On Tue, 22 Jan 2008 11:27:05 GMT, (Doug Miller) wrote:

In article , (Charles Bishop) wrote:

I don't think that's what she said. If you want the temp to be 70, then
set the t-stat to 80. It will cycle once or twice, then set it to 70. This
is from a "cold" start where the temp is way below the setting. It takes
time to bring the furnishings, &c up to temp, so having the t-stat set at
80 helps with this. Yes?

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.

Not quite. Technically a heat pump thermostat is a 3 state device, off,
refrigeration heat and resistance heat. Anyway,

More to the point, the thermostat responds primarily to AIR temperature and not "room
temperature". Room temperature, as felt by us humans is a mixture of air temperature
and radiant heat transfer back and forth between the masses in the room. Furniture,
walls, etc.

When the thermostat is turned up, the air is heated first. Heat transfer into the
mass becomes greater but is sill slower than air heating. The air reaches the
thermostat's setpoint and the heat turns off. Meanwhile the mass continues absorbing
heat from the air because it is still colder. And occupants feel cold because the
mass is also absorbing heat from their bodies.

The air quickly cools, the thermostat calls for more heat and the cycle continues
until the mass in the room also warms to the setpoint. Then the heat only runs to
make up for losses.

Turning a plain thermostat up initially certainly works to warm things faster, both
because it causes the air to get hotter, speeding heat absorption by the mass, and
because it minimizes the heating plant's off time. In instruments and controls, this
is known as rate or derivative action, instituted manually.

If the operator pays attention and understands thermodynamics and I&C (intuitively,
if not explicitly) then he can turn the 'stat down at the right moment to minimize
overshoot. Just like real rate action does in a process controller.

If the thermostat has an anticipator that is properly adjusted (most 'stats have 'em
but few are adjusted properly) then the anticipator will reduce the time needed to
bring the room up to normal.

If the thermostat is a smart electronic unit then it may have the capability to learn
the room's thermal dynamics and tune itself to maximize warm-up while minimizing
overshoot. Mine does. It takes about 2 cycles after I change the batteries for it
to learn my house. It also optionally inhibits resistance coil operation during
warm-up. This minimizes the loss of economy that heating with resistance heat
instead of the heat pump entails.

Really smart thermostats without outside temperature sensors combined with
multi-stage heating/cooling units, tune themselves to both inside and outside
temperature. I had that setup in my restaurant. It was remarkable how my power bill
dropped when I installed that combo in place of the two single stage heat pumps.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
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Tellico Plains, Occupied TN
Daddy, why doesn't this magnet pick up this floppy?