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re: residential heat recovery ventilator
13 jan 1996
still ruminating on the fact that air infiltration may make for half the heat
loss in a superinsulated house, reducing heat storage time in the 10,000 lbs
of hot water in a solar closet to 80 hours, if, say, the 2000 ft^2 house needs
200 btu/hr-f in phila...

how does one accurately calculate the air infiltration loss of a house in
the winter, vs just using the raw 50 pascal blower door test data?

50 pascals is 0.2" of water, corresponding to a 20 mph wind trying to blow cold
air into every side of a house at the same time, or 20 times the stack effect
pressure in a 2-story house with an indoor-outdoor temperature difference of
80 f. the nrel book says the average windspeed in concord, nh, in december is
3.0 m/s <--> 6.3 mph <--> 0.02" h20. the average nrel windspeed in phila is
4.2 m/s <--> 8.8 mph, but we have measured an average closer to 5 mph on the
flat roof of our 4-story building, in between gusts that blow down trees :-)
the very coldest times are often dead still, with no wind, at night. part of
the lesson here may be that superinsulated houses should have good trees for
windbreaks, or be underground.

i wrote of an avis modular house:

>the house has a remarkably low air infiltration rate of 0.0125 ach,
>based on a 50 pascal air infiltration rate of 0.25 ach.

i divided the 50 pascal spec by 20, following a rule of thumb in nisson and
dutt's 1985 _superinsulated home_ book, which is the only place i've ever read
about this. they were not very definite about this rule of thumb. perhaps more
is now known about this relationship? 

page 43 of the book says:

  note that the 50 pascal leakage rate is obtained according to the
  _exaggerated interior-exterior pressure difference_ created by a blower
  door. by available methods of calculation, a 50 pascal leakage rate of
  3.0 acph translates into about 0.15 acph average leakage rate under
  _natural, everyday air pressure differences_. a leakage rate of 0.15 acph
  means that air leakage is very low. a 50 pascal leakage rate of 1.0 acph
  translates into about 0.05 acph leakage rate under natural conditions. 

page 221 says:

  measuring the leakage at several pressure differences can be used to
  calculate another indicator of a house's leakage, it's equivalent
  leakage area (ela)... an accurate prediction of the natural air
  infiltration equivalent to specific blower door measurements is far
  less useful for a superinsulated house than for a conventional house,
  because natural air infiltration in the former case is very small--
  most of the air entering or leaving the envelope is accounted for
  in the controlled ventilating system. for this reason, even a crude
  formula relating pressurization leakage to natural air infiltration
  should suffice for a superinsulated house. such a crude relationship is:

    typical winter natural          average leakage at 50 pascals (acph)
    air infiltration rate (acph) =  ------------------------------------
						    20
  the natural air infiltration rate is roughly one twentieth of the
  leakage measured by the blower door at an inside-outside pressure
  difference of 50 pascals. we call this the "divide by 20" formula.

artificial ventilation seems like less of a concern, because it can come from
air-air heat exchangers or earth tubes. jonathan sawyer just posted some
measured data from his earth tube + hrv system that indicates 95% efficiency
in heating ventilation air... he used a 250' long x 1' diameter earth tube
buried 6' in the ground, under a utility trench, with a measured airflow of
175 cfm at less than 0.1" h20 pressure drop. perhaps this could have been done
with a basement floor, and a downward duct from a basement window, and an
upward duct from near the floor to the top of the house, with a fan, or two
concentric tubes, as a counterflow air to air heat exchanger, which would also
serve as ac in the summer, with airflow into the house in the outer tube only,
powered by a sunspace or solar chimney.  

nick



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