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On Oct 29, 8:22 pm, "Ed Huntress" wrote:

I've seen those studies for years, Don, starting with a book I read in the
'70s, titled something like _Low-cost, Energy-efficient Shelter_. What the
DOE says is widely known.

--
Ed Huntress

What the DOE says is widely known, but incorrect.

No, it's dead on, and based on a lot of actual experiments.

Don has explained
the problem correctly.

But he explained the wrong problem. He explained a theoretical
thermodynamics problem. The question is whether the savings would be worth
it if you only shut off the furnace for a relatively short time. And the
answer is, unless you live in a lightly-built and underinsulated trailer,
probably not.

Thermal mass and insulation determine the
thermal time constant, which affects how much saving results. But
has no effect on whether there is a savings.

Again, you're talking about a vacuum-bottle experiment. The question is
whether the savings are worth coming back to a 50 deg house.


As Pete C. points out there are some heating systems that change
efficiency depending on the demand. Heat pumps are one case.

Approximately 8% of homes.

Another
case is modulating furnaces. These will be less efficient at higher
loads. But the common furnace located in a non heated area, will be
somewhat more efficient as the furnace will run for a longer time
before shutting off and loosing heat to the unheated area.

All of this is very nice for armchair philosophizing, Dan, but you have to
know the actual *values* involved in the practical problem to know whether
they're significant issues, in terms of your monthly heating bill. DOE has
done the work, and I've shown what their actual experiments show. The
savings are quite small in a typical house unless you leave the furnace off,
or the temperature set low, for a long enough period for the STABILIZED
temperature to be maintained for a significant portion of the total cycle.

Thermal mass works against you, by extending the ramping-down and -up
portions of the cycle. Likewise, insulation.

--
Ed Huntress