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re: home power question
3 oct 1999
i wrote:
>manfred humphries wrote
>> the original question was whether he should turn
>> the power off completely or leave it at a comfortable
>> temperature.
>to be more precise, the original question was:
>> > > is it cheaper to set the temperature in a room to
>> > > something comfortable and leave it there or less
>> > > expensive to turn the power off when we are out
>> > > and back on when we are using a particular area
>> > > of the house.
>the latter is cheaper, no matter how long it takes
>the room to warm up.
>> in a colder region the walls will cool and stay cold
>> without a few days of "cranked" baseboard heat.
>wrong. a room with 1/2" drywalls and 0.5 btu/f-ft^2 of thermal
>capacitance and a surface air film conductance of 1.5 btu/h-f-ft^2
>has an approximate time constant rc = 0.5 btu/f/1.5btu/h-f
>= 0.33 hours or 20 minutes. if the initial wall temperature is 50 f,
>and the thermostat's set to a reasonably comfortable 70 f air
>temperature, the walls should warm to 68 f in approximately
>-20ln(70-68)/(70-50) = 46 minutes.
we might speed this up by making the air warmer to start with...
i've read that a 1.5 f difference in wall temperature is equivalent
to a 1 f difference in air temperature, for human comfort, so we
night sense the wall temp or radiant temp and adjust the room temp
for "constant comfort" while heating the room up more quickly...
twall 68-twall 1.5(68-twall) ta ta-twall heatflow/ft^2 duration
----- -------- ------------- ---- -------- ------------- --------
50 f 18 f 27 f 95 f 45 f 67.5 btu/h 1.33 min
53 15 22.5 90.5 37.5 56.25 1.6
56 12 18 86 30 45 2
59 9 13.5 81.5 22.5 33.75 2.67
62 6 9 77 15 22.5 4
65 3 4.5 72.5 7.5 11.25 8
68 0 0 68 0 0 -
the room temp setpoint ta would be 68+1.5(68-twall). with a
1.5 btu/h-ft^2 surface air film conductance, the heatflow into
the wall would be 1.5(ta-twall) btu/h-ft^2, and the 0.5 btu/f-ft^2
wall would warm 3 f in 60/(ta-twall) minutes, reaching 68 f in a
total of about 19.6 minutes.
does anyone make "constant comfort" thermostats? :-)
these might work well in low-thermal mass rooms with
occupancy sensors like those used for lighting...
nick
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