Setting Back Thermostat

[sb1835]

I have a question that is related to heating.

I was having a discussion with a collegue (sp?) about the cost of oil for oil heat. Specifically, we were talking about the benefits of a programmable thermostat and setting the temperature back when the zone was not occupied. He made the statement that if you set the thermostat back more than 4 degrees it, it takes more fuel to bring the room back up to temperature than if you set it back just 4 or fewer degrees. It doesn't make any sense to me, but then again I'm not an expert on the subject. Can anyone comment. Thanks.

 

[buzzp]

I think it will depend on how long the room is unoccupied. If it cools too much, then the room will take more energy to get back to the original temp than if you were trying to sustain the original temperature. This is okay for a general guideline, I guess, but there are a lot of factors in this: efficiency of heating system, how quickly the house or room loses heat, how long the room is occupied or unoccupied, etc. I am not a HVAC person but being an outsider, this is probably not a rule of thumb that I would use too much.

 

[DDC]

sb1835
Setback is a great thing if you don't over do it. I have used 5 degrees as a rule of thumb for a maximum setback. As buzzp mentioned length of time is a factor as well as your system.
My experience is with large buildings and I have seen extreme cases where it has taken 3 days (until Wednesday) before a building actually warmed up after being turned off for the weekend. I tracked the system closely and the steam valve for the hot water converter stayed wide open for about 32 hours before it began throttling back in control.
This is very inefficient way to operate. The boiler load was very high, pumps were at full capacity, valves wide open, and customers very unhappy.
I suggest experminting with your specific system to see how it responds and recovers. Keep some simple records and compare recover time vs fuel consumption, run time etc.
Good luck

 

[Goorah]

Of course it depends on the building and contents thermal capacitance and the length of the setback period and the activities/processes once the occupied period resumes.

For an analytical approach, computer programs such as TRACE or the DOE programs can provide guidance. Given the relatively low cost of programmable Tstats, a trial-and-error method would be a cheaper way to go.... almost certainly SOME setback would be beneficial.