Mark Lloyd wrote:
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.
I've found that it does help, even if it shouldn't. At least I've seen
this with a particular gas furnace. When expected to raise the
temperature 20 degrees of so, the furnace would cut off well before
reaching the set temperature.
Again, unless there are different heating rates available dependent on
the difference between measured and setpoint, it's thermodynamically
impossible. If it turned off, it simply either reached the demand point
_at the location of the thermostat_ or there was/is another interlock on
the system coming into play.
The thermostat turns the unit on; it runs until demand is satisfied.
Heat input is fixed while the unit is on; off when it isn't. Loss to
fixtures, etc., is dependent on the surface temperature differential and
the local heat transfer characteristics and has nothing whatsoever to do
w/ the setpoint.
It is true that if one overshoots an end temperature significantly (say
80F instead of 70F) and lets that reach an equilibrium there will be
more retained heat in fixtures, etc., than there is at 70F which, some
of which will gradually be re-radiated and convected back into the room,
but the overall heat input required will be greater than if simply set
to the desired end setpoint.
The only way the setpoint matters on rate is if there is a variable
input source that is dependent on the temperature differential between
setpoint and actual.
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