Sizing an ERV
So I think I am starting to be able to read and understand the ratings in the HVI spreadsheet.
The HVI ratings include net airflow at the max SRE
Max SRE appears to occur at lowest flows and temperatures.
It looks like SRE declines as outside temp increases – this makes sense since the delta T or temp difference is lower.
It looks like SRE declines as airflow increases – this also makes sense as the residence time in the core is reduced with higher airflows
In my area (zone 5, western MI just north of Grand Rapids), code requires airflow based on square footage and room count. After reviewing floorspace and room count with my local mechanical inspector it looks like I need to move 75 CFM continuously. I would like to be able to move more on demand to exhaust humidity from bathrooms. I plan to exhaust from three bathrooms and a powder room. Fresh air will be returned close to an existing cold air return. Two baths are in the basement.
ERV will be in the attic. Ducting is all 6 inch rigid with an 8″ oval to get from basement to attic. From the ERV to the fresh air inlet and the stale exhaust is relatively short, perhaps 15-20 feet of ducting with a couple of bends.
The stale air inlets will be on much longer runs to get to the ERV with a max effective length of 220 feet (after factoring in pipe length, bends and turns etc.).
Based on other suggestions found here, I plan to also put a stale air inlet in the attic and put vents from the living room cathedral ceiling to the attic (conditioned space, 5″-6″ closed cell foam on the roof deck). Mechanical inspector indicated both of these aspects would be acceptable.
It seems to me that I need to find a unit that moves 75 cfm at its lowest speed and accept that the only time I get the max SRE is when it is really cold out. Most of the time the achieved actual SRE (or TRE in summer) will be closer to the lower levels advertised for each unit.
When I run the boost after a shower, the SRE or TRE will decline as I am pushing more air through the core.
Am I understanding all this correctly?
Thanks for any comments.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part