This may be old hat to some readers, but I'll put it out there anyway. If you have had a similar experience, please feel free to share it.

Bought a four level split recently, located in S.W. Ontario where we get a wide variation in outdoor temperature over the heating season. Original furnace had been replaced 5-6 years ago with "mid-efficiency" unit equipped with a "once-through" humidifier controlled by a return air duct-mounted humidistat. Replaced duct-mounted humidistat with a programmable humidistat, which I mounted on the wall of the L.R. Programmable humidistat is the same brand as humidifier and it has an outside thermometer/thermocouple that senses the outdoor temperature and adjusts the indoor humidity by controlling the ON/OFF operation of the furnace mounted humidifier, according to the selected program (based upon the vintage of the house as a function of air infiltration).

It maintains the optimal indoor humidity for comfort and mold prevention and eliminates the formation of condensation on windows. Very convenient as it precludes the need to monitor outdoor temperature and manually adjust the old return duct mounted humidistat frequently to increase/decrease indoor humidity.