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Re: Air to Air Heat Exchange Ventilator?

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Posted by nicksanspam on October 6, 2006, 10:12 am
 


An HRV will work to control Humidity, while an ERV does not, in the sense
that an HRV can reduce excess humidity in a perfectly airtight house.
 

That depends on the size of the house, no? A 2400 ft^2 1-story house might
naturally leak 4/20 = 0.2 ACH (using "the rule of 20"), 0.2x2400ft^2x8'/60
= 64 cfm on a winter day, enough for 4 full-time occupants, but on a mild
day, it may not leak any fresh air, so even air-leaky houses can benefit
from a mechanical ventilation system, eg an exhaust fan with a humidistat.
 

Not for a typical air-leaky US house. A mythical US house in Madison
Wisconsin that only leaks 2 cfm (built to the Canadian IDEAS standard,
with 2x60/2400/8 = 0.006ACH, ie 0.125 ACH50) might waste (70-16)(15-2)
= 702 Btu/h in January, with an exhaust fan to supply the rest of its
15 cfm fresh air requirement, like a 200 W bulb running 24 hours per day,
about $00 per year, with electric heat at 10 cents/kWh, or $0, with
a heat pump with a COP of 3, or $.00, with high-performance solar heat.

An HRV with 90% efficiency might reduce this to 70 Btu/h if condensation
and freezing of moisture from outgoing air were not a problem, which it
might not be if the occupants tried hard to avoid making moisture with
latex suits and ERV mufflers for breathing and covered cooking pots and
no floor mopping or plants, but if the outdoor humidity ratio wo = 0.0017
pounds of water per pound of dry air with Pa = 0.08 "Hg, warming it to
70 F would make the RH 100Pa/0.748 = 11%. Most people would find that
uncomfortable.

How would we make an ERV breathing muffler? Cloth mufflers seem to work
well when it's freezing outdoors, condensing water when we breathe out
and re-evaporating when we breathe in. Steve Baer does an interesting
experiment: breathe out into a 1/4" ID x 3' long glass tube and watch
condensation travel halfway down the tube, then breathe in and watch
it disappear in the other direction. I can't breathe out fast enough to
make more than 2' of condensation. The air that comes out of the end of
the tube might be close to 70 F and 100% RH.

With indoor air at 70 F and 50% RH and wi = 0.00778, maybe the best we
can do with an HRV or an ERV is to keep the outgoing air temp above 32 F,
which limits the sensible heat recovery to (70-32)(15-2) = 494 Btu/h.
With an HRV, we lose an additional (15-2)x60x0.075(wi-wo)1000 = 356 Btu/h
of latent heat. We might lose less with an ERV, but not much less, since
it's hard to evaporate cold water into cold air. But we seem to require
the cold air to condense the water.

For 100% energy recovery in this mythical house, maybe we need something
more like an ERV breathing muffler, with evaporation and condensation or
adsorption of water at higher temps, something like two Scandinavian
breathing walls with a periodically-reversing fan in a room partition
between them? With Typar membranes vs vapor barriers? The in-breathing
wall might be part of a solar air heater with an infinite R-value :-)
How can we avoid lots of condensation and frost in the insulation?

Nick


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