wanttaja@ssc-vax.UUCP (Ronald J Wanttaja) (10/15/84)
I'm throwing out a question that's be bothering me for a long time: On a furnace thermostat, there is a control labeled "Anticipator". This apparently controls the cyclic rate of the furnace. Duty cycle (ratio of off time to on time) remains constant. It is much more comfortable to operate the furnace with a short cyclic rate, for instance, 5 minutes between turn-on times vs 10 minutes between turn-on times, as the short cyclic rate maintains an even temperature. My gut feeling is that the short cyclic rate is less energy efficient, however (I have a gas furnace, with electric blower). Does any one know what the relationship between cyclic rate and efficiency is? A short rate should have some gains, in that less energy is required to reheat the ducting every time the furnace turns on. Any comments/suggestions? Ron Wanttaja (ssc-vax!wanttaja)
dwl@hou4b.UUCP (10/19/84)
Maximum efficiency (minimum fuel for a given indoor average temperature) is reached with long on-time duty cycles. This is because the furnace gets the opportunity to run at maximum temperature, where maximum heat transfer occurs. It's internal limit switch should be set to keep it from getting hotter than that temperature. As the original news item pointed out, rapid cycling produces the most uniform temperature (minimum deviation from average) which is probably the most comfortable. It "correct" anticipator setting will therefore be a compromise between comfort and efficiency. In general, the thermostat manufacturer's recommended compromise setting can be obtained by setting the anticipator control to the number (in milliamps, usually) corresponding to the current rating of your gas control valve. Starting with this setting, vary it a little at a time to find comfort, if the original setting is not comfortable. The anticipator is there because your furnace keeps producing heat after the thermostat opens. The anticipator causes the thermostat to open a little before the temperature increases to the set point, in "anticipation" of the continued furnace output. If you have a very old or a very large furnace (lots of iron) you will probably need to set the anticipator to a lower number of amperes, so that it will anticipate more than average thermal inertia. -Dave Levenson AT&T-ISL Holmdel
5131eds@hound.UUCP (E.SHIPLEY) (10/19/84)
The efficiency of a furnace is not determined by heat transfer from the furnace to the house air, but by the fraction of the generated heat that goes (or doesn't go) up the chimney. To minimze the heat going up the chimney, one should keep the plenum as cool as possible, to maximize transfer to the plenum. (I hope plenum is the right term.) I believe rapid cycling would do the best job in this regard. Ed Shipley houxm!hound!5131eds
bmt@we53.UUCP ( B. M. Thomas ) (10/19/84)
Another consideration on cycle time (if you have an oil or gas furnace) is the fact that a short cycle time will cause more condensation in the heat exchanger, shortening the life of your furnace.
2141smh@aluxe.UUCP (henning) (10/21/84)
**** **** From the keys of Steve Henning, AT&T Bell Labs, Reading, PA aluxe!2141smh > The efficiency of a furnace is not determined by heat transfer > from the furnace to the house air, but by the fraction of the > generated heat that goes (or doesn't go) up the chimney. To > minimze the heat going up the chimney, one should keep the plenum > as cool as possible, to maximize transfer to the plenum. (I hope > plenum is the right term.) I believe rapid cycling would do the > best job in this regard. The same amount of heat goes up the chimney when the furnace is on, however every time it shuts off a chunk of residual heat goes up the chimney. The more times it shuts off, the more residual heat goes up the chimney. Thus the more it cycles the more you loose. This assumes the on-time is the same. This why automatic dampers are energy saving devices that you can buy. They eliminate this "chunk".