Share 'T-stats without fan-only control'

When the air handler is only used for heating, they often don't have a fan-only control.  That's a problem for setting up the house in worst case conditions for combustion safety because the leaks in the ducting, etc. can really impact the house pressures.  Some furnaces have a control on the high limit switch that can be used to turn the blower on.  But if that's not available my strategy has be…