Hysteresis on the Honeywell old-style bulb thermostat

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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?
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On 12/26/2015 12:37 PM, Kirk Landaur wrote:

The hysteresis is set on the thermostat. I used to have one of those old Honeywell units and as I recall it had a calibrated sub-dial that set the hysteresis temperature range. Something like 0.5 degrees to around 3 degrees (F). You simply moved this sub-dial to the over/undershoot range you desired.
Of course this info may well be searchable, rather than depending on memory...
John :-#)#
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On 12/26/2015 3:55 PM, John Robertson wrote:

That is the "anticipator", not hysteresis setting. It is actually a heater that warms the thermostat in anticipation of the room heating up and minimizing overshoot.
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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/Thermostat_Temperature_Response.php
Cheers, Dave M
Kirk Landaur wrote:

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Dave M wrote:

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

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Tony Hwang wrote:

They are interactive.
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Tony Hwang wrote:

Yep, I understand that the "Spring controls the position of bulb depending on temperature". That was, I thought, my intention to describe. When the spring bends far enough to tip the bulb, the weight of the mercury in the bulb swings the bulb a bit farther , requiring the temperature to cause a greater swing in the other direction to make it switch back. That's where the hysteresis comes from. That was the question from the OP, which is what I was trying to answer.
cheers, Dave M
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On 12/26/2015 3:58 PM, Dave M wrote:

Yes!
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On Saturday, December 26, 2015 at 3:58:56 PM UTC-5, Dave M wrote:

That would be how I would think it would work too. Once the mercury rolls in one direction, it's going to take X amount of temp change to make it shift enough to roll back the other way. That's the hysteresis. As othere have said, the anticipator just heats the thermostat so that it opens a bit early, to avoid overshooting. The anticipator can be set. I haven't seen an old mercury type thermostat where you could set the hysteresis.
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They are related. The weight of the mercury provides an Amount of hysteresis that is usually excessive. The anticipator heater resistor allows you the reduce the hysteresis to the desired value. The anticipator provides negative hysteresis if you want to think of it that way. To the OP. Simply take a note or photo of where the setting is now? then move it and see what happens. If it does the opposite of what you want move it the other way. M
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On Sat, 26 Dec 2015 12:49:29 -0800, Uncle Monster wrote:

Is this the anticipator?
http://i63.tinypic.com/280nvqs.jpg
Mine is currently set at 0.55 I think (if I'm reading the right thing).
Notice the distances get spread out non-linearly to the one end: (1.2, 1.0, .9, .8, .7, .6, .5, .4, .3, .25, .2, .15, .12, .10)
It has a confusingly labeled slider that has "LONGER" and an arrow indented on it.
The LONGER indent and the arrow fight each other.
On the one hand, the LONGER on one end *implies* that's the end where the slider makes the hysteresis longer; but, on the other hand, the arrow points in the opposite direction, which implies the hysteresis is longer in the other direction?
Do you have experience with this confusing setup?
Mine is currently set to one side, but I want the hysteresis to be greater.
To increase hysteresis, should I slide the slider toward LONGER? Or should I slide the slider more towards the direction of the arrow?
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Kirk Landaur wrote:

General rule of thumb on anticipator is matching the number on dial same as current draw on your gas valve. You can play with setting it little higher or lower. I used to set it slightly higher than gas valve rating considering the length of wiring from furnace to thermostat. Again go digital.
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On 12/26/2015 4:14 PM, Kirk Landaur wrote:

Yes

The anticipator has nothing to do with hysteresis. It's adjustable to account for different currents that are drawn by different "heaters". Once adjusted to the current that your heater draws, it will put heat into the thermostat to anticipate the room heating up.
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On Sat, 26 Dec 2015 21:14:41 +0000, Kirk Landaur wrote:

This seems to be the correct URL to the picture.
http://oi63.tinypic.com/280nvqs.jpg
Notice the "LONGER" and "<------" fight each other.
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Kirk Landaur wrote:

No they don't. Following your picture in the URL above, the numbers on the scale go from 1.2 to .10 (left to right).
The 'LONGER' label indicates what happens when you move the pointer in the direction indicated below it by the arrow. Moving the pointer to the left, to a higher number say from .2 to .3, in the direction of the arrow, makes whatever you are adjusting 'LONGER'. No ambiguity there at all, except for what that 'LONGER' adjustment is specifically doing (I guess that would be in the manual).
S.
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Kirk Landaur wrote:

What are you trying to accomplish? To make things worse? 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. With this you can adjust some things in the service mode at initial installation.
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On Sat, 26 Dec 2015 14:20:25 -0700, Tony Hwang wrote:

I'm simply trying to make the hysteresis of longer duration.
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Kirk Landaur wrote:

Then move the anticipator to higher number from what it is now. When we were running 4 stores in the past, one store had that old thermostat, I played with it during summer cooling, winter heating months. After all that time spent, I drew a conclusion original setting was best optimized setting. Eventually I replaced it with digital 7 day programmable thermostat which realized some savings on utility bill.
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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.
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Ralph Mowery wrote:

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)
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