"Joe" wrote in message
...
On Fri, 30 Oct 2009 12:31:30 -0400, "Ed Huntress"
wrote:


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

[snip]
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.

Really?

If you missed the link I gave in an earlier message, this is where that
number comes from. There are something like 400,000 units sold per year, but
only around half of those are residential. So the percentage now could be
between 9% and 10% of homes:

http://www.eia.doe.gov/emeu/recs/rec.../alltables.pdf


I'm not going to contradict you, since I've never seen any data, and
heat pumps are probably not very effective in areas where winters stay
pretty cold, but here in the sort-of-south (SC Appalachians), heat
pumps are installed in nearly every house that is worth more than
about 80 grand. My old 10 SEER unit works fine down to about 38F. If
8% is correct, then I'd say that a lot of people in the more temperate
regions are missing out on a fair amount of savings, especially now
that NG is about the same cost as electricity per BTU, oil is higher
and LPG is outrageous.

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.

I don't know what intervals the OP was thinking of (he didn't say),
but I drop my setpoint for the 10 hours I'm gone to work, and for the
8 hours I'm asleep (well, not as much sleep nowadays, but, you know).
I've done no studies to measure my savings, but even 10% would be
great. If a large proportion of the population believes that they can
save half of their energy bill by occasionally dropping their
setpoint, well, that says a lot about our citizenry.

Joe