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Community and Q&A

Sizing an ERV for a larger home and optimal vs code minimum ventilation

mark_gil | Posted in Mechanicals on

Hi All!

I’m in the process of specifying a full remodel for our house in the San Francisco Bay Area (Climate Zone 3).  

As we remodel (back to the studs) the house (c. 5800 sq ft) we’re working hard to ensure that we’ll dramatically improve the air tightness of the envelope (fully adhered house wrap, attention to continuity of the water and air control layer and sealing at the foundation etc). 

We’d rather avoid lots of separate bath fan perforations in the envelope and prefer the idea of balanced valuation; and not venting all our often (expensively) cooled air to the outside; reason one for the HRV/ERV.

Secondly we rented a house in the UK which (we found out later was super well sealed and had a Zehnder ERV and) both my wife and I noticed have never had such a clear head and good nights sleep as we did staying there.

Anyhow, while ASHRAE 62.2 etc., set a minimum (e.g. 0.35 ACH which will equate to c.300 cfm), clearly this is a ‘code minimum’, and we’re trying to figure out what would be optimum (family of 5, often socially hosting others).  We’re thinking there’s a ‘standby, no one home’, a ‘normal’ and a boost (e.g entertaining, perhaps during/after the morning shower routines) level that we should target.

Does anyone have good insight/data/sources?

Also recommendations for manufacturers at this end 300cfm capacity range; also at this range would folks recommend zoning and multiple systems or one large system?

many thanks



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  1. user-2310254 | | #1


    If you like the Zehnder system (and most people do), I'd let them design and size the system you need for such a massive house.

    That said, this article might be helpful:

    1. mark_gil | | #2

      Thanks Steve,

      The ASHRAE / Joe & BSC-001 debate is super interesting, but I think comes at the ventilation problem from a balancing ‘safety’ with moisture and thermal efficiency question, rather than optimal wellness - I don’t think it is really reflecting on 1) optimal ventilation for the ‘human experience’; or 2) dealing with the fact that an ERV will help manage the energy and moisture penalty of bringing outside air in.

      Great suggestion on calling a vendor, but not clear to me that they are unbiased or science based as they make recommendations ;)

      Outdoor CO2 concentrations are typically 350-450 ppm; with indoor concentrations being really variable (up to 3000 commonly). There’s good research that cognitive function starts to be impacted in the 500-1000 ppm range, and that it becomes generally perceptible in the 1000-2000 range, with headaches and nausea potentially becoming more common above 2000 ppm.

      What has surprised me is that while there’s a lot of IAQ discussion about using Co2 as a proxy for IAQ, which I think is as much because it’s relatively easy to measure as because folks are focused on CO2 (i.e. it's used as a proxy for CO, PM2.5 etc risk); there’s little on the rates of ventilation required to manage Co2 at various levels.

      What we’re really doing with an ERV is diluting the extra CO2 (at 38,000 ppm CO2) that each person may be producing raising the internal concentration of CO2 in a building with outside air at 350-450 ppm. In a simple ‘sealed box’ I guess the math wouldn’t be impossible to do myself…

      I’m assuming the ASHAE 3,5,7 cfm/sq ft plus an allowance per person (or bedrooms +1) must be making some assumptions about the amount of CO2 its diluting and the effective concentrations it’s looking to maintain, adding a margin of safety (someone in a home gym will produce more than someone sleeping!); and then the sq/ft amount is making an assumption about mixing and diffusion to add a further margin (or similar).

      My question is really that the standard has moved around quite a lot in terms of recommendations (with debate about ‘leakage credit’, ‘mixing credit’ etc), but is necessarily recommending a ‘minimum’ for ‘acceptable’ indoor air quality. I’m trying to get underneath ‘optimal’ for ‘great’ indoor air quality - for example what would be necessary to sustain CO2 concentrations for busy family of five at <500 ppm (or perhaps more technically not more than 100ppm above external air) and how much might that have to increase if entertaining (e.g. holding a party, which will materially increase both the number of people and concentration/density of people within particular parts of the envelope !). It's fascinating that the Harvard Cog fx study showed measurable improvements in cognitive function (decision making) doubling ventilation from 20 to 40 cfm/person!

      There are a lot of variables e.g. number of people breathing in a house, level of activity, heck even presence/absence of plants and time of day/light level; and ease of internal diffusion, quality of mixing (also impacted by number of inflow registers and returns etc). But am surprised that there doesn’t seem to be a good solid model out there.

      The best introduction to the topic I’ve found so far is

      Thanks again for the response! Welcome any other folks ideas/resources!

  2. user-2310254 | | #3


    If you are comfortable with a higher energy cost, the healthy approach is to opt for a higher level of ventilation. If it were my house, I would design a balanced ventilation system with individual supplies serving the bedrooms and main living areas. I'd put returns in the bathrooms and kitchen to mitigate moisture, smells, and pollutants. I'd size the ERV to support the most likely maximum level of ventilation. (A large house might need more than one system.) Last, I'd use air quality monitors to provide metrics for informing how I operated the system.

    FWIW. I recently moved into an existing home with a leaky ventilated attic and no mechanical ventilation. So far the indoor conditions have been okay even without an ERV. Humidity in CZ3A sometimes get somewhat higher than I'd like, but the indoor CO2 levels are generally around 500 ppm. If the house was tighter, I'd probably see higher numbers.

    Just from reading GBA, people seem to get into trouble when they build tight houses with little or no mechanical ventilation. Or sometimes they have some ventilation but keep doors closed at night.

    FWIW, here is an article by Allison Bailes that might be helpful: (Energy Vanguard could help you with designing the best solution for your particular needs.)

  3. Jon_Lawrence | | #4


    Zehnder designed mine to meet the IRC minimum continuous mechanical ventilation requirements, so 25 CFM for the kitchen and 20 CFM for bathrooms, plus exhaust for the mechanical room, mud room, and basement. The total exhaust for these rooms determined the size of the unit, so in my case a Q600. Whether that will keep the CO2 concentrations below the level you desire, I can't say for sure, but we are a family of 4, large house, all electric, super-insulated, etc., and the CO2 levels in our open space kitchen/family room area stay below 500 when the unit set to medium. When we have company over, I set the unit to boost, but I have not bothered to check the C02 levels. Btw, my exhaust flow rates are 153 on low, 203 on medium and 286 on high. The supply rates are about 10 CFM higher on each setting as the guy who commissioned the system likes to slightly pressurize homes.

    One option is to oversize the unit if possible, so if your design calls for a Q450, you could instead install a Q600. There really is not much of energy penalty for that and you can run a higher CFM rate on the medium setting if you need to get your CO2 levels down.

    1. mark_gil | | #5

      Thanks Jonathan,

      I love the idea of slightly (<5 pa) pressurizing the house (it would be almost impossible with the pre-remodel envelope which was so leaky we couldn't even get to 50pa for a blower door test with the door blowing on full!), but given wildfires, smoke and managing IAQ, a slight positive pressure to 'keep the outside out' makes a ton of sense.

      Thanks! ~Mark

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