Hysteresis on the Honeywell old-style bulb thermostat

[pfjw@aol.com]

For the record, 99-44/100ths of all residential forced air systems do not, repeat!, do not have an independently forced return. They are closed systems with what is best described as a "plenum" (central) return. Usually it is a large floor duct somewhere near the furnace. Essentially, it is the make-up air for the supply.

There is a great deal of bad information out there. Please DO try to verify any information from a reliable source before taking any irreversible action(s).

Peter Wieck
Melrose Park, PA

 

[Tom Miller]

<pfjw@aol.com> wrote in message
news:ac45991f-249a-4b8d-a508-d10a3d4c653b@googlegroups.com...


" In such cases, the stat should respond very nearly to ambient temperatures
(and should be on an inside wall not facing a radiator)."

Peter Wieck
Melrose Park, PA

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

And don't forget to consider what is inside the wall. Such as hot and cold
water pipes, return air ducts, wiring,...

 

[Tony Hwang]

Jerry Peters wrote:

In sci.electronics.repair Tony Hwang <dragon40@shaw.ca> wrote:
Ralph Mowery wrote:
"Kirk Landaur" <kirk@example.com> wrote in message
news:n5mtr6$8v0$1@news.mixmin.net...
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.

 

[Tony Hwang]

Jerry Peters wrote:

In sci.electronics.repair Tony Hwang <dragon40@shaw.ca> wrote:
Ralph Mowery wrote:
"Kirk Landaur" <kirk@example.com> wrote in message
news:n5mtr6$8v0$1@news.mixmin.net...
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.

Sounds funny, then your system will blow cool air before warm air start
blowing out.

 

8:31 PMpf...@aol.com wrote:
"The 'anticipator' lives on the stat. Not in the system. 'Lowest' setting. It is still between the gas valve and the valve's
power source. That is the specific issue.
- show quoted text -"

Most of us here know where the anticipator is located.

 

[pfjw@aol.com]

On Sunday, December 27, 2015 at 10:41:45 PM UTC-5, thekma...@gmail.com wrote:

8:31 PMpf...@aol.com wrote:
"The 'anticipator' lives on the stat. Not in the system. 'Lowest' setting. It is still between the gas valve and the valve's
power source. That is the specific issue.
- show quoted text -"

Most of us here know where the anticipator is located.

They do. But, not to be snarky, there are vanishingly few residential systems out there with forced returns. Few, even in commercial forced air systems. I was involved ONCE (1 time) with a system that had fully ducted supply and returns, this in an academic building of 425,000 square feet. It also had oxygen and CO2 sensors in the classrooms, ran on a VAV based system using constant-velocity variable-vane axial fans on both the supply and returns. It was a high-volume, low pressure system to reduce noise and maintain high air quality. It could vary from a minimum of 10% 'new' air to 100% new air. That, for the record, is the only forced-return system that I have been directly involved with - others exist, I am sure - but mostly in commercial/institutional applications, and most of those also VAV based.

Peter Wieck
Melrose Park, PA

 

[Jerry Peters]

In sci.electronics.repair Tony Hwang <dragon40@shaw.ca> wrote:

Jerry Peters wrote:
In sci.electronics.repair Tony Hwang <dragon40@shaw.ca> wrote:
Ralph Mowery wrote:
"Kirk Landaur" <kirk@example.com> wrote in message
news:n5mtr6$8v0$1@news.mixmin.net...
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.

 

[Jerry Peters]

In sci.electronics.repair Tony Hwang <dragon40@shaw.ca> wrote:

Jerry Peters wrote:
In sci.electronics.repair Tony Hwang <dragon40@shaw.ca> wrote:
Ralph Mowery wrote:
"Kirk Landaur" <kirk@example.com> wrote in message
news:n5mtr6$8v0$1@news.mixmin.net...
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.

Sounds funny, then your system will blow cool air before warm air start
blowing out.

For perhaps 30 seconds or so, then the secondary heat exchanger starts
warming the air while the primary is heating up.