From: al@qiclab.scn.rain.com (Alan Peterman)
Subject: Re: need programmable thermostat
Organization: SCN Research/Qic Laboratories of Tigard, Oregon.
Distribution: usa
Date: Sun, 4 Oct 1992 19:23:30 GMT

In article <69873@hydra.gatech.EDU> gt7865a@prism.gatech.EDU (Comer,Matthew Brian) writes:
>I am looking for a digital programmable thermostat that supports a heat pump. 
>All of the models I have seen at the local home stores say "Not for use with
>a heat pump." Brands, models, and sources would be appreciated.

Lots of brands and models available - again see the Grainger catalog for the
best listing of possibilities.  Some I see at a quick glance include:
White Rodgers 1F91-71, 1F95-71, 1F92-71 and 1F94-71
Honeywell T8621 and T8611

That's for digital units..

>Also, I would very much like if someone were to explain to me the electrical/
>control system of a heat pump. I am an EE by trade so I am familiar with 
>electronics in general, but I am unsure as to what sort of signals a heat
>pump expects, and why a thermostat for one should be different in some way.

The heat pump varies by one big way - it has an extra stage of heating
(usually electric resistance heat) that kicks in when the heat pump
is unable to hold temperature by itself.  There are also some extra
indicators for the elcetric heat being on, switches for overiding the
heat pump and going to "emergency" (resistance) heat alone and so
forth.  Also some hea pumps even have multiple stages of heat augmentation
so it might kick in 20 amps of resistance heat when the temperature is
2 degrees below the set point, and another 20 amps if it falls to 4
degrees below setpoint.  That's why heat pump thermostats are often
called "multistage" thermostats.  If you look at a mechanical one,
you'll see 2 mercury switches, with a slight difference in tilt to
allow for the temperature differential (hint - adjust it for about
4 degrees so you use resistance heat less often).  In electronic
ones it requires some more electronics and since the number of heat
pumps has been relatively small the costs are high (like $300+)

I suggest finding a good one from a contractor who has removed a 
heat pump and put in more conventional heat (it happens).  I got a 
like new Honeywell T8611 for very little from a fellow who was getting
rid of his heat pump.  Now if I could only get around to installing it..

BTW - I also added an extra switch to our heat pump thermostat.  It
kicks in the resistance heat at the same time as the heat pump.  Why
would I want to do this, you may wonder.  Well I think that when the
weather is such that the heat pump evaporator is going to freeze
up (like near or below freeing and precip) I am better not letting
that happen, and would rather get the heat as quickly as possible,
even if it means running the strip heating.  It also can be used if
I want "hotter" register heat and am willing to pay for the less
efficient strip heating.  Anyone have comments on this modification?


-- 
Alan L. Peterman                                 (503)-684-1984 hm & work
                       al@qiclab.scn.rain.com
It's odd how as I get older, the days are longer, but the years are shorter!
Newsgroups: misc.consumers.house