I'll offer some of my thoughts and experiences with conservation by
temperature set back during the night and during unoccupied times.
(1) The general idea follows from Newton's Law of Cooling that
thermal-energy loss is proportional to the temperature difference. That
being the case, the closer your inside temperature is to the outside
temperature, and the longer it stays that way, the more money you'll
save. This would imply you should turn the heat as low as possible when
you are not home and, as low as you can tolerate when you are asleep.
(2) In my house it does not do any good to turn the thermostat below 60F
on a typical day when the outside temperature is 25F or above (which is
most of the time during the heating season) because I have so much
thermal mass in my house that the inside temperature does not fall from
69F to 60F during the set-back period. My house is an old Victorian
brick building with walls that are three bricks thick with 1-inch air
space between the brick layers. I realize this is unusual and that many
houses will heat up and cool down faster than mine.
Anyway, I have two zones (upstairs and downstairs) and I have setback
thermostats in both places. My wife and I both teach, and are gone from
8am to 5pm. If we setback to 60F at 8am, and have it reset to 69F at
4pm, then if I go home to check it at 3pm I usually find the temperature
has only fallen to 64F or 65F when the outside temperature is hovering
near freezing.
Thus I cannot reap much benefit from setback on normal days because I
never get down to a reasonable setback temperature and Newton isn't
helping me much.
(3) If the house does get down to about 64F by 4pm when the heat comes
back on, then by 5pm when we get home, the house is only warmed up to
66F. My wife doesn't care much for that. Thus, we don't bother to run
the setback program because we just don't benefit much from it on a
normal day, and we suffer from being a bit cold unless we reset to 69F
at least 2 hours before we get home.
(4) Part of the reason we recover so slowly is because we have hot-water
heat and I don't want to run the water temperature too hot. If I set
the water temperature at 140F then the furnace is more efficient than if
I set it at 180F. It appears that setting the water at 180F and having
a setback uses more natural gas than setting the water to 140F and
keeping the house temperature steady. Of course that is one of the
things that is good about gas-fired hot-water heating systems. If you
keep the radiators warm with fairly low-temperature water, your overall
heating efficiency is high, and your temperature stays nice and even.
Trying to do a setback routine is counterproductive (if you have to
increase the water temperature) unless you can set back for a long time
(like if you're going to be gone for a week) and then take a long time
to recover with low-temperature water.
If you have a gas-fired hot-air system the equivalent of my water
temperature would be your bonnet temperature. To get faster recovery,
you might be able to increase your bonnet temperature, but doing so will
reduce your overall efficiency.
(5) I think some people in some homes can benefit from setback. The
biggest savings is if you setback permanently... that is, get used to
65F all the time instead of 70F. I could live with that, but my wife
would shoot me. I had to fight her to get it setback from 70F to 69F.
She wears sweaters and thick socks, which is okay. But she has problems
with her hands, and does not like to grade papers or type on her
computer while wearing gloves. We've done some experimenting with local
heaters, but it does not appear that any savings (which seem elusive) is
worth the inconvenience.
(6) Heat pumps have been mentioned. That really makes electrical
heating less expensive than natural gas if you are in an area where the
temperature stays at or above freezing, or if you have a geothermal heat
source feeding heat to the evaporation coil. If you have 50F outside
(or 50F geothermal) and 70F inside you could theoretically get a
coefficient of performance (COP) of about 25 (25 joules of energy
brought into the home for each one joule expended in the compressor).
Of course that COP is not realized and a COP of 7 to 10 is more
reasonable for these temperatures. But even then you are getting 7 to
10 kWhr of heat for each 1 kWhr of electricity used. Today that makes
these systems considerably less expensive than natural gas as long as
you don't get eaten alive by maintenance costs such as compressor
failure or Freon leaks.
The colder it gets, the less well the heat pump works and at some point
you need a back-up. Not too many people have heat pumps around here.
Those that do use geothermal via wells or ground-loop systems.
If you have a heat-pump system, setback is difficult to recover from
because the discharge temperature is not high, so it takes a long time
to recover to the desired non-setback temperature. This is similar to
me wanting to keep my radiator temperature low in order to maximize
furnace efficiency.
Bottom line... It's complicated.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu