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re: erv selection criteria
17 may 2002
larry brasfield   wrote:

>as mr. pine noted, an erv may not make
>sense if your building leaks enough air
>that additional ventilation is not needed.

we can't count on that, on a still day with an outdoor temp near the
balance point of the house, but i suspect that condition is rare enough
that an erv won't save much energy compared to a ventilation fan, for
all but a few airtight houses like my late friend dr. bill freeborn's
15 year old underground house near phila, which just sold for a very 
good price to a bubbly young couple, with its $58 annual (not monthly)
electric resistance heating bill :-)

can we do this above ground, with less-expensive construction?

bill was a gastroenterology prof with a bridgeport milling machine in
his home shop. he designed his passive solar home with a sunspace on
the south side and 12" prefab concrete slabs under 12" styrofoam slabs,
covered all over with epdm rubber and several feet of soil. it has
a very long time constant. bill used to prepare for winter by closing
the windows around thanksgiving to raise the house temp from 70 to 72 f
over 2 weeks. 

>...for a passive solar home located where it is difficult
>to get 100% solar heating during parts of the year, (which
>covers much of planet)...

some homes are 100% solar-heated by design. the less-passive homes of
norman saunders store solar heat for several cloudy days in a row. the
sun shines about 50% of the time in phila. if a home can keep itself warm
on a day with an average amount of sun, adding 1 day of heat storage can
raise its solar heating fraction from 50% to 75%. two days (in this coin-
flipping model) can raise it to 1-1/8 = 0.875... 3 make it 0.94, 4 make
it 97%, and 5 make it 98.4%, so it only sees (1-0.984)4954 = 77 heating
degree days in phila's 4954 hdd climate. that might come from electrical
energy consumed in the house. (altho bill pulled the main breaker when
he went south one winter, and left a max/min thermometer inside, and
the house never dropped below 68 f.)

>i would endeavor to get the infiltration low enough that
>an erv would be supplying most of the ventilation.

that can be very labor-intensive and expensive.

>i forget the details of percentage loss for ventilation relative
>to other heat losses, but it is significant at code mandated air
>exchange rates...

a very airtight superinsulated house might have a conductance of 200
btu/h-f, including about 30 for 30 cfm of ventilation. on an average
jan day in phila, it needs (70-30)200 = 8k btu/h of heating, including
(70-30)30 = 1200 btu/h (15%) for ventilation. we might just include
this in the heating budget when designing a 100% solar-heated house.

>...an erv can eliminate the need for supplemental heating for
>a large fraction of the days that it would be needed absent the erv. 
 
there may not be a large fraction. then again, maybe a house needs a
"shurcliff lung," vs an erv. bill shurcliff proposed attaching a bag
to the outside of a house, in principle, with a fan that periodically
inflated and deflated the bag with air from the house, thus turning all 
the cracks and crevices in the house envelope into bidirectional heat 
exchangers, with latent heat recovery, as in a camel's nose. a simpler
variation with an infinite virtual bag would divide the house into two
partitions, with a fan between them that periodically reverses. this
could be very efficient, done slowly, with lots of heat exchange area.

how do we know when this is needed? as moisture accumulates in a mythical
airtight house? the average outdoor humidity ratio wo = 0.0025 in phila
in january. if 2 people put 8 pounds of water vapor per day into the house,
with 30 cfm of ventilation, 30x60x0.075(wi-wo) = 8/24h, so wi = wo+0.00247
= 0.00497 pounds of water per pound of dry air, vs ws = 0.0159 for 70 f air
at 100% rh, so we should ventilate when the rh rises to 100wi/ws = 31% 
(vs the usual practice of winter humidification :-)

this criterion doesn't work well in the summer, when making the indoor
humidity ratio more than the outdoor ratio would make the rh in the house
close to 100%. we might use a co2 monitor or open the windows at night
in summertime... 30 cfm for 24 hours is 43k ft^3 of air, vs 16k for a 2k
ft^2 house. would anyone suffocate if the house were completely airtight
during the day? the ashrae standard of 15 cfm/occupant began when owners
of 19th c coal mines noticed that miners became unconscious with less than
5 cfm, but they were hard workers with large cooling and oxygen needs,
and competing fumes. 

>...i will caution that it is easy to have air exchange through open
>doors exceed ventilation requirements (and make the erv sometimes
>needless) unless you have an airlock (double-doored small entry).

a non-airlock door might admit 16.6x16ft^2sqrt(4'x(70-30)) = 3400 cfm
in january, a whole day's worth of ventilation in 43k/3400 = 13 minutes. 

>on cold days, where the prospects for 100% solar heating are worst,
>it only takes a few minutes of total open door time to blow the heat
>budget and make an erv useless when there is no airlock.

it might not blow the budget, but it could make an erv useless. 

nick




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