I wondered if anyone knew if there were, aside from price, bells 'n whistles, major differences between thermostats used for home furnace/air-conditioning systems.
I have a White-Rodgers thermostat and have noticed that if it's 72 degrees inside, and I set it go go on at 70 (that is, in the on position, but nothing actually running), the display will, in short order--say, 20-30 minutes--drop a few degrees and the furnace goes on.
If, on the other hand, I leave it in the off position, the temperature may remain the same for hours, or even rise a degree.
It's happened without fail, over and over, for the two and a half years I've been here. Even now that I've just replaced the furnace, the phenomenon persists.
Pure speculation here (this IS usenet, after all...) but perhaps with the switch in the off position the device remains in a power save mode longer or is in a different power save mode, as it is not responsible for any machinery control, and so it doesn't update the temperature display as often. Have you tried hitting it with a warm air stream for a few minutes when it's on and off (after reaching equilibrium in each state) to see if there's a difference in the response with a (sort of) known stimulus?
Ok so far except that it sounds like your house isn't very well insulated. eg When you turn off/down the thermostat setting the house cools far too rapidly.
So with the thermostat turned down you still see a temperature rise. That may also be possible.
Possible causes...
a) There is a fault somewhere and the heating is still coming on...
Does the furnace also heat or supply hot water? In the UK boilers (=furnace) also heat the hot water. They use a valve that diverts the hot water from the boiler either through the heating system or through the hot water storage tank. If the valve fails then some water can be diverted to the heating loop when it's meant to be heating the hot water storage tank loop.
b) There is another source of heat.
For example the fridges, washing machines or simply the sun warming the house.
c) Thermostat hysteresis (Temperature or Time) exists.
This is what I think you are seeing...
Temperature hysteresis : Old style bimetallic strip thermostats never switch on or off at exactly the same temperature. The actual switching point depends on the direction in which they are going. For example if you set it at 70 it will switch ON at 68 and OFF at 72. This prevents the thermostat oscillating and the contacts arcing when the air temperature is exactly the same as the set temperature. It also stops the boiler constantly switching on and off which is bad for efficiency and its life. It does however allow some under/overshoot. eg the air temperature can rise to 72 before the boiler switches off or fall to 68 before it switches on.
Time: Some modern electronic thermostats have a smaller or no thermal hysteresis . Instead to prevent oscillation they have electronics that limit the number of boiler cycles per hour. This can be as low as 6 or 10 cycles per hour. eg When the air temperature is the same as the set temperature it switches the boiler ON and OFF about every 6 mins (=10 cycles per hour). This normally results in much less over/undershoot than with the old bimetallic strip type thermostats. With some types of thermostat you can adjust the maximium number of cycles per hour but this is very rarely needed. Sometimes only the installer can change this. Some models are badly designed and only sample the air temperature or the setting every 6-10 mins. This means they don't appear to respond when you adjust the set temperature! The solution is to adjust the setting then wait at least 20 mins and see what the boiler/furnace does. Making small frequent adjustments to the set temperature can produce very confusing results with the boiler appearing to have a life of it's own.
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