Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!floyd!harpo!seismo!hao!hplabs!sri-unix!Schauble@MIT-MULTICS.ARPA From: Schauble@MIT-MULTICS.ARPA Newsgroups: net.physics Subject: Re: Why don't thermostats work? Message-ID: <16509@sri-arpa.UUCP> Date: Tue, 14-Feb-84 01:18:00 EST Article-I.D.: sri-arpa.16509 Posted: Tue Feb 14 01:18:00 1984 Date-Received: Fri, 10-Feb-84 03:20:13 EST Lines: 24 I haven't ever really studied control heory, but I have a lot of practical experience with heating system. Unfortunately, the Usenet time warp has delivered this reply without delivering the original message, but I'll try answering anyway. 1) Most home thermostats of whatever type do contain an anticipator that works as jeff describes. But not for that reason. The problem is that the heating system has a considerable thermal capacity that has to be warmed before it begins to delivery heat to the room. After the furnace is shut off, this heat storage capacity causes the system to continue delivering heat. This stored heat would make the room too warm. The anticipator warms the thermostat and causes it to shut the furnace off early, thus compensating. 2) The description of the "error term" that the thermostat generates is accurate. But this effect is small, perhaps a degree for a good thermostat. Too small to be really noticed. The perceived coldness of the room is a subjective effect that does not depend on room temperature. This is because the room walls are colder, increasing body heat loss to radiation, and because the indoor air is dryer, thus increasing heat loss to skin evaporation. If you live in a cold climate, both of these are correctable, by wall hangings and by humidification. Paul