On Tue, 26 Mar 2013 09:10:56 -0400, Scott Dorsey wrote:

BUT, if you want to replace the device with a retrofit one, you need to
know what that mysterious input signal is.

All I know of the HVAC input signal is that it's a 2.0 to 7.8 VDC
signal from the HVAC controller, presumably to correspond to the
various levels of the fan blower motor speed.