Smarty wrote:

As to whether a Honeywell Humidicalc with no outdoor sensor sitting in a
basement cold-air return plenum can infer enough from the surrounding air
and plenum temperature / humidity to make really appropriate guesses about
what the dew point is remains to be seen in my opinion...

It's easily seen, given the frost control knob feedback, if it measures
the furnace duty cycle, ie how often the duct air is moving. A system with
an outdoor temp sensor and no user feedback might do a lot worse, with
no knowledge of window R-values.

... I am sorry for the digression which occurred in this thread earlier
regarding energy needed to evaporate water.

That's the important part, you pompous ass :-) Caulking and humidification
can both increase indoor humidity, but caulking reduces fuel consumption,
and humidification can dramatically increase it.

Nick