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Set Back Thermostat To Save Energy?

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Posted by John Gregory on October 26, 2005, 12:57 pm

I've teetered on this issue for years and finally settle on leaving the
house at constant temperature to conserve heat. Now, faced with expected
gargantuan increases in the cost of natural gas, I find my self revisiting
the issue.

My four leave cap cod (1800 sq ft prox) is set at about 70 degrees. My
setback thermostat was reloaded with its battery two days ago so it would
drop the temperature 5 degrees from midnight to 6:30AM. Because it has two
sets of set back detents, it also drops the temperature from 4 PM to 5:30PM;
that was as narrow a period I could get by placing the start/stop detents
side by side.

 When the heat came on the past two day, it seemed my furnace ran
continuously from 6:30AM to about 8AM in order to regain those 5 degrees.
Now. I haven't measure this by tying on some measurement devices that would
capture the number of cycles and their duration (is there such a devise
available retail?) but I'm sensitive enough to recognize that furnace
blowing air throughout the house and it sure seems like it's on full blast
for an hour and a half. (I realize it can't be but the cycle times are
awfully short while it's trying to satisfy the 70 degree setting).

What do the mathematicians have to say about this. those with knowledge of
this heat loss/regain problem?

Posted by barry on October 26, 2005, 1:42 pm

This really has been beaten to death.  (Insert obligatory suggestion to

My Honeywell programmable t-stat of about 5 yrs back supposedly runs
adaptively.  And, it does seem to do so. Meaning, when approaching
scheduled temp rise, it starts early, cycling according to its own
scheme, to be at set temp at scheduled time.

That seems to be similar to what you're seeing.

In fact, it can be shown mathematically that the best overall
efficiency can be obtained by having the furnace either on or off for
longest blocks of time possible.  Assuming that burner firing rate is
just sufficient to make it on rise to setpoint- that's the sticky part.
Net effect can be measured most simply by reading your gas meter- note
consumption over several days with each scheme, with similar outside
temps. (When meter seems broken and/or monthly bill is tiny, you're
onto something.)

Multitude of variables make calculating all this "interesting"- firing
rate, airflow rate, fan-switch settings, presence/absence of flue
damper, etc., etc. Definitely of theoretical interest.

Bottom line: so long as you're not relying on resistive-electric
heat-strips to recover temp, you're best off with max setback possible
short of freezing pipes.  Works for me.

Low firing-rate helps.  Ditto flue-damper, clean air-filter,
throttling/closing registers where heat is needed less and it doesn't
affect t-stat.  Ditto alternate heat source, such as clean, efficient
wood, pellet, whatever stove. Ditto max insulation and sealing all
leaks possible.  Etc., etc. (Many blanks here for others to fill.)

Enjoy staying warm. May it be a rewarding challenge rather than an


Posted by nicksanspam on October 26, 2005, 3:01 pm

Good. Night setbacks save energy. So does zoning.


Posted by SQLit on October 26, 2005, 4:51 pm


Set back detents?  sounds like a pretty simple stat. Maybe not what is
needed in your situation. Which of course we can not see.   My utilities
make large bones about more than 5 degrees for more than 8 hours saves 10%.
But I am air conditioning.
An adaptive stat is a really good idea. The stat tends to learn how long it
takes to make set point.

Check your local utilities web page. Regional issues are a factor.

Posted by nospam.clare.nce on October 26, 2005, 7:26 pm

On Wed, 26 Oct 2005 12:57:08 GMT, "John Gregory"

Get a GOOD digital setback thermostat, like a Honeywell, with a
totalizer on it. It will tell you how many minutes per day the furnace
runs. Gives you today, yesterday, and total season.

Real easy to see how much difference the setback makes that way.

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