I wonder if the thermistor could be removed from a modern Tstat
(like the RTH221B1000)
Honeywell at Lowe's: 1-Week Basic Programmable Thermostat
And re-connected using a twisted-pair?
If you could locate the thermistor inside the warm-air collector, the Tstat
could be used in the AC mode. Coming on whenever the temperature
inside the collector got above ~75-85 deg..
I think these things have about three degrees of Hysteresis..
So the fan was moving a lot of air, it would shut down pretty quickly,
as the collector cooled off a bit.
~~
In the early evening, you could try using the heating mode. If the outdoor temp
dropped below 70, air from the house would be sucked into the collector
and cooled down a little.. (Assuming the house was still warm).