Doug Miller wrote:

In article , Wayne Whitney wrote:

On 2008-01-22, Doug Miller wrote:


No. A thermostat is an on-off switch. As long as the room
temperature is lower than the thermostat's setpoint, it's on. When
the room temperature reaches the setpoint, it turns off. That's all.

The point is that different parts of the house heat up at different
rates, because they have different heat capacities. Air has a low
heat capacity, so it heats up quickest. Denser parts of the house,
e.g. drywall, concrete or plaster, heat up more slowly.

In the usual case, a furnace is just trying to maintain a set
temperature. The heat loss to the outside will be fairly constant,
the temperature differences between different parts of the house
fairly small, and the different rates (heat capacities) of the
building materials not so important.

But if the house starts off "cold", say everything at 50 degrees, and
you want to raise the temperature significantly, say to 70 degrees,
then the different heat capacities have an effect. If you set your
furnace thermostat to 70 degrees, it will shut off when the air around
the thermostat reaches 70 degrees. Other parts of the house that have
higher heat capacities will not have reached 70 degrees yet, so they
will continue to absorb heat and cool the air down.


At which time the furnace will come on again.

That is, even though the thermostat shut off, the average temperature
of all the materials in the house is not yet 70 degrees--only the air
temperature is at 70 degrees. As the cooler parts of the house
continue to absorb heat from the air, the air temperature will drop
and cause the furnace to run again to bring the air temperature up to
70 degrees. This process will repeat until all the materials in the
house reach 70 degrees.

Now if you initially set the thermostat to 80 degrees, and then reset
it to 70 degrees, you will initially overshoot your target temperature
by 10 degrees as far as air temperature. But at this point the
average temperature of the materials in the house will be closer to 70
degrees. You will reach equilibrium with everything at 70 degrees
sooner.


No, you won't. Not unless you've discovered some new laws of thermodynamics.

Which law of thermodynamics is contradicted, and in what manner?

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .