How does the hysteresis work on the Honeywell old-style bulb thermostat?
My house has two of those tan round things, with a dial that has
only two indicators.
1. The desired temperature on top, and......
2. The current temperature on bottom.
I understand that the heat overshoots on the current temperature
and that it lets the current temp go below the set temperature,
so that the gas furnaces are not constantly turning on and off
exactly at the set temperature.
That makes sense (from a wear and tear and noise standpoint).
I call that delayed on and off time the "hysteresis" (but you can
call it whatever it's really called).
Pulling off the cover, I see a mercury bulb inside, which is at the
end of a curved metal strip (bi-metallic perhaps?), which explains
on/off mechanism is from the expansion and contraction
of the coiled flat strip kicking the mercury switch on and off.
This can't be the actual on/off of the furnace, because hysteresis
decrees that the on time of the furnace itself is after the mercury
turns it on and so is the off time of the furnace being after the
mercury turns it off.
I can easily test this, simply by turning the thermostat to a
high or low temperature, where the actual on/off of the furnace
blower (and later, the heat) is something like a couple of
I get all that - but what I don't understand is *where*
hysteresis is built in? Is it in the computer? Is there a dial
that sets the temperature range of the hysteresis? Is there
How do we *change*
how much hysteresis there is?
Specifically, how do I get *more*
hysteresis in my furnace?