Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10 5/3/83; site cubsvax.UUCP Path: utzoo!watmath!clyde!floyd!cmcl2!rocky2!cubsvax!peters From: peters@cubsvax.UUCP Newsgroups: net.misc,net.physics Subject: Re: Why don't thermostats work? Message-ID: <164@cubsvax.UUCP> Date: Mon, 6-Feb-84 22:09:18 EST Article-I.D.: cubsvax.164 Posted: Mon Feb 6 22:09:18 1984 Date-Received: Thu, 9-Feb-84 14:13:34 EST References: <877@ihuxl.UUCP>, <194@heurikon.UUCP> Organization: Columbia Univ Biology, New York City Lines: 49 heurikon!jeff's commentary on this question is excellent... I just want to amplify a bit. (By the way, I used to work for Owens/Corning Fiberglas, and though I didn't work directly in energy conservation, some of my friends were experts in "HVAC" [afficionados will know the acronym], and I picked up some things informally... I also had to use PID controllers (see below) to regulate processes.) Jeff's comment about adding an integrating function to compensate residual errors is correct. A proportional controller will in general settle down or oscillate about a temperature different from the setpoint; the integrator amounts to an automatic reset function. Even this, however, works well only in steady-state conditions; in this case, that would mean constant outdoor temperatures, essentially. By the way, such controllers are called "PI" controllers -- for Proportional Integrating. For non-steady state systems, such as when the temperature varies outdoors, or in a chemical process, a ton of cold reactant is added to the kettle halfway through the process, a third function is added, which responds to the rate of change of the error, and adds an extra boost of heat if all of a sudden, say, someone opens the door on a cold winter day. "PID" controllers, where the D stands for "Differentiating," incorporate this as well as the P & I functions, and let me tell you they are a son-of-a-you-know-what to tune to a process! Home thermostats are really only on-off sytems -- not even Proportional! -- and these tend to oscillate around the set point quite severely. What the anticipator does is to heat up the bimetallic element while the couple is calling for heat, to compensate for the time-lag involved with the room air diffusing into the thermocouple box. A guy I worked with wrote his Ph. D. thesis about modelling a home furnace/thermostat system. As earler articles have pointed out, it's *very* complicated. I believe there were ten or fifteen terms in his model. Now, a few more comments about how to make it better. Industrial heating systems work differently. In, say, an office building, in, say, the winter, the periphery of the building (that means near the windows, for all you hackers) is always being heated. The room air coming from the ceiling vents is switched between heated and chilled air to either "buck" or augment the peripheral heating system. Without that, it would always be very cold near the windows, due to radiative heating (i. e., of the cold walls by warm bodies). These systems, unfortunately, are also difficult to "tune," or "balance," and maintenance people don't usually know enough to do it. Eventually, thermostats will have a "learn" cycle, in which they record temperature changes, etc., and adjust their own parameters, perhaps on the fly. In additiion, if they have access to outside temperatures, together with the information that heat flux is proportional to (T[in] -T[out]), they should be able to do very well indeed. {philabs,cmcl2!rocky2}!cubsvax!peters Peter S. Shenkin Dept of Biol. Sci.; Columbia Univ.; New York, N. Y. 10027; 212-280-5517