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Old 05-08-2010, 12:22 PM   #1
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Thermostat Adjustable Tolerance


Hello,

Here is my situation. The temperature has recently dropped and my furnace is back running (I was hoping I was done for the season). Anyways, the furnace cycles on and off too frequently.

I did some searching for the tolerance (hysteresis) on my thermostat and could not figure it out. So, I did some tests using another thermometer. I have the furnace set to 64 degF. I measured for ten on-off cycles and it looks like it is turning on ~63.7 degF and turning off ~64.2 degF. So there is only 0.5 degF tolerance.

What I am looking for is a new thermostat that has a looser (or even better adjustable) tolerance (hysteresis). I want to set it to 64 degF and have the furnace kick on ~61-62 degF and kick off ~64 degF ... give or take. I would like to have 2-3 degF between the on/off temperatures.

I would like to minimize the furnace cycles because the furnace is inefficient (80%) and it takes the furnace ~5 minutes from the time the furnace kicks on to where I feel warm air through the vents.

Any recommendation? And please not the obvious ... buy a higher efficiency furnace (unfortunately, not in the budget).

Thanks in advance.

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Old 05-08-2010, 02:18 PM   #2
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Thermostat Adjustable Tolerance


If its a Honeywell thermostat, it probably has a CPH setting. Might want to see what its set for. And lower it one setting.

EG: If its set to 4, set it to 3.

White Rodgers makes thermostats that do somewhat like you want.

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Old 05-08-2010, 02:51 PM   #3
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Thermostat Adjustable Tolerance


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Originally Posted by arctic View Post
turning on ~63.7 degF and turning off ~64.2 degF. So there is only 0.5 degF tolerance.
That's the hysteresis.
The center value of your readings is 63.95F. If the room temp is actually 64.95F and your 'stat has a tolerance of +/- 1F, then it is just barely within tolerance.

Last edited by Yoyizit; 05-08-2010 at 02:54 PM.
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Old 05-09-2010, 08:42 AM   #4
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Thermostat Adjustable Tolerance


You could buy a 2 stage thermostat . With nothing hooked to w1 it will drop another degree to energize w2. Some of the Robert Shaw thermostats have an adjustable second stage so you could set it to drop 3-4 degrees before tuning on w2.
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Old 05-09-2010, 09:13 AM   #5
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Thermostat Adjustable Tolerance


arctic, it would help if you would post what you currently have for a thermostat. Is this a "heat only" thermostat?

If it's an older manual thermostat, such as a round analog Honewell T87 model you can adjust the heat anticipator.

http://homerepair.about.com/od/heati...nticipator.htm

If you have a digital thermostat, post back what model it is and I'll see if it has an adjustable heat anticipator.

I've been in the HVAC trade for 24 years and have never heard the term "hysteresis" used in conjunction with the explanation of the operation of a thermostat, but maybe I'm missing something.
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Old 05-09-2010, 09:20 AM   #6
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Thermostat Adjustable Tolerance


Quote:
Originally Posted by fabrk8r View Post
arctic, it would help if you would post what you currently have for a thermostat.

I've been in the HVAC trade for 24 years and have never heard the term "hysteresis" used in conjunction with the explanation of the operation of a thermostat, but maybe I'm missing something.
Yes, I think the term used in the business is "anticipation". Most older mechanical thermostats have a manual anticipator like the one in the picture. Many newer digital thermostats have a computerized anticipator or "cycle rate" adjustment ("CPH" or Cycles Per Hour).

Last edited by sktn77a; 05-09-2010 at 09:24 AM.
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Old 05-09-2010, 07:08 PM   #7
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Thermostat Adjustable Tolerance


Quote:
Originally Posted by fabrk8r View Post
arctic, it would help if you would post what you currently have for a thermostat. Is this a "heat only" thermostat?

If it's an older manual thermostat, such as a round analog Honewell T87 model you can adjust the heat anticipator.

http://homerepair.about.com/od/heati...nticipator.htm

If you have a digital thermostat, post back what model it is and I'll see if it has an adjustable heat anticipator.

I've been in the HVAC trade for 24 years and have never heard the term "hysteresis" used in conjunction with the explanation of the operation of a thermostat, but maybe I'm missing something.

What is currently there is a Honeywell Pro 4000. It is a programmable heating/cooling digital thermostat. It is still the original thermostat that the builder installed. I assume he just got a bunch of them in bulk for cheap.

