it is called heat transfer. more heat in air at 80 will put more heat into
the walls, getting them up to 70 faster. You won't save energy, you will
save time. But, the heat loss out the windows will be greater with a temp
up to 80.
Umm, my understanding is that for convection and conduction, heat
transfer rate is proportional to the temperature difference. So there
is a big change in heat transfer rate for 70F versus 80F air.
[E.g. for 60F building materials, the temperature difference is double
with 80F versus 70F air.]
Look at the original question this way--you want to get the average
temperature of a building from 70F to 50F. Based on all the different
materials and their heat capacities, this will require some number of
BTUs; based on the furnace output rate the furnace will have to run
some number of hours to put out the required heat. Say 2 hours.
Air temperature will be a leading indicator of average building
temperature. So if you set your thermostat to 70, the furnace will
run less than two hours, then it will cycle on and off for a while as
the building catches up to the air temperature, until the total
running time reaches two hours. Or you could set the thermostat
artificially high for two hours and then reset it to 70 degrees. The
two hours of furnace time required occurs all at once.
Clearly the latter strategy causes the building to reach equilibrium
sooner. That's all I'm claiming, not that it is a good idea, will be
more comfortable for the occupants, or that it is more efficient.
Even if so (and I think it would take a pretty exceptional house design
for it to make any discernible difference in any practical sense), it
will certainly be more expensive and the time to reach the initial
setpoint is still the same so at best it's a period after that initial
warmup at most that can be affected at all.
I don't see why, unless you've got a heat pump with resistive backup.
First order, you need to put the same BTUs in either way to start the
same mass at the same temp and end it at the same temp (although I will
admit to the fact that the path can have a small effect due to the
variation in delta-T's to the outside which affects losses along the
way, but work with me here and assume that the heat needed to go from
cold to warm is most of the energy used and that losses during the
short time involved are second-order).
and the time to reach the initial
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But, it's as real as the heat going into the material is and at higher
dT the loss proportion goes up at the same rate as the gain portion so
you can't win -- you might come close to breaking even, but you can't
win. Well, actually I guess you could possibly break even in one
specific instance but it would take really detailed measurements or
calculations to come to that point--if you were to be able to find the
time at which the exterior wall temperatures would first reach their
steady-state temperature and cut the extra input at the time when the
heat input on the inside surface would then be transferred to reach that
exterior temperature, then it would be the break-even point. Once the
interior temperature is higher than that, then the exterior temperature
also would rise above its steady-state value and then the previous
conclusion would also hold.
The point in my view is that the two paths are identical owing to the
fixed input until the lower setpoint is reached so there's absolutely no
advantage there. The only question is whether then raising the
temperature above the end setpoint perhaps aids a little _from that
point_ in "creature comfort" -- my opinion is that unless the house is
one that is actually designed as a thermal mass rather than conventional
likely to be essentially unchanged although it just might aid a little
bit in "taking the chill" off in comfort level. But it can't help but
be more energy-costly and can't help the initial recovery.
Nit-pick: four state: off, refrigeration/heat, defrost (self
nitpick: via controller) and supplementary heat.
The supplementary heat doesn't _have_ to be resistive. Ours (in
a previous house) wasn't. If we go HP again, it won't be.
Ours (with gas backup) had two sensors. I don't think it did a lot of
smarts with them, but it was remarkable how low the heating/gas bill
was, even in the great white north.
Age and Treachery will Triumph over Youth and Skill
Not true for at least 1 version of mechanical and 1 (probably many)
versions of electronic thermostats, which have means of compensating for
overshoot, and therefore switch at differing points under differing
conditions. Said adjustments are often out of whack if ignorant persons
mess with a thermostat they don't understand the subtleties of, or
install a new one without reading the directions. Thus the automatic
adjustment that someone else mentioned for a newer electronic 'stat.
Actually, the classic heating thermostat anticipator is a tiny heater that
warms the sensing element a degree or two above the room's air temperature.
The purpose is to cause the burner of the furnace to shut off just before
the room reaches the desired temperature. This works because even after the
burner shuts off, the heat-exchanger in the furnace and the blower continue
to supply heat to the room for close to a minute longer. When properly
adjusted, the burner will shut off just before the room air reaches the
setpoint and the stored heat in the hot heat exchanger will continue and the
room temperature will 'coast' up to the setpoint just as the blower shuts
This feature avoids an overshoot of the room temperature, but doesn't do
anything to '...reduce the time needed to bring the room up to normal.'
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