In sci.electronics.repair Tony Hwang wrote:
Jerry Peters wrote:
In sci.electronics.repair Tony Hwang wrote:
Ralph Mowery wrote:
"Kirk Landaur" wrote in message
...
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).
and
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?


Part of it is in the house its self. It takes a while for the air and walls
in the house to heat up. Then the thermostat cuts off, but the air handler
will blow for a while to cool off the frunace heat chamber. The furnace
should have a control for this near the heat chamber. Say the house over
shoots 2 degrees during all of this. Then it cools down and the furnace
starts back up.


That is why blower does not come on with flame on, does not go off with
flame off(this going off delay is usually adjustable at the control board)

That depends on the furnace. MY blower comes on as soon as the burner
ignites, it's a high efficiency furnace, so the blower must start to
cool the secondary heat exchanger.

How high efficiency? Ours are 96% 2 stage one. There is built-in delay
for blower to come one. Off delay is adjustable by dip switches. Inducer
blower purges vent already before ingnition comes on.

93%. It's 30 years old, no digital controls. There's a standard
fan/limit thermal switch, but it only turns the blower off.