What’s the break-even for ERV?
I’m in the design stage of a new construction house and I just got the Manual J from my mechanical engineer. He has assumed an ACH50 of 5, and has calculated that infiltration will provide 62% of the necessary ventilation in winter and 36% in summer.
My thinking is to make the house tighter, which means that more of the ventilation comes from mechanical ventilation. I’m also thinking of using an ERV to capture the embodied energy in the conditioned air that exhausted. Obviously there’s going to be a point of diminishing returns, a bigger ERV costs more and requires bigger ductwork, and improving the sealing of the house becomes more expensive as the house becomes tighter.
I’m just curious if people have a ballpark idea of where the break-even point lies, where we should be shooting for.
The house is in Washington DC, climate zone 4.
Thanks.
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An ACH50 of 5 isn't even code in my area. That's biblically terrible. There should be locusts arriving shortly after that, and water turning to blood.
You should be shooting for below 2, and ideally below 1.5...
My house ended up at .6.
The ERV break even was a few years for my house compared to just an HRV.
His design is not code compliant in DC either. DC has adopted the residential portions of the 2015 IECC so 3ach50 is the maximum allowed.
Who knows if it gets tested properly though? I'm in Maryland and I asked my local single family home inspector how they verify. They just ask the builder to provide a certification. Hopefully the spec builders are taking this seriously and hiring 3rd party testing agencies.
Relying on infiltration to provide any of the ventilation is a mistake. It's dependent on environmental factors you have no control over. Build the house as tight as you can and size the ERV for the entire ventilation requirement, plus at least 50% oversizing to accommodate for parties, etc.
Thanks Trevor. That's actually what I was hoping to hear, I didn't want to lead the witnesses though.
This is somewhat of a subjective question. ERV/HRV have one job and that's to help provide comfort for the occupants of a fairly tight house. Comfort is subjective and the ERV/HRV isn't the only contributor. Window choices, roof overhangs, house orientation, air sealing methods also contribute to comfort so you really can't assign a value to the ERV/HRV only.
Now it's possible to calculate the break-even in terms of energy usage but in order to do that you'd have to determine the amount of energy lost on a house which was not as tight*. For example I live in leaky house that runs on average around $120 month to heat/cool. I think that's cheap but after 20 yrs I can say it has never really been comfortable. Mitigation would've cost about $30k or $1500 yr over 20 yr period. I doubt my energy bills would drop to zero.
*Reduced costs to build (reduced air sealing, cheaper windows, etc) but higher costs to heat/cool once completed.