I have searched the internet and it looks like the CPH is not adjustable. I am not even convinced it has CPH because I know the furnace has cycled at least 10 times in a single hour (and it was not even that cold outside).

Anyways, I think I might have confused things. I believe I finally have this figured out. I was using the term I was used to (hysteresis). I actually work on aerospace heating/cooling systems and what we call hysteresis it looks like is commonly called "swing" on thermostats. After figuring out the proper term is looks like there are thermostats with adjustable swing.

Now that I know the proper term... any recommendations on thermostats with adjustable swing? The furnace is a single stage gas furnance. My searches always seem to end up at Lux. Anything against them?

Thanks
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Old 05-10-2010, 06:14 AM   #8
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Thermostat Adjustable Tolerance


Did your furnace cycle this much in the fall when you first started using the heat.

The 4000 does have an adjustable CPH. But it sounds like your furnace may be cycling on its high limit. Have you changed or cleaned your air filter recently?

Set up function 5, is the CPH setting for heating.

Is your furnace, oil, gas, or electric? If electric, they may have set it for the recommended 9 CPH.
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Old 05-10-2010, 08:12 AM   #9
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Quote:
Originally Posted by fabrk8r View Post
I've been in the HVAC trade for 24 years and have never heard the term "hysteresis" used in conjunction with the explanation of the operation of a thermostat, but maybe I'm missing something.
The backlash of mechanical systems and hysteresis used in this sense both have a dead zone, but in a system with hysteresis, at some point the input loses control over the output. The system 'fires'.

Last edited by Yoyizit; 05-10-2010 at 01:32 PM. Reason: Didn't like my answer
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Old 05-10-2010, 09:29 AM   #10
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Thermostat Adjustable Tolerance


Quote:
Originally Posted by Yoyizit View Post
In mechanical systems it's called backlash.
In HVAC mechanical systems "anticipation" is the term used to describe the temperature overshoot/undershoot that is built into the control device (in this case a wall thermostat) to prevent a system from trying to maintain an exact temperature without any tolerance while still providing comfort for the occupants.

arctic, regarding the recommendation of a thermostat, my choice would be a Honeywell T87. They have been around for a long time and are proven. I would just use an analog model, but they are also available with a digital display. If you are looking for a programmable thermostat I would just buy an inexpensive one without a bunch of bells and whistles. I've had no problems with any brands I've ever installed.
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Old 05-10-2010, 09:53 AM   #11
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The old a;most reliable T87F is going the same way as lead based paint did.

Nothing wrong with his current thermostat. Its probably either not set up right. Or his air filter is dirty.
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Old 05-10-2010, 06:19 PM   #12
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Thermostat Adjustable Tolerance


Okay. Thanks for all the help.

Where I messed up is I was looking at the user manual, which makes no mention of the CPH and other setup functions. I finally, realized to look in the installer manual and found all the settings I was looking for.

It was set to 9 for a gas furnace (the installer screwed up because I never played with any of these settings, although I guess I should of known better and never trust anyone ). It was this constant cycling all fall/winter, but I just kind of ignored it because my neighbor's operated similar (same HVAC system and installer) . I set the CPH to the recommended 5. I guess I will see if this improves things.

Thanks again for all the help.
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Old 05-10-2010, 06:44 PM   #13
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then help your neighbor, he'll let you borrow stuff
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Old 05-10-2010, 06:51 PM   #14
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Set it to 3.

5, is on and off, 5 times an hour. So it would work out to: on 6 minutes, off 6 minutes, on 6 minutes, off 6 minutes, etc(this is at 50% of your systems ability).

3 provides better efficiency.
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Old 02-08-2012, 10:33 AM   #15
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A long chat with a helpful HP engineer reveals that:
- NONE of the domestic thermostats allow the user to control the "swing" (aka hysteresis, degree range, comfort control, differential, dead band, anticipation). All are set to +- one degree.
- you can indeed set "Function 5" to adjust the "heating cycle rate". My informant disagreed with "beenthere", saying that "9" would produce LESS cycles than "5" ... haven't tried it myself. My informant suggested that this function is conservative - i.e. if you slow the cycle rate with Function 5, the thermostat will heat to a HIGHER temperature to slow the cycles down. Counter-intuitive, so maybe wrong? One would expect Function 5 to simply delay switching the heating on - which would allow descent to LOWER temperatures.
- commercial thermostats are available with adjustable temperature swing (and many other sophistications), but are expensive and ugly. Cheapest HP example quoted was the T775A2009, over $200 and clunky, but does everything except make the coffee.

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