Thermostat offset

[the-commuter]

I have a question that has been bothering me for a while. Gas and Electric companies suggest temperatures to set your thermostats at in the summer and winter. My question is what offset is most efficient. My thermostat originally had a 3 degree offset meaning that, in the summer, with my thermostat set to 78 deg, it would allow my home to get to 81 deg before turning on the AC. The AC is then on for quite a while before the house gets down to 78. Is it more efficient to set the offset to 1 deg and have the AC run more frequently but for less time?

[David_Simmons]

*Originally posted by the-commuter *
**I have a question that has been bothering me for a while. Gas and Electric companies suggest temperatures to set your thermostats at in the summer and winter. My question is what offset is most efficient. My thermostat originally had a 3 degree offset meaning that, in the summer, with my thermostat set to 78 deg, it would allow my home to get to 81 deg before turning on the AC. The AC is then on for quite a while before the house gets down to 78. Is it more efficient to set the offset to 1 deg and have the AC run more frequently but for less time? **

Sort of a WAG. I don’t think a difference, if any, would be detectable. The amount of fuel used depends upon the temperature difference maintained between inside and outside and the conductivity of the roof and the walls, including windows and doors.

[Popup]

*Originally posted by the-commuter *
** My thermostat originally had a 3 degree offset meaning that, in the summer, with my thermostat set to 78 deg, it would allow my home to get to 81 deg before turning on the AC. The AC is then on for quite a while before the house gets down to 78. Is it more efficient to set the offset to 1 deg and have the AC run more frequently but for less time? **

If the hysteresis in the thermostat is 3 degres, then on average the temperature will be 1.5 degrees warmer than the desired temperature.
If the offset is 1 degree the average temperature will be 0.5 degrees warmer than the target temp.
This 1 degree temperature difference will probably make an appreciable diference in energy consumption. (At least a statistically verifiable difference.)

[sailor]

I think we may be confusing two different things here. In general what utility companies advise is that you set back (“offset”?) your thermostat at certain hours when you do not need the precise temperature like when you are out during the day or when you are in bed asleep. A setback of, say, 10ºF will make quite a difference in the bill.

I have never heard any utility company give any advice regarding the hysteresis of the system. In general terms the amount of hysteresis is not going to make much difference in the bill but, also in general terms, you do not want the system cycling too often. Some people don’t mind a given amount of hysteresis but I have known others who just cannot wait for the A/C to cycle off and they have to do it by hand. You could fill a volume with a basic study of heating, cooling spaces. It depends a lot on the inertia, lag, capacity and other factors of the system.