Ventilation Rate Quandary
Hope everyone had a great day!
I was recently talking to an HVAC installer about newly constructed home and its ventilation system. The mechanical ventilation is ducted through an ERV into the heating and cooling system. The ERV runs based on monitored CO2 levels. There are 3 bedrooms, and 4 occupants. According to the homeowner, the CO2 monitor almost never reaches the threshold to activate the ERV. When I was energy auditing for the Weatherization Assistance Program here in Virginia, we always ran an ASHRAE 62.2 calculation for existing dwellings, and used infiltration credits from blower door tests and local ventilation inputs from fan flow readings in kitchens and baths – the installer did some work for the program back then, and asked me what I thought about the house in question. So I had the installer send me some more information and told him I would take a look at it, as it seemed like a fun evening of entertainment. Upon closer inspection, it has turned into a bit of a head-scratcher.
The home has a volume of approximately 70,000 cubic feet within the conditioned envelope (timber frame with SIPS on a walkout basement). The heated square footage from the plans is just under 4000 square feet, but there are multiple areas with tall ceilings, great rooms, etc., and if I normalize the square footage by dividing the total cubic feet by the average ceiling height, I come up with an adjusted square footage of about 7750. Running everything through RedCalc, I get a ventilation requirement of 50 cfm if I use the 4000 sq. ft. number, 87.5 cfm if I use the adjusted square footage of 7750, AND if I refer to the 2018 Virginia Mechanical Code, which appears to require .35 ACH (table 403.3.1.1 Minimum Ventilation Rates: Private Dwellings – Living Areas), I come up with a required ventilation rate of over 400 cfm (but maybe I did my math wrong after baking in the heat all day).
I guess ultimately I have two questions:
1) Has anyone run into large volume homes (relative to heated square footage) like this one in the past, and how did you calculate the ventilation?
2) How did we get into such a mess with ventilation standards?
Thanks!
-Nat
***Update – dove into Virginia Residential Code this morning and found yet another formula – (.01 CFM per sq/ft) + (7.5 x (# of Bedrooms+1)). Thisstill leaves me wondering if there is benefit to using an adjusted square footage for homes with a large volume to living area ratio. Quick math tells me the difference with this formula is 35 CFM (70 CFM vs. 105 CFM). I think that given the presence of the ERV, it is safer to err on the side of more ventilation, rather than less, as there is less penalty for over-ventilating.
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Replies
I always just go with the code that is required by the AHJ (authority having jurisdiction). If the Virginia Residential Code is what applies to the build then I would go with that but if the IMC applies then I would use it or if ASHRAE 62 applies I would go with that. In the end I've found them all to be somewhat similar with the end result which seems to match with your results except for the 400cfm value. The code council may want to rethink using ACH to calculate ventilation rate since it can vary widely based on ceiling heigh, this case being an example. If the house had 8' ceilings throughout it would be a different story (no pun intended) (32,000 cubic ft and ~200cfm of ventilation required). I'd go with the Virginia Residential Code regarding this matter since it is likely the applicable code.
It sounds like the house has a large enough volume compared to the occupant load that the ventilation due to infiltration is high enough that CO2 concentrations never really build up. Maybe if they have a bunch of people over for a party or something they might monitor the ERV to see if it fires up. Sounds like there really is no issue here other than the anomaly with the calculation of the ventilation rate due to the high ceilings if you were to use the mechanical code which is really intended for commercial construction anyhow.