Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10 5/3/83; site heurikon.UUCP Path: utzoo!watmath!clyde!floyd!harpo!seismo!uwvax!heurikon!jeff From: jeff@heurikon.UUCP Newsgroups: net.misc,net.physics Subject: Re: Why don't thermostats work? Message-ID: <194@heurikon.UUCP> Date: Sat, 4-Feb-84 03:28:07 EST Article-I.D.: heurikon.194 Posted: Sat Feb 4 03:28:07 1984 Date-Received: Wed, 8-Feb-84 08:24:30 EST References: <877@ihuxl.UUCP> Organization: Heurikon Corp., Madison WI Lines: 55 I'm not a thermostat expert, and it was a long time ago that I took control theory, but I'll take a stab at answering your questions: 1) The bi-metal strip thermostats have something called an "anticipator" in them. It's a little heater element which runs off the 250 ma or so of current used by the furnace relay. When the thermostat is calling for heat, the heater element warms up the strip, thus anticipating the warming of the room. If this were not done, there would be wild changes in temperature - it would get much warmer in the room before the thermostat shut off. You'd be uncomfortable because of the wider temperature swings. (Look inside your thermostat and you should find a little calibration screw or lever, labeled in milliamps.) (For you Californians who - through lack of use - don't know what a thermostat is, it's that little box with levers under your doorbell. Sorry, I just *couldn't resist that.) 2) A closed loop control system (such as a room-thermostat-furnace-room) regulates the "process" by creating an error term. A thermostat is a relatively dumb control unit. There must be an error in room temperature in order for the thermostat to call for heat. The thermostat does not know how much heat is leaving the room, it only knows that there is an error in desired the temperature. The average error will be proportional to the rate of heat loss in the room. So, on colder days, the average room temperature will be lower. Look at it this way: If a (dumb) thermostat *was* able to get the room temperature *exact*, the error would be zero. If the error were zero, the thermostat would not call for heat. But if it doesn't ask for heat, the room temperature will fall. So the error *can't* be zero. For the system to reduce the error to zero, there must be additional intelligence. The system must be able to compute the heat loss. When you manually increase the setting on a cold day, you are adding that "intelligence" by adding a constant to the error term to compensate for the extra rate of heat loss. There are some uproc based thermostats on the market. I saw one (Heath?) which is smart enough to realize that the error term isn't going to zero and increase its call for heat. In control theory, the extra feedback term which is needed to completely reduce the error to zero is one which integrates the error signal over time. So, to regulate the error to zero, you need two feedback terms: one which is proportional to the error itself, and the other which is the integral of the residual error. Thermostats lack the latter. Your idea of using a simple knob instead of a thermostat would allow you to get the exact desired temperature as long as the room's heat loss rate was a constant. Simply raising the thermostat setting does the same thing and also compensates *some* for changes in the loss rate. Now, is there a control theory expert out there who can confirm any of this? -- /"""\ Jeffrey Mattox, Heurikon Corp, Madison, WI |O.O| {harpo, hao, philabs}!seismo!uwvax!heurikon!jeff (news & mail) \_=_/ ihnp4!heurikon!jeff (mail - fast)