PC-based Thermidistat

This may be a bit off topic for a weather forum, but I would
appreciate your comments about a project I have been pursuing in one
form or another for a while - one which includes amateur weather
station gear.

I live in Houston, which is a subtropical climate. For about 7 months
of the year we are forced to run air conditioning, since the dew point
(DPT) is above 70F with an ambient temp above 80F. In fact today, May
1, 2007, is the first day of summer for us. This will last until the
end of November.

If I use a conventional thermostat, my runtimes for the a/c are about
8 hours per day under peak load. If I use a thermidistat (either
homemade or one provided by Carrier) I can get the runtimes down to 6
hours and maintain the same comfort level. The reason is because the
Thermidistat is designed internally to control the DPT.

I have mine set such that it controls at 60F DPT. That means the
internal living-space temp gets to about 80F and the vapor-compression
(freon) a/c unit chills the air by 20F degrees at the discharge.
Because discharge air is coming off the evaporator, it is at the DPT.
Hence I control at 60F DPT.

I want to build a system that I can control with my PC. Companies that
sell amateur weather stations supply most of the equipment at an
affordabble price - a transmitter that connects with the PC, and an
indoor remote sensor that measured temp and RH. I can calculate the
DPT from those two measurements.

I need a remote control relay to switch the 24 VAC a/c relay on and
off. I do not need wireless as I can do the wiring in the attic.

I also need manufacturer-supplied application software that lets me
call APIs to get the data directly from a C language program. I will
write to do the control algorithm. I do not want to interface with a
spreadsheet or any other intermediate application.

I could buy a relay card for the PC but they tend to be expensive. I
could use the game port to create the contact closure, but I would
have to build support electronics, which I would rather not do. So I
seek a remote control relay that is supplied by the vendor of the
weather station equipment.

I wrote Lacrosse Products and Mitchell Products but neither has
responded. Perhaps some of you have either built this kind of system
or have knowledge of components.

--

"Perhaps the meek shall inherit the Earth, but they'll do it
in very small plots - about 6' by 3'."
-- Robert A. Heinlein
.

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