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From: rpw3@fortune.UUCP
Newsgroups: net.physics
Subject: Re: Re: Thermostats - (nf)
Message-ID: <2438@fortune.UUCP>
Date: Sun, 5-Feb-84 07:04:57 EST
Article-I.D.: fortune.2438
Posted: Sun Feb  5 07:04:57 1984
Date-Received: Wed, 8-Feb-84 08:14:11 EST
Sender: notes@fortune.UUCP
Organization: Fortune Systems, Redwood City, CA
Lines: 38

#R:alice:-256700:fortune:8600011:000:1901
fortune!rpw3    Feb  5 02:20:00 1984

The use of "anticipator" resistors also has the effect of transforming
a simple first order feedback loop into a "proportional controller"
(assuming the anticipator is set right), also known as a pulse width
modulator (often seen these days in power supplies). The net result
(which also very much applies to floor mounted glass panels, b t w)
is a MUCH more closely controlled temperature.

The thermostat (and therefore the heating system) just sits there cycling
on and off at a period (hopefully) shorter than the time constant of the
room or house. What you feel is then the long-term average of a bunch
of "heat pulses" rather than the surge of the heat pulses themselves.

Oviously, this has got to be adjusted correctly to work right, for it
has to interact (in forced-air furnaces) with the fan turn-on/turn-off
threshold temperatures (which delay the fan starting until the air in the
duct is above a certain temparature, etc.). This puts yet another completely
independent term inside the feedback loop, so adjusting it is tricky for
the average handyperson. A rule of thumb I have used is: On a "normally"
cold day (one that doesn't threaten to overload the heating system), the
fan should come on about half way (or later!) through the gas "burn".
That way, the big pulses the gas makes get smoothed out a little by the
fan/duct system before hitting the house. Another way of saying it is
that the demand for heat (thermostat) should be about "90 degrees out
of phase" (imprecise term) with the response (hot air from the duct)
when the system is delivering about half it's maximum output.

As I noted before, not something the average bloke gets right without
calling the repair crew (who may not know how either!).

Rob Warnock

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