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From: seifert@ihuxl.UUCP (D.A. Seifert)
Newsgroups: net.misc,net.physics
Subject: Why don't thermostats work?
Message-ID: <877@ihuxl.UUCP>
Date: Thu, 2-Feb-84 08:05:49 EST
Article-I.D.: ihuxl.877
Posted: Thu Feb  2 08:05:49 1984
Date-Received: Tue, 7-Feb-84 15:12:51 EST
Organization: AT&T Bell Labs, Naperville, IL
Lines: 67

{ This should really be in net.mechanical_engineering,
  or net.control_theory, but there isn't any, so... }

Question: why don't thermostats work?

I'm talking about the standard bi-metallic strip which opens
and closes electrical contacts to control a furnace (or ac)
and supposedly maintain a steady inside temperature dispite
a varying outside temperature. (fig 1) What usually happens
is that as the outside temp falls, the inside temp also falls,
(see fig 2) and you have to adjust the thermostat to maintain
a reasonable inside temp.  With the current weather conditions
(30F below one day, 40F above the next), this is quite a pain.

inside  |		ideal thermostat set at 70F	 /
temp	|		(furnace only, furnace runs	/
     80 |		 out of BTUs at -10F)	       /
	|					      /
     70 |	     ________________________________/
	|	    /
     60 |	   /
	|	  /
     50 |	 /
	|	/
	|      /
	|__________________________________________________
		|	|	|	|	|	|
	      -20F      0	20	40	60	80   outside temp
	
				fig 1	
	
	

inside  |		real thermostat set at 70F	 /
temp	|		(furnace only, furnace runs	/
     80 |		 out of BTUs at -10F)	       /
	|					      /
     70 |	                            _________/
	|	     		  _________/
     60 |	   	   ______/
	|	     _____/
     50 |	    /
	|	   /
	|	  /
	|__________________________________________________
		|	|	|	|	|	|
	      -20F      0	20	40	60	80   outside temp
	
	
			fig 2
			
(the graph in fig 2 should be a smooth line, not a stair step,
but until we all get graphics terminals...)

Anyone know why this happens? The thermostat doesn't know what
temperature it is outside, it should only know "it's below 70
in here => turn furnace on".  But somehow it's getting fooled
into thinking that the inside temp is 70 when it's really 60.

Sometimes it seems that a knob controlling the duty cycle
(with no feedback) would do just as well. -sigh-
-- 
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	       /_____\		from the flying doghouse of
	      /_______\			Snoopy
		|___|	
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