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Tracking HRV energy

user-1135248 | Posted in Mechanicals on

So, I’m wondering how many folks who are keeping energy stats
remember to track the energy picture around the ventilation
system. This is twofold: the electrical energy used *by* the
HRV/ERV, and the losses inherent in exchanging conditioned
interior air with whatever’s outdoors.

The former should be fairly miniscule, as ventilators don’t
draw huge amounts of power … but my Fantech does pull 0.6
amps on low fan so that invariably sends a few watts in a
couple of directions. Assuming that most of the load is fan
watts and thus dissipated right into the air streams they’re
handling, would it be safe to assume that about half of the
overall power is being sent outdoors through the exhaust duct,
and the other half is adding to heat content indoors? Control
power is probably in the noise. Even on the 15 minutes-per-hour
and lowest speed settings giving about 0.1 ACH average, I feel
like I have to account for some reasonable portion of the half
kilowatt-hour per day that this unit consumes.

The latter can be more significant, depending on the unit’s
efficiency. Not everyone has a Zehnder, so we’re looking at
maybe 70% recovery but that still means that some heat is being
exhausted outside and cool air brought inside [assuming winter
season throughout this, as that produces the greatest deltas].
Knowing the airflow, typical run-time proportion, and the unit’s
rated recovery efficiency should allow a sensible-heat calculation
of BTU per hour per degree F that can plow into an additional
heat-loss factor over recorded heating degree-days. For example,
I calculated mine to be losing 8.1 BTU/hr/degF out the exhaust
which doesn’t seem like a lot but when multiplied by (24 * HDD)
starts to rack up in daily total BTUs that should be accounted for.
Especially in efficient buildings with low heating loads already.
That loss bypasses the envelope and is unavoidable unless you want
to shut down your ventilation system, so anyone trying to calculate
overall enclosure loss can likely see a *better* btu/hr/F figure
for it by subtracting out the HRV factor. Oh, and I’m ignoring
the fact that there are two cold [but insulated/tight] ducts
running some short distance inside the at least semi-conditioned
space of the basement.

Am I the only one nuts enough to care about this?


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Am I the only one nuts enough to care about this?"

    A. No, of course not. Energy experts have been computing HRV energy use for over 30 years. For an in-depth discussion of these issues, see Are HRVs Cost-Effective?

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