[time-nuts] Temp/Humidity control systems?

Bill Hawkins bill.iaxs at pobox.com
Mon Oct 31 01:18:52 EDT 2016

Looking at it as a problem in thermodynamics, which has equations for
the flow and storage of heat, it might have a simple solution.

If you can have your equipment closet hotter than the basement will ever
be, we can use basement air for cooling the closet.
The basement air can be held to about +/- 5 degrees F with conventional
heating and cooling. You should dehumidify to 50% or less if that is a
problem. 30% is a reasonable minimum. You should have a fan or two to
stir the basement air.

Install the biggest standard air filter you can find in an inside wall
of the closet, which will filter incoming air. Install one or more
exhaust fans on the wall opposite the filter. At least one of the fans
must be variable speed. Or you could use an array of small fans with
individual switches to get controlled air flow - better yet, make two of
the fans in the array variable speed. Shouldn't be a problem if you use
common 12 VDC fans.

Now for the control system. You need sensors for closet temp and
basement temp and humidity, and also one for the power flow into the
closet. Electrical power leaving the closet on 50 ohm cables is assumed
to be negligible. You need a D/A converter to control the speed of the

You know the temperature of the closet air (maybe want an average) and
you know the temp and RH of air available to cool the closet. The RH
will affect the heat capacity of the air. Now you calculate the amount
of basement air needed to balance the heat flows at the desired
temperature, and adjust the fans to provide it.

Think of the fun you'll have determining the heat flow model constants
for the system. In particular, there's no air flow sensor because they
are expensive. You'll need to determine the relation between fan speed
and air flow.

I'd do this myself to determine the attainable precision, but I live in
an old folks apartment now. Let us know how it turns out.

Bill Hawkins

I know you wanted a COTS solution, but I think this is what you need for
1 degree control. You can't do it with on/off control.

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