Differential settings

Unknown

I was hoping a little ignorance on my part could be rectified. The differential knobs on hi/low limit controls such as the Honeywell L8124A has become the topic of discussion among some of the service techs where I work.

Does anyone know where I could find some information on the subject? Manual sheets that come with the controls dont provide enough information to wet our appetite, and I havent been able to find anything on honeywells site, or in trade school books.


I was taught that, for example, if yout low is 160 and high is 180, a differential knob set at 10 will cause the burner to fire when the boiler temperature drops from 180 to 170.

Is that correct?

Thanks for your help.

- Norm

Michal

IF i AM NOT MISTAKING

the high and low will maintain the water temperature in the boiler when the system is not calling for heat. The differential will control the circulator cycling example you gave the circ will come on when heat is called for at 180 and shut off at 170. While when the system is not calling for heat it will let the water temperature cool down to 160 before it turns on to heat it back up to 180. best place to get that info is in the box itself, there is literature attached with new control. But thats how I alwatys understood it? anyone care to share???

Alan R. Mercurio_3

From my friends at Honeywell and how I explain it in my Oil Tech Talk Wiring Workshop.

OPERATION of the Tripple Aquastat

High Limit Controller


The high limit opens and turns off the burner when the

water temperature reaches the set point. The high limit

automatically resets after the water temperature drops

past the set point and through the 10°F (6°C) differential.

Low Limit and Circulator Controllers


On a temperature rise, with the adjustable differential at the minimum setting of 10°F (°C), the burner circuit (R-B) breaks and the circulator circuit (R-W) makes at the control set point.

On a temperature drop of 10° F (6°C) below the set point, the R-B circuit makes and the R-W circuit breaks.

At any differential setting greater than 10°F (6°C),the R-B make temperature and R-W break temperature will remain the same (control setting minus 10°F (6°C). The R-B break and R-W make temperature will be the set point temperature plus the difference between the differential setting and 10°F (6°C).

EXAMPLE:Set point of 140°F (60°C) differential set at 25°F(14°C). On a temperature rise, R-B will break and R-W will make at 155°F (68°C).On a temperature fall, R-B will make and R-W will break at 130°F (54°C).

On the L8124M, the burner and circulator circuits are
controlled separately.the low limit controls the burner circuit(R-B) as described above, while the low voltage thermostat directly controls the circulator circuit

Setting

Follow the boiler manufacturer recommendation when making
the L8124 settings. The high limit setting must be at least 20°F (11°C) higher than the low limit setting. Place the indicators over the selected temperature marks. The low limit differential is set by turning the differential adjustment knob to the desired amount of differential.

Now is all that as clear as mud?


Your friend in the industry,
Alan R. Mercurio

www.oiltechtalk.com

Ron Schroeder

I just took apart an old 8124 to see how it worked. All the temperature settings are controled by springs of one kind or another. I am convinced that the calibration of each spring drifts with age. In other words, be advised to maintain a good temperature difference between the circulator setting and the burner limit setting, i.e. at least 15* plus the differential. If you want accurancy change it to a 7224.

Unknown

So if I understand this correctly, the reason why you would have the differential set to anything above 10 is to delay the circulator comming on untill you have hotter water in the boiler.

What are some circumstances where you would the boiler to be controlled this way?

Thanks,
Norm

Ron Schroeder_3

Hi Norm,

A wider differential on the low limit will reduce short cycling and number of cycles when there is no call for space heat. Unfortunatly the high limit doesn't have an adjustable differential to allow you to reduce short cycling durring a space heat call.

Ron