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From: jhc@alice.UUCP (JHCondon)
Newsgroups: net.physics
Subject: Re: Thermostats
Message-ID: <2567@alice.UUCP>
Date: Fri, 3-Feb-84 01:19:05 EST
Article-I.D.: alice.2567
Posted: Fri Feb  3 01:19:05 1984
Date-Received: Wed, 8-Feb-84 02:28:56 EST
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 26

Thermostats are more complex than most people realize.
A simple bi-metalic strip sits there making and breaking, sensitive
to vibrations, creating radio and tv interference.
You avoid that by putting in some hysterisis; two forms are popular,
a little magnet on the contact so that there has to be several degrees of
heating before the contact opens again, or if it is a sealed Hg switch
you mount the switch on top of a spiral of bi-metal so that once the
switch falls to one side it wants to stay there.
BUT the hysterisis now causes the temperaure in the house to have big swings
since the furnace must overheat the house in order for the stat to turn it
off, and additionally there is still more heat in the furnace to come out
even if it's turned off.
Well, humm, what to do. AH HA the ANTICIPATOR. Believe it or not
there is a resistor under the bi-metal that is in series with the thermostat
contacts. It "knows" the furnace is on and supplies a little heat to the
bimetal in anticipation of the heat from the furnace. Well different furnace
realys draw different amounts of current, and houses are different so that
resistor is usually a variable one.
Now if the anticipator is has to high a resistance it overanticipates
and cuts the furnace off to soon, making the behaviour ihuxl!seifert describes.

OK you software types get your little screwdrivers out. What's that. What's a
screwdriver? Oh well I should know better.

The above explanationdoes not apply to houses with heating coils cast into
a slab floor. Those are bizarre.