Does a HRV or ERV pay for itself in a pretty good house?
artisanfarms
| Posted in General Questions on
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.