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Community and Q&A

Natural vs. Mechanical Ventilation

Lynne_Mc | Posted in Green Building Techniques on

Hi,

I’ve been reading quite a bit regarding air tight construction and the subsequent need for mechanical ventilation.  One of the articles written many years ago “Are HRVs Cost-Effective?” surprised me, as I had always assumed the only option for an air-tight home was either HRV or ERV.  I never knew about exhaust-only and supply-only ventilations.  Also surprising was that HRV/ERVs are not always cost-effective, especially in a climate such as mine.  Given this, I am now questioning the true energy savings of air-tight construction.

Given that exhaust and supply-only ventilation systems provide no heat transfer between exhaust air and supply air, then how are air-tight homes with these systems any more energy-efficient than comparable natural ventilation (ignore for now the IAQ)?  In other words, if we built a house that was somewhat non-leaky, and just under the threshold for requiring mechanical ventilation, and this system provided naturally-ventilated fresh air at about the same rate a supply-only system would provide, then how is the supply-only system any better?

Would appreciate any insights.  Thank you,
Lynne

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Replies

  1. charlie_sullivan | | #1

    Some disadvantages of relying on leaks for fresh air include:

    1. If it's leaky enough to get enough air on mild days, you'll get way more than you want on the coldest days, when it will hurt your energy use and comfort the most.

    2. The air may come from yucky smelly places, and might bring radon along with it.

    3. The exfiltration can carry moisture into your envelope and deposit dew or frost over the winter leading to rot and mold in the spring.

    Mechanical ventilation can solve some of these problems even without heat recovery, but then there's still the energy cost and the comfort issues of supplying ventilation air at outdoor temperatures and humidities.

    For me, a key advantage of a mechanical ventilation system was the ability to put in high quality filtration. The need for that depends on your local pollution levels and your sensitivity to allergens.

    Overall, I think that once you consider the comfort issues, the cost is pretty easy to justify.

    1. Lynne_Mc | | #2

      Thanks for your response Charlie. Your first comment is an excellent rationale I didn't previously understand. Your second and third comments, however, don't appear to be mitigated by exhaust-only or supply-only systems. In those cases, air is still forced through your building envelope (through leaks, cracks and holes), right? So, I think that means the IAQ still suffers. But, maybe I'm still confused. But regardless, I was more curious about the energy efficiency aspects. To pay extra for air tightness, then pay more again for the mechanical ventilation, then have to pay again for a dedicated duct system (to get the full IAQ) and HRV/ERV. Difficult to justify the upfront costs if the overall energy savings are small. If we decide to install an exhaust or supply-only system instead, then we lose the health benefits as well. It's tough to know the best course to take. Thanks again.

      1. charlie_sullivan | | #4

        Yes, issue 2 remains for exhaust only and issue 3 for supply only. It is possible to have a balanced mechanical ventilation system, with a supply fan and and exhaust fan, so neither the entrance nor the exit is through accidental leaks, but omit the heat recovery. It's rarely done, but it does offer the cost savings of omitting the heat exchange core, and also the cost savings of not needing as much ductwork (if any), because the supply and exhaust needn't come together.

        Are you familiar with the ductless HRV/ERV units such as Lunos? They aren't cheap but they do avoid duct installation cost. You can also go with a few smaller ERV units in a big house rather than one big one.

        One other option is supply or exhuast only with an explicit passive vent for inlet or outlet, to avoid problems 2 or 3. In theory, you could even do passive vents only, and open and close them according to measured air quality and air flow through them, adjusting them automatically or manually according to the ventilation need and wind/stack effect conditions.

  2. acrobaticnurse | | #3

    This is something I've been thinking about as well, with a goal of building a "pretty good house" in zone 4a. With good outdoor air quality I wonder how much could be accomplished by a slab on grade foundation with a physical radon barrier and no gas appliances. With my relatively mild but humid climate if I use any kind of air handler I could see it being a dehumidifier that can bring in outdoor air and filter it like an ultra aire. I'm still interested in zip sheathing and taping seems but I do wonder about the sweet spot for a relatively mild climate with clean air, especially if just 1000 to 1500 square feet.

    1. Expert Member
      MALCOLM TAYLOR | | #5

      Eli,

      A lot of builders here in coastal BC are opting for a balanced ventilation system with no heat recovery. Even in this mild climate I do hear a lot of complaints about comfort, as the code requires the air-supply to be into the bedrooms.

      The big advantage of mechanical over natural ventilation supplied through leaks or open windows, is as Charlie said consistency. The amount of air moving though a house that is naturally ventilated fluctuates wildly.

      1. Expert Member
        BILL WICHERS | | #6

        I would add that not only does mechanical ventilation provide consistency, but it also allows for CONTROL. Control lets you vary the amount of ventilation with conditions, if required. There is no way to "adjust" leaks...

        Bill

  3. Lynne_Mc | | #7

    Thank you everyone for your comments, which were very helpful.

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