Subject: Humidity control rambling

From: [email-masked] (Nick Pine)
Date: 7 Oct 1995 06:20:43 -0400

It's easy to just open a window in wintertime. Or turn on an exhaust fan
with a humidistat. I use a dehumidifier with a COP of 1.6...

My house is fairly airtight, so I have to work hard to keep the humidity
down in the winter. Makes sense, no? People, showers, plants, cooking, etc.
Houses should be airtight, with air-air heat exchangers.

In summertime, perhaps we could use the sun to dry out a powdered dessicant
like the stuff sold in bags, that people put in closets to absorb water, and
then put in the oven to dry out again. Silica gel? Baking soda? How quickly
does this stuff absorb water, or dry at 130 F? I guess it works better when
it is very dry, the air is very wet, and there is a large surface exposed
to the air. I want numbers.

Could we use two alternating solar thermal panels, ie two glazed boxes,
air heaters with some powder in the bottom, one box being dried by the sun,
and the other absorbing humidity from the house air? When the absorbing box
becomes too moist, ie the exiting air becomes too humid, switch the boxes,
and start using the dry box while evaporating the water from the wet one.
Doesn't concrete also controls humidity, adding or taking water out of the
air, until the concrete becomes too damp to do that?

Perhaps this could be done with one box, a solar chimney made with a thin
layer of concrete, with an 8 hour time constant (?) so it dries out in the
sun during the day, and absorbs water from cool air that flows down the
chimney into the house at night. There must be a simpler and more continuous
process to do this. At any rate, we don't want to heat the house air in the
summer, while trying to dry it out. So the house air should somehow pass over
cool, dry dessicant. Perhaps this means that after drying out the wet box,
it needs to cool down for a while, before we try to use it to dry house air.

We could dry air in the summer by exposing it to some house surface that
is a little cooler. My auto radiator full of 55 F well water condenses
humidity out of my 80F house air in the summer. A basement floor with a
dip-tube air duct and a fan to blow air up into the house would do the same,
with a vapor barrier and a floor drain, and perhaps a dehumidifier near the
floor drain. Baruch Givoni's new book, _Passive And Low Energy Cooling of
Buildings_, addresses systems like these, without the dehumidifier. 

Herbach and Rademan (800-848-8001) sell excellent Navy surplus humidistats.
$4.95 for a really fine little machine, their part number TM89HVC5203, with
a setpoint range of 20-80% humidity and a 3-6% on/off differential. It has
a microswitch rated at 115V, 7.5 A, that opens on humidity rise, but can
be rewired to close on humidity rise. Carefully made and packed for the Navy
in sealed, canvas bags, lined with foil. I imagine the Navy must have paid
at least $300 each for these, but now they make very nice tinkerer's toys.

I installed one with a solenoid valve (H&R also sells cheap solenoid valves)
and a needle valve to help a friend raise winter humidity levels in her
New York City apartment, by venting steam into the room air from her radiator,
which sometimes frightens the cats :-) Since there was only one thermostat 
in her 25 unit apartment building, this comes off of a new 1/8" pipe in series
with a new Danfoss mechanical thermostat, that blocks the flow to the steam
trap on her one-pipe steam radiator unless the room is too cool. It's not
exactly independent control of heat and humidity, but it works. Removing the
other radiator in the apartment and insulating the three bare steam risers
that passed through the rooms finished the job. Now she can close the windows
in the winter, and even wear sweaters. What about the other 24 apartments?

Or one might use a humidistat to turn on a small water pump in a fountain,
eg the garden fountains that make a thin globe of water, like a transparent
basketball, when a room gets too dry. I've heard that fountains in shopping
malls, hotel lobbies, etc, are sometimes controlled this way. The fountain
needs a float valve and a tap water connection to keep it full. Grainger sells
cheap float valves. The fountain might go on top of a toilet tank, like the
hand washing bowls sold by Real Goods...

Nick