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?


Re: Hysteresis on the Honeywell old-style bulb thermostat
On 12/26/2015 12:37 PM, Kirk Landaur wrote:
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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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(Please post followups or tech inquiries to the USENET newsgroup)
John's  Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
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Re: Hysteresis on the Honeywell old-style bulb thermostat
On 12/26/2015 3:55 PM, John Robertson wrote:
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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.


Re: Hysteresis on the Honeywell old-style bulb thermostat
Bob Engelhardt wrote: "That is the "anticipator", not hysteresis setting. "

Yes!  Digital programmables have replaced
the heat anticipator with a set of "firing length"
options: 1 for shorter firing time, 2 for normal,  
and 3 for longer.

Re: Hysteresis on the Honeywell old-style bulb thermostat
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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Re: Hysteresis on the Honeywell old-style bulb thermostat
Dave M wrote:
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  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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Re: Hysteresis on the Honeywell old-style bulb thermostat

Tony Hwang wrote:
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   They are interactive.

Re: Hysteresis on the Honeywell old-style bulb thermostat
Tony Hwang wrote:
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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  



Re: Hysteresis on the Honeywell old-style bulb thermostat
On 12/26/2015 3:58 PM, Dave M wrote:
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Yes!


Re: Hysteresis on the Honeywell old-style bulb thermostat
On Saturday, December 26, 2015 at 3:58:55 PM UTC-5, Dave M wrote:
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The voice of wisdom speaks. Thank you Dave. If you take the thermostat off  
the wall and tilt it left and right you can see the heavy mercury drop flop
 back and forth causing the hysteresis in the temperature by-metal spring.  
An elegant design from old school physics that does not require microproces
sors and such. I would add more reliable as well.

Re: Hysteresis on the Honeywell old-style bulb thermostat
On Sat, 26 Dec 2015 12:49:29 -0800, Uncle Monster wrote:

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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?


Re: Hysteresis on the Honeywell old-style bulb thermostat
Kirk Landaur wrote:
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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.

Re: Hysteresis on the Honeywell old-style bulb thermostat
On 12/26/2015 4:14 PM, Kirk Landaur wrote:
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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.



Re: Hysteresis on the Honeywell old-style bulb thermostat
On Sat, 26 Dec 2015 21:14:41 +0000, Kirk Landaur wrote:

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This seems to be the correct URL to the picture.
http://oi63.tinypic.com/280nvqs.jpg

Notice the "LONGER" and "<------" fight each other.


Re: Hysteresis on the Honeywell old-style bulb thermostat
Kirk Landaur wrote:
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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.



Re: Hysteresis on the Honeywell old-style bulb thermostat
ssinzig wrote: "arrow, makes whatever you are adjusting 'LONGER'.  No ambiguity there at all, except for what that 'LONGER' adjustment is specifically doing (I "

Should be pretty obvious what is happening
for a "longer" period of time: the boiler or
furace is running longer!  Moving that slider  
toward Shorter results in shorter, more frequent
run times of the boiler or furnace.

Assuming setpoint = 68.0F
Longer means swing from 66 to 70,
but house gets hot and cold.
Shorter means swing from 67.8 to
68.2, but heat cycles on and off
constantly.

Just right(anticipator setting measured  
with meter) and you should stay between
67.5 - 68.5F.

Same concept with digital "Firing length"
3-4 position menu setting.

Re: Hysteresis on the Honeywell old-style bulb thermostat
snipped-for-privacy@gmail.com wrote:
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Okay, that seems reasonable. Although I would argue that it is not
'pretty obvious' what is happening for a 'longer' period of time,
without already being familiar with its operation or consulting an
operating manual.

Is the scale (1.2 to .10) measured in seconds, minutes, or hours?
Is it a multipler, ie. 0.8 x burn duration?
Is it the time the burner stays on?
Is it a time delay before the burner comes on?
Is it a time delay before the burner turns off?

Everything is always 'pretty obvious' when you are
already familiar with how it works.

S.



Re: Hysteresis on the Honeywell old-style bulb thermostat
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It's the current draw for the solenoid valve.

Re: Hysteresis on the Honeywell old-style bulb thermostat
ssinzig wrote:  "- show quoted text -
Okay, that seems reasonable. Although I would argue that it is not  
'pretty obvious' what is happening for a 'longer' period of time,  
without already being familiar with its operation or consulting an  
operating manual.  

Is the scale (1.2 to .10) measured in seconds, minutes, or hours?"
No time bearing whatsoever.  It's VOLTAGE.
"Is it a multipler, ie. 0.8 x burn duration? "
No.
"Is it the time the burner stays on? "
Yes!!
"Is it a time delay before the burner comes on? "
Sort of.  You're starting to get it.
"Is it a time delay before the burner turns off? "
Getting warmer!(pardon the pun).  It's function
is determined by voltage(the numbers on the
anticipator slider are in Volts).

A heat anticipator generates 'false heat' - it tricks a
traditional bulb-stat into firing the boiler in a more
energy efficient manner, by generating a small
amount of local heat within the wall unit itself.

The correct amount of false heat both prevents
the thermostat from calling for heat too soon after
it drops below set point(what you the user set it for),
and prevents thermostat from running so long that it
overshoots that set point by significant amount.

At its extremes, a heat anticipator could cause too
frequent and short boiler runs, or not so frequent boiler
runs between which the house gets too cold, and then
gets too hot before the boiler turns off.  Set properly, the
anticipator will turn the unit off just before reaching the
setpoint, so that remaining hot water(or air) in the system
can be pushed through the system without a significant
overshoot.  

Succinctly, you want neither a scenario where the boiler
turns on and off every three minutes, nor where it turns
on for one half hour, and is then off for nearly one half
hour.  The anticipator, when properly calibrated, prevents
both situations, and keeps the temp. within a tolerable
range.

"Everything is always 'pretty obvious' when you are  
already familiar with how it works. "

S. "

That's why we're here to share knowledge.  ;)

Re: Hysteresis on the Honeywell old-style bulb thermostat
Kirk Landaur wrote:
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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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