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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#41
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Hysteresis on the Honeywell old-style bulb thermostat
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. Sounds funny, then your system will blow cool air before warm air start blowing out. |
#42
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Hysteresis on the Honeywell old-style bulb thermostat
Tony Hwang wrote: "- show quoted text -
Sounds funny, then your system will blow cool air before warm air start blowing out. " Ever hear of a 'return' in a force air system? That should engage first, before the hot end of things does. |
#43
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Hysteresis on the Honeywell old-style bulb thermostat
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.
Peter Wieck Melrose Park, PA |
#44
Posted to sci.electronics.repair
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Hysteresis on the Honeywell old-style bulb thermostat
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 |
#45
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Hysteresis on the Honeywell old-style bulb thermostat
8:31 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. |
#46
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Hysteresis on the Honeywell old-style bulb thermostat
On Sunday, December 27, 2015 at 10:41:45 PM UTC-5, wrote:
8:31 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 |
#47
Posted to alt.home.repair,sci.electronics.repair
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Hysteresis on the Honeywell old-style bulb thermostat
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. |
#48
Posted to alt.home.repair,sci.electronics.repair
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Hysteresis on the Honeywell old-style bulb thermostat
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. 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. |
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