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re: furnace humidifiers; was re: not needed--pyjamas
22 dec 2001
 wrote:

>trivia-  most houses do not expand to accommodate the larger volume of
>air.  a certain portion of the heated air exits through cracks and
>vents (bathroom vent, range top vent), leaving *gasp!* less moisture
>in the remaining interior air.

warmed indoor air contains less water per unit volume than outdoor air
before warming, but the humidity ratio (% water by weight) and vapor
pressure of water remain unchanged unless water vapor is added by some
means, eg a humidifier or evaporation from humans or plants. more airtight
houses require adding water at lower rates to raise indoor humidities. 

humans alone can humidify a sufficiently airtight house to any degree,
but an air-air heat exchanger may be needed in very cold weather, when
the maximum air-infiltration rate required to maintain humidity is less
than the amount of fresh air needed for breathing. for instance, keeping 
indoor air at 70 f and 50% rh with 0% outdoor air by means of 4 humans
who evaporate 16 pounds of water per day requires that air infiltration 
be less than 18.8 cfm, but the current ashrae standard requires 15 cfm
per occupant, ie 60 cfm altogether, so it looks like we need a latent
air-air heat exchanger. 

the driving force to evaporate water from plants or skin or noses is
the difference between the saturated vapor pressure at the warm moist
surface (which only depends on its temperature) and the vapor pressure
of the surrounding air (which only depends on its vapor density.) the
absolute amount of water contained in house air (the total number of
pounds of water vapor within the envelope) is not important for health.
the water vapor density is.

>actually, this is kind of neat and helps to explain why houses with
>relatively constant baseboard or flooring heat may feel just a tad
>more moist than houses where the temperature of the air fluctuates by
>five or more degrees each time the heating unit cycles.  during all
>parts of those cycles, the house has to breathe, exhausting hot air in
>the heat-up phase and inspiring cold air in the cool-down phase.

it seems to me that airtight houses do not have to breathe when the
temperature inside changes, and the indoor air temp needn't fluctuate
by as much as 5 f. it might be constant to 1 f over time (vs space)
with proportional vs bang-bang control of forced air or baseboard heat.
and this breathing might be a good thing for a nearly airtight house,
if it turns all the crevices in the house envelope into bidirectional
latent and sensible heat exchange surfaces, like camel's noses. 
 
>it could be interesting to find the relative importance of a short
>heating cycle in a 10 x 20 x 50 foot uninsulated box which had a
>pressure relief vent in the center of the ceiling, similar to a
>bathroom vent.

we might say it has a hole at the top, for simplicity. how long is the
cycle, and how big is the hole, and what's the outdoor rh? we might say
it's a 15 minute cycle with a big hole and 0% outdoors, for starters.
i'd say insulation does not matter.

>start with the air in the box at 72 degrees, 54% rh, and do a
>series 20 cycles, graphing the results.  i would anticipate a
>1 degree drop cycle x 20 cycles would retain far more humidity
>than a 10 degree drop cycle x 20.

sure. (so it's a 10 degree cycle now?) it would be interesting to see
how the temp-change-induced rate of dehumidification compares with the
human humidification rate or the dehumidification resulting from natural
air infiltration with constant temperatures. what's the equilibrium rh,
with 4 humans inside an airtight box with a large hole at the top and
a 15 minute 10 f cycle and 0% rh outdoors?

>the pure math figures would be sanguine, since hot air stratifies and
>the warmer air can hold more moisture.  the air exits through the vent
>would hold a disproportionate amount of the box moisture, compared to
>cooler air at floor level.

not much more. "humid air rises," but diffusion tends to overcome that
in a more powerful way than temperature stratification. i've noticed
that a single window ac can lower the rh in every room of a house even
though some rooms are almost as warm as the outdoors. and the fact that
warm air can hold more moisture does not guarantee that it will. 

>this phenomenon could also affect the overall effectiveness of setback
>thermostats.

that kind of pumping seems less important, given the long cycle time.

steve baer mentions a natural native american "blowhole" in arizona.
it's about 1 square foot, with a grate on top, and it appears to lead
to a large underground cavern. sometimes it sucks, sometimes it does
nothing, and sometimes it blows all day at a seemingly-constant rate,
depending on weather...

nick




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