Does a HRV or ERV pay for itself in a pretty good house?
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I’ve been trying to learn about HRV and ERV systems and have found some payback analyses for larger commercial systems, and am wondering if they ever pay for themselves in a single family house with ~22,000 cuft living space (including the finished portion of the basement), or if it would be cheaper to just run a small ventilation fan occasionally, or crack a window?
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Replies
Artisan,
The answer can be found in my article, "Are HRVs Cost-Effective?"
Thanks for the link.
A Pretty Good House is relatively airtight--ideally no more than 1-2 ACH50, so mechanical ventilation is required--both by most building codes and by common sense. The question is whether you need balanced ventilation, exhaust-only, or supply-only. Ten years of crowd-sourcing information and opinions for what makes a PGH says that balanced ventilation is required, whether or not it has an attractive payback period.
While exhaust-only ventilation systems can work, they also depressurize the home's interior, leading to things like increased radon infiltration, makeup air entering through random gaps and cracks rather than through a known filtration system, water infiltration and backdrafting of combustion equipment. There are work-arounds to all of those, but with what we now know about the importance of indoor air quality, and the availability of many good balanced ventilation options, if you're building a Pretty Good House, you should include an ERV or HRV, even if it's not the most cost-effective option.
I'm not trying to be difficult, but what's wrong with cracking a window and running a small exhaust fan when you're in the house vs. spending thousands for a HRV or ERV?
My opinion is maybe in zone 6 & 7 where you will see very differentials between the indoor maybe 100° at times then yes, I think a dollar return on investment is possible.
In zones 1 & 2 absolutely no way to see a dollar return on investment with a 30° differential.
Walta
The math on this is pretty easy.
For example, I'm at 7200 heating degree days. A typical house will need around 100CFM of ventilation.
Exhaust only ventilation the cost of providing this is:
7200 HDD *24 hours * 100CFM * 1.08 (btu/cfm)=186 Therms.
With an 80% efficient HRV/ERV that drops to:
7200 HDD *24 hours * 100CFM * 1.08 (btu/cfm) *(1 - 80%)=37 Therms.
So it definitely saves a fair bit of energy, even when heating with a heat pump, that adds up.
The question is does a $20k ERV setup make sense? I would have a hard time justifying that, ROI is pretty much never. There are some ancillary benefits to a fancy ERV, so for some that might be worth the cost.
Instead of a top of the line install, getting a higher efficiency ERV installed using simplified or hybrid ducting to the air handler return should be a reasonable ROI in colder climates.