Dave M wrote:
It's the weight of the mercury that moves the bimetal coil spring such that
it takes a higher temperature to bend the bimetal coil spring so that the
mercury flows back to the other end of the bulb. When the mercury moves to
one end of the bulb, it makes the spring reposition itself so that it takes
a much higher (or lower, depending on heat or cool mode) for the spring to
return back to its original position.
Here's a link to a web page that describes the operation very well, saving
me a lot of typing.
http://inspectapedia.com/heat/Thermo...e_Response.php

Cheers,

Dave M

I think you are thinking backward. Spring controls the position of bulb
depending on temperature. Not the other way around.


Kirk Landaur wrote:
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
the *initial* 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
minutes delayed.

I get all that - but what I don't understand is *where* the
hysteresis is built in? Is it in the computer? Is there a dial
that sets the temperature range of the hysteresis? Is there
a potentiometer?

How do we *change* or *set* how much hysteresis there is?
Specifically, how do I get *more* hysteresis in my furnace?