Posted by jtown on October 02, 2001 at 06:16:10:
In Reply to: Re: Turning Back Thermostat posted by Kathy on October 01, 2001 at 16:49:35:
The bulk of heat lost from your house is mostly directly proportional to the average difference between inside and outside temp. Conductive q=UA(Ti-To) plus air infiltration q=mCp(Ti-To). Then you have chimney losses (none for electric people), radiation, and allowances for convective loss onto and off of the walls etc. Say you have average Ti=70 and To=30F and then setback Ti to 65F(setting to 60 may avg 65 dropping and rising back), you go from a difference of 40 to 35 and spend 35/40 (87.5%)on the bulk of losses. Those people at the water cooler might think that you have to reheat the boiler and walls etc. You really have to look at the control surface - like a big zip-lock bag that your house is in. You look at losses going out of the surface. Like the wc crowd thinking high cathedral ceilings are bad. You just have to ensure that the 100F up there is not leaving to be reheated. The negatives with set back are the rate at which the house cools and the heating system warms up. If your house cools slowly then you don't get down to where you save and if it does cool quickly, it is probably wasting way too much all the time. And if your heating system heats fast it might have excess loss from oversizing. You do have to replenish the heat in the walls and contents of the house. Also, if steam piping and rads get cold it will condense a lot more water until it gets hot. If your layout is marginal it could mean a lot of banging and hissing. Occupants have to weigh the comfort factor with a test run. And keep a crdigan or zipped sweatshirt handy for the warm up time.