have pretty low expectations. Some of what you say is true in the most basic applications. The Atlantic coast region benefits with a higher ethlapy recovery rate. This is so you can get the flow coming in but leave more moisture outside. You’re probably used to the units with 50% enthalpy recovery. I agree, there’s a significant amount of moisture traveling with that air. Get up to the higher numbers.

    Also, these higher performing units like you see here don’t perform well unless they are operating in the right flow range and in balance. That means not connecting to HVAC air handlers. You’re right its impossible to balance. The unit in the picture is in my home and its balanced. It self balances and I can see the flow and fan speed on my iPhone. Pretty easy. Without balanced flow you can’t get heat or enthalpy recovery. Not even the modest 50% – let alone 68% – 80%.

    And yes, do not use paper cores. Good units don’t offer paper cores because they are good units.

    Lastly, the picture in this article has an extra steel box on the supply. It has a MERV 15 filter in it. Thats in addition to the MERV 13 pre-filter that’s already in the unit. I have the supply air monitored. The current status in the home for US EPA PM2.5 = 2. Getting down to the really small stuff – its showing Particles <0.3um = 167. Those are pretty low numbers.

    So, yes you can have airflow with less moisture, you can balance or let the unit self balance, not get mold on your core, and put some pretty strong filtration in. You just can't do by using inexpensive low performance equipment connected to an HVAC system. Its that old "you get what you design for" thing.

  3. This is where I’m with Terry. I’m on my own journey with IAQ and the lack of industry knowledge is absolutely shocking. I’m 3 weeks into an install of a dedicated ventilating dehumidifier and it took me 9 months to find a contractor who understood how to do it right. There seems to be more than a bit written up on best practices, but none of that filters down to the general HVAC community.

    For me, my journey continues. My humidity is under control, but I’m genuinely puzzled why my VOCs spike on certain days (measured by a FooBot). I can’t yet find any correlation between weather/temp/humidity. And ventilation — not sure yet. My crawls space is encapsulated, but I’m sure I still have plenty of stack effect going on.

    My point in all of this is like Terry, I’d like to attack the problem once and for all, but the industry needs to get out of the mid 90s. I could trying and keep shooting in the dark to find the right expert to tell me what is going on, but it seems like the standard for the “experts” is still pretty low.

  4. Dr. Bailes,

    It’s interesting that this article was published just one day after I finally got my ventilation system installed! It’s the supply-only type. And its working great at it’s intended purpose: reducing CO2. VOCs are down too, which is nice.

    Last fall, after installing IAQ sensors (AirThings brand), I came the conclusion that my 2012 built house was too tight and spent the fall, winter and spring pondering the best solution for my situation.

    ERVs look enticing, but since I don’t have a basement, that would mean an attic installation and unfortunately the attic is mostly occupied by the resident HVAC ductzilla.

    Ventilating dehumidifiers looked good until I noticed that unit I was quoted would not meet the ASHRAE recommended ventilation rate for the square footage it would be dehumidifing. Furthermore, the unit in question was nearly the size of a Fiat and the models with enough ventilation CFMs were even bigger.

    So I opted for the simple supply only ventilation. I realize its low on the totem pole of solutions, but it was the only practical option. But regarding your (and Terry’s) concerns:

    Humidity: supposedly the fan will shut itself off if it detects too much humidity. However, should it do that, then I’ll just do what I did prior to installing it: open the windows.

    Energy: the max draw is 0.7A, not much really. And I don’t care anyways: I’ll gladly pay more money in order to keep my daughter from being subjected to >1100ppm CO2 throughout the night.

    Pollution: I had the unit installed with a wall control that will allow me to turn it off should the neighbors be grilling, letting off fireworks, or should our subdivision be attacked with chemical weapons. Otherwise, the typical pollutants in the outside air can’t be much worse than the VOCs indoors. Additionally, the fan accepts filters up to MERV 11 which I’ll installing shortly. Plus the HVAC has its own MERV 8 Media filter, so I think we’ll be ok.

  5. Matthew – Interesting. How tight is your home? You mentioned the crawl had been encapsulated, but was the envelope tested for leakage?

    1. Genry, I wish I had a before my encapsulation blower door measurement when I encapsulated 5 years ago. But as part of the sizing process for my dehu installation 2 months ago, my contractor did a blower door test. Results: Depressurize 5.43 ACH 50 Pressurize 5.66 ACH 50. My house was built in 1993, so from what my contractor told me, I’d fail current building code standards. But based on houses built during that era, I’m not in terrible shape.

  6. Don’t know what’s going on with UltraAire 70H units, but in three of them in a row, the compressor failed and they were nearly empty of refrigerant. Yet another warranty replacement unit is still sitting in its box. The currently installed unit is providing ventilation only, a standalone dehumidifier in the basement is doing its thing with no issues. Considered a Panasonic Intelli-Balance but the MERV filters are nowhere to be had. Anyone have a source?

  7. I’m in Florida and I am looking to remodel my 1980 house. I am redesigning the entire HVAC system with focus on ventilation. My plan currently is as follows.

    1) Zehnder ERV for balanced ventilation (bath exhaust, bedroom supply)
    2) Whole house dehumidifier with dedicated return and supply ducting
    3) Variable speed Trane AC

    The reason for this combination: I am looking for balanced ventilation for bathrooms exhaust to take moisture out of the house. Additionally I want to supply fresh air to the bedrooms to keep the CO2 levels and VOC low. I realize the ERV will in general add moisture to the house. Therefore I am installing a whole house dehumidifier. I looked into the CERV ventilating heat pump option but for some reason I am not sold on that particular product. So I am hoping my setup will allow me to achieve balanced ventilation and humidity control in a hot humid climate. A ventilating dehumidifier won’t allow me to be balanced so ill either end up with too much pressure (dehumidifier on) or too low pressure (bathroom fans on) in the house.

    I’d love comments or advice on this planed setup.

    Thank you

    1. @Niko, as you point out, ERV’s add moisture to the house, although not nearly as much as other ventilation methods. However, ERV’s do a poor job of removing shower moisture and should never be used for that purpose. Since steam from a hot shower has a higher dew point than outside air, the ERV will dutifully recycle a high percentage of shower moisture to the rest of the house, bedrooms in your case.

      It’s OK to exhaust the ERV from the bathrooms as long as you use a conventional bath fan while showering. If you want to recover heat from shower exhaust, you’d need an HRV, but I can’t imagine how that would ever be cost effective.

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