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re: decreasing inside humidity on cold days with two humidistats
9 nov 1997
lopes@microtec.net (mel lopes) pondered briefly, and wrote:

>some preliminary experiments i made, have indicate that two humidistat
>connected in series may be a simple and inexpensive solution to control
>the humidity during the winter season in cold areas of the country.

paul milligan  pondered briefly, and wrote:
 
>	i think what the hell are you talking about ?

maybe mr lopes was talking about a way to control a system (hrv, vent fan, 
or humidifier?), to limit the maximum humidity inside a heated house on a
cold winter day to a value less than that which results in condensation
on the inside of the windows, along the lines of the window condensation
sensors using strips of metal that we have been discussing. 

>one of the humidistat is installed in the standard manner.  the other
>should installed on the same environment as the first one, but be
>cooled by the outside temperature.

so the uncooled one senses the relative humidity of the house air at
room temp, and the cooled one senses the rh of the house air when cooled
to the approximate outside air temp. the glass of a single pane window
would be close to the outdoor temp, if there is any significant wind... 

>i have solder  3 feet of copper tubing on the inside box of the unit
>and placed the tubing outside but the humidistat inside.

a cold conductor, exposed to outside air, with no air flowing through it?

a 0 c copper tube with a thermal conductivity of 385 w/mc that is perfectly 
insulated inside the house, which has a diameter of 1.25 cm (1/2") and a
length of 1 meter [a small house] and a wall thickness of 1.5 mm (0.060"?)
and conductance of 3.14(0.0125)(0.0015)385/1m = 0.023 w/c, connected to
an 8 cm (3") cubical humidistat box with a surface of 400 cm^2 and still
airfilm->metal conductance of 0.04x10 = 0.4 c/w in a 20 c room would only
make the box 18.9 c, using ohm's law for heatflow, if i did that right. a box
mostly covered with metric r2 foamboard (2") would be about 9.1 c, close
to a single pane window glass temp, with no wind. maybe it's better to mount
the box on the inside of the window glass with some silicone glue, since
that's the temperature we really want to sense, for condensation purposes,
and the glass makes a good conductive fin with a large contact area and
a short path length from the glass to the humidistat box. 

>as i was soldering the tube i noticed that the humidistat was
>indicating a lower percentage of humidity with the heat.

makes sense. as temperature increases, rh decreases, for the same air... 

>the nights in this part of the country  (canada) are not cold enough
>yet fora full test, but it is clear that at night the humidistat
>indicates a much higher percentage of humidity.

seems like the cooled humidistat would. i guess it "indicates,"
when the switch opens or closes as the control knob is turned
past a certain rh setpoint.

>the first unit should be set for some arbitrarily figure, like 40 %

that might be the desired maximum indoor humidity level.

>and the second for about 80 %.

that might be the rh level at which condensation begins to appear
on the inside of the windows. 

>this way the cooler the outside temperature the lower the humidity inside.

a parallel combination of humidistats "turn on the vent fan or dehumidifier
when the uncooled humidistat senses 40% rh or more, or when the cooled
humidistat senses 80% or more." a series combination might "turn on the
humidifier when the uncooled humidistat senses 40% or less, and the cooled
humidistat senses 80% or less." 

maybe h&r (800) 848-8001 still sells navy surplus hair element humidistats
in 1x2x5" solid brass boxes full of holes, with a 20-80% range and 3-6%
differential and a spdt 7.5 a 125 vac microswitch that can be wired to
open or close on rising humidity, ie their catalog number tm89hvc5203,
at $4.95 each.

nick



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