Thermostat Puzzle

Fred Nickols [951130.0550 EST]

Bill Powers recently posed a thermostat puzzle.

    [From Bill Powers (951128.0805 MST)]

    A rather neat example of spurious cause and effect showed up this
    morning. The living room was pretty chilly when we got up, so we turned
    on the solar heating register by turning up the thermostat to the
    daytime setting. This was half an hour before dawn. Then a nice clear
    day dawned, the sun came up, and Mary opened the curtains to let the
    sunshine in through windows and the patio door (all Thermopane) and warm
    up the room faster. Half an hour after that, the living room was cold
    again. So we found empirically that letting the sun shine into the room
    caused the room to get colder! Can you guess what mechanism was
    responsible for this negative effect of heat input on temperature?

My guess is that the direct rays of the sunlight
fell on the thermostat. This altered the thermostat's
perception of the room temperature, making it think
the room was warmer than it really was. Its design is
such that it does not control for room temperature but
only for temperature in its immediate vicinity. In any
case, the thermostat is responsible for maintaining the
appropriate room temperature and it didn't. For such
sloppy work the thermostat should receive a damning
performance appraisal, placed on disciplinary warning
and, if its performance does not improve, it should be
terminated. The only mitigating factor that comes to
mind is that the thermostat might be like my daughter,
really groggy in the morning hours--grumpy, too.