Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site fortune.UUCP Path: utzoo!watmath!clyde!burl!we13!ihnp4!fortune!rpw3 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 UUCP: {sri-unix,amd70,hpda,harpo,ihnp4,allegra}!fortune!rpw3 DDD: (415)595-8444 USPS: Fortune Systems Corp, 101 Twin Dolphins Drive, Redwood City, CA 94065