# Thermostat Adjustable Tolerance



## beenthere (Oct 11, 2008)

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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## Yoyizit (Jul 11, 2008)

arctic said:


> 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.


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## Marty S. (Oct 31, 2009)

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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## fabrk8r (Feb 12, 2010)

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/heatingcoolingrepair/ss/adj_anticipator.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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## sktn77a (May 11, 2009)

fabrk8r said:


> 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).


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## arctic (Sep 29, 2009)

fabrk8r said:


> 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*.
> 
> ...



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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## beenthere (Oct 11, 2008)

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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## Yoyizit (Jul 11, 2008)

fabrk8r said:


> 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'.


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## fabrk8r (Feb 12, 2010)

Yoyizit said:


> 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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## beenthere (Oct 11, 2008)

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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## arctic (Sep 29, 2009)

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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## tpolk (Nov 7, 2009)

then help your neighbor, he'll let you borrow stuff


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## beenthere (Oct 11, 2008)

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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## carrt7 (Feb 8, 2012)

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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## hvac5646 (May 1, 2011)

Your *informant* i does not make coffee?????


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## beenthere (Oct 11, 2008)

9 cycles per hour would be on 3.33 minutes, off 3.33 minutes, on 3.33 minutes, off 3.33 minutes, etc.

3 cycles per hour is a lot less cycles. And the slight temp increase before shut off, and slightly lower temp before coming on, won't raise or lower the temp displayed on the thermostat, since its less then a .1°F change.

I've spent a lot of time installing and setting up Honeywell thermostats for customers, and have 6 different models of them here at my house that I experimented with. So I know how the CPH effects them.

White Rodgers may have a model like you want.


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## KevinRhodes (Nov 30, 2012)

Sorry to revive and old thread, but I was looking for some info on adjusting hysteresis and came across this thread on the first page of my search. There is a little bit of incorrect information regarding the following. I thought some of you may be interested, so here it is.:thumbsup:



fabrk8r said:


> 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.


I graduated from RSI in Phoenix, AZ, Feb. 12, 1986. That is where I learned the word "hysteresis".



fabrk8r said:


> 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.


This actually describes hysteresis.

An overshoot/undershoot example in cooling mode would be- You set the thermostat to 70* and it cuts off at 72*, that would be undershoot. If it cuts off at 68* that would be overshoot....it is either undershooting or overshooting its target temperature.

Hysteresis is this- You set the stat to 70*, the unit cools the house down and the unit shuts off. Now the house starts to warm back up...does the unit come back on at 74*, 76*, 78*, etc? Obviously, @ 74*, you have 4* of hysteresis, @ 76* it is 6*, etc.

Does that make sense?


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## vect0r (Apr 23, 2013)

Is there an ideal setting that's the most energy efficient? I have my thermostat set to a range of 1°, but I'm not sure if more or fewer cycles is better on the electric bill.


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## beenthere (Oct 11, 2008)

Fewer cycles saves more money. So a wider spread is more economical, but also a little less comfortable.


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