re: air/air heat exchanger
28 feb 2001
abby normal wrote:
>i am now missing about half of the posts in this thread.
too bad. i wonder why...
>a while back i seem to remember you saying 60 cfm of infiltration is
>adequate ventilation. where did you get this from?
i've read that the ashrae residential standard is 15 cfm per occupant, eg
60 for 4 people. i haven't actually read the standard (62-1989, ventilation
for acceptable indoor air quality?), but kreider and rabl's book says
there are two ways to comply: provide 15 cfm of outdoor air (which may have
to be treated to improve quality), or monitor and control several indoor
air contaminants. the limits for particles, so2, co, and o3 (1h), and no2
are the epa limits for outdoor air, and the limits for "contaminants of
indoor origin," co2, chlordane, o3 (continuous) and radon are generally
higher, eg 3x the outdoor concentration for co2.
"dilution is the solution," mixing with outdoor air: ci = co+vdp/vdo,
where ci is the indoor pollutant concentration, co is its outdoor conc,
vdp is the volume rate of indoor pollution, and vdo is the volume rate
of outdoor ventilation.
kreider and rabl say "the minimum outdoor air rates in table 11.1 have
been set to avoid stuffiness from body odors, rather than being determined
by oxygen demand. in fact, at these outdoor airflow rates, the oxygen
content changes only a fraction of a percent as a result of metabolism."
so this standard is meant to control something that isn't even mentioned
in the standard, like a lighting standard based on ambience that only
talks about safety. (it appears the amount of light we need is also
subject to inflation :-)
the 15 cfm standard replaced a 5 cfm standard based on productivity. it
was experimentally determined in the 19th century that 5 cfm was the min
airflow rate needed to prevent workers from passing out in coal mines...
("passed out--nonproductive." :-) the current standard requires 20 cfm
per occupant in dining rooms and offices, 30 in hotel rooms, and 60 in
public smoking areas.
>i am used to residential mechanical ventilation codes where on average
>perhaps 120 cfm is required for a house. this 120 cfm is 'above and beyond'
>what naturally infiltrates.
sounds expensive, especially the part about "above and beyond."
>an r2000 home, old technology in canada would have an air change rate of
>perhaps 0.07. so for a 2000 square foot home with say a 9 foot ceiling
>that's 21 cfm.
on a day with some wind or an indoor-outdoor temperature difference...
we can't count on those in a short-term microclimate.
>these homes need heat/energy recovery ventilation in order to
>keep humidity in check and to maintain iaq...
i wonder about the humidity. an andersen window brochure on condensation
says the activities of a family of 4 evaporate 2 gallons per day of water,
ie 0.0111 pounds per minute... 70 f air with 50% humidity has about 0.008
pounds of water vapor per pound of dry air and weighs about 0.075 lb/ft^3,
so we could maintain 50% rh on a cold day with 0.0111/0.075/0.008 = 18.5
cfm of outdoor airflow.
>a home built to current ontario building code has sealed vapour barrier
>however they do not have to pass blower door tests for their air tightness
>and an infiltration rate of 0.37 could be expected...
it's surprising that the ontario building code doesn't require blower door
tests, given canadian coolth and building energy forethought and frugality.
phila is a lot warmer, but new public housing gets blower door tested.
>with the same size home approximately 111 cfm is infiltrating in. if
>there was no mechanical ventilation, these homes would still have
>a serious humidity problem.
on a very cold day, 50% rh may cause condensation on windows...
>exhaust fans, fresh air intakes into the return air and heat\energy recovery
>ventilation will all provide adequate ventilation to keep humidity in check.
humidity doesn't accumulate instantly. we might run fans less on colder
days to save energy (and replace all the indoor green plants with cacti--
then again, green plants eat co2...)
>with hrv's there are usually 2 apparent efficiency ratings. one rating at
>say 0 degrees c and a second rating at -25c. to calculate the efficiency of
>these units you have to allow for defrost.
turning off an inflow fan to let outgoing warm air melt frost can be more
efficient than turning on electric heaters...
>120 cfm of ventialtion air with a minus 30f ambient would add about 3.8 kw
>to the heating load. in an area similar to "international falls", that 3.8
>kw could add about 6200 kwh to the annual energy consumption. i believe heat
>recovery ventilation has its merits in a cold climate.