Sizing and Locating ERV
I am looking at putting an Energy Recovery Ventilator into my 6000 sq. ft. home (calculated by measuring the exterior on Google Earth and doubling it for the number of floors; seems high. It’s a 4-bedroom house.).
Our house has spray foam insulation and no ventilation, so if we don’t run the bathroom fans the CO2 level sits at around 2000ppm. If we run a 110 cfm upstairs bathroom fan constantly, it drops to around 450-500ppm after about 4 hours.
The home has separate HVAC units for upstairs and downstairs. The upstairs unit is a 3.5t heat pump, the downstairs is a 2.5t heat pump. The upstairs unit runs probably 2-3x as often. The upstairs is the main living area and has windows on all sides. The downstairs level is partially underground (about 3-4′) and only has windows on one side. We live in Delaware.
I’m looking for any thoughts — we can only afford to mount one ERV unit (quoted approx. $6k cost due to difficult ventilation work to the outside). It will work through the existing return ducts. Should we mount it on the downstairs unit or the upstairs unit?
I’d rather mount it downstairs if possible, because I work all day in the same room as the upstairs system, and I don’t want to add to the noise level. But I’m not sure that it will work well enough if the downstairs system doesn’t run all that often.
Also, we were quoted 3 options for ERV units, all about the same price and all from Carrier/Bryant:
– ERVCRLHB1200: Enthalpic transfer medium; 80-198 CFM; 41-60 sensible recovery efficiency (86-72 apparent); .47-.60 LMT
– ERVCRSVB1100: Enthalpic transfer medium; 35-105 CFM; 60-67 sensible recovery efficiency (75-79 apparent); .48-.61 LMT
– ERVXXNVA1090: Enthalpy paper; 52-128 CFM (vent-mounted); 55-60 sensible recovery efficiency (69-76 apparent); .48-.56 LMT
Which one makes the most sense?
I’m asking because the company we are planning to use admits that they don’t really know what they are doing with ERV units, but that’s true of others around here too apparently. I tried calling around and other shops said that they won’t even do ERV units or just never returned my call.
Any help is greatly appreciated!!
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Replies
You can't get a reasonable size estimate using google earth. However, in my limited experience, the number of people is a far bigger factor than square footage. For four bedrooms, I would suggest you want around 125cfm for medium setting, with at least two other settings including a boost of at least 200cfm. Even more headroom is better.
Those units you list are all terrible in terms of efficiency. The sensible should be at least 70 or 75%. Check out different models at the HVI.org product listing page.
Having the ERV on only one floor is really not ideal, to put it mildly. But if you are married to that idea, having one anywhere is better than none, put it on the floor with the bedrooms. Try to get some return air coming from the other floor, although I can't imagine how to do this using the existing ducts. As for the noise, it might be possible to locate the ERV downstairs and connect the ducts to the upstairs system. It all depends on where all your utilities are. If you have a separate room upstairs where the hvac is located, an ERV in there is not going to be very audible outside of that room.
Thanks! That was super helpful. That link HVI.org has nice apples-to-apples comparison data.
It looks like it's tough to find a unit that meets the CFM requirements and has good efficiency. The S&P TR20o seems to be the closest fit:
https://www.grainger.com/product/6XAU0
That one hits 78 sensible effectiveness at 180cfm, which seems like it is in the right range at least. It doesn't go over 200 cfm, but if I wanted one that did that I would have to drop down to around 70 sensible recovery.
The downside is that it is not a Carrier model like my air handler, and my air handler has weird smart thermostats (variable speed compressor) so I'm not sure the thermostat connections will be compatible. I imagine not.
Please let me know if you have any thoughts on that model!
It's got pretty high electric power consumption and it looks like it doesn't have variable speed capability, so I'm not that excited about it.