On Saturday, December 26, 2015 at 1:20:38 PM UTC-8, Tony Hwang wrote:
Kirk Landaur wrote:
How does the hysteresis work on the Honeywell old-style bulb thermostat?

My house has two of those tan round things...
Pulling off the cover, I see a mercury bulb inside, which is at the
end of a curved metal strip (bi-metallic perhaps?)

That is just dumb thermostat. Only thing you can set is anticipator.
And remember metal fatigue, bimetal strip calibration goes off with
age. I'd just replace them with decent smart digital programmable ones.

I think this sounds like new-paint disease. No, there's no metal fatigue
going on, and 'decent smart...' devices don't have a half-century MTBF
like a mercury switch mechanism.

Because this IS a dumb thermostat, the timing adjustments that are wanted
might reside in the furnace room, not the thermostat.