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