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#33850 01/28/04 08:55 PM
Joined: Mar 2001
Posts: 85
I have a programmable thermostat which I have set to come on for an hour in the morning before I get up, and it comes back on about an hour before I get home to work. My furnace(gas) then runs until about the time I go to bed, and the thermostat shuts it off for the night. My question is what should the lowest temperature be that I set my thermostat to? The temperature here right now is dropping to minus 40 at night, and my house temperature is dropping 20 degrees during the day. So basically at what temperature drop difference does it not become economical enough to heat my house back up. Make sense to anyone?

#33851 01/28/04 09:36 PM
Joined: Dec 2003
Posts: 886
That's a complicated question dependent on a lot of things. Basically you are asking at what point do you use more fuel- maintaining a constant temperature or lowering the temperature and bringing it back up.

Factors include the R value of your house, the outside temperature, the inside temperature differentials and length of time, the efficiency and recovery rate of the heating system etc, etc. Also, because the outside temperature varies greatly, what might save on one day might waste fuel on another.

A heating engineer could probably give you an better idea than we could.

#33852 01/28/04 11:08 PM
Joined: May 2001
Posts: 376
The Honeywell i installed last week had default settings of 62 and 68 degrees.It also has short cycle timers to force the furnas to run a minimum of 10 minutes regardless of temp so the heatexchanger does not rust and a 5 minute lag for the A/C Leaving the temp at a constant 66 will save up to 30% in gas was the also in the labbeling on the box.Good luck with the wife and kids.

#33853 01/29/04 12:46 PM
Joined: Dec 2003
Posts: 15
Rob Offline
I have been told 2% savings for every deg.
its turned down from your normal set point.
I wouldn't lower it more than 10 deg myself.
If your furnace is sized properly it should run constantly on a "design day" for your region. Check out
Lots of info on their forum.

#33854 01/30/04 02:16 AM
Joined: Dec 2003
Posts: 886
...I have been told 2% savings for every deg. its turned down from your normal set point... if your furnace is sized properly...

Don't care what anybody says it's not that simple. My own experience is that if I set back the temperature 6-8 deg. from say 10PM to 6AM and the temperature is approaching zero outside the system will run all day from 6AM until 10PM trying to catch up. No savings there!

I set it back a few deg at night for comfort not savings.

I should add that there is a real danger of frozen pipes, both domestic water and heating (if you have hydronic heat) if you allow the temperature to decrease for a period of time. Don't forget that the temperature within walls, basements, crawl spaces, etc and even at floor level are normally lower than the temperature of the living space. I see it all the time, somebody sets the temperature back over night and the pipes and baseboard heat freeze up and cost them $$$. Now where's the savings?

[This message has been edited by hbiss (edited 01-30-2004).]

#33855 01/30/04 04:39 AM
Joined: Jul 2003
Posts: 394
The rule of thumb I have seen in several places is no lower than 62F. If you drop 10 degrees and it takes all day for you system to make the difference back, there are likely other problems like insulation or undersized unit. It is a fact of life that, if your furnace was installed by the builder, it is the dead minimum that will meet code and likely will not meet your comfort needs. A lot of frozen pipes are crappy design or build issue that are just masked over by dumping more heat at the problem.

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