set and find differential???

[dum one]

setting a low pressure switch. i find different techs setting differentials crazy ways.
refrigerant type 404a
find your temp you want 35f
the range you want to have cut in at 40f convert to 85psig
cut out at 30f convert to 70psig
now subtract the two pressures and there is your differential. 15psig
does this sound correct?

[rdm]

Probably not. To begin with, ask yourself the question, "what temperature is the pressure corresponding to?" Box temp? wrong. Refrigerant temp? Aha! Then, as far as cut out temperature in concerned, ask "what refrigerant temp do I need to achieve Box temp". That depends. What are you cooling? Fresh flowers, or packaged meat? It makes a world of difference. My guess is that with a cut out temperature of 30 degrees, you might be dealing with window A/C temps, and not product refrigeration. I hate using "rules of thumb", but try Product temp minus 15-20 degrees = refrigerant off temp. Experiment with your own equipment, not a customers, though.

[dum one]

makes no sence according to pt chart cut in, cut out.

[lennoxman]

I have found that if possible add a pump down solenoid and penn stat. Pressure controls can be a real headache. A stat is usually an easier way to contol temp.

[dum one]

it is always good for safetymeasures.

[bob hvac]

that does not sound right.what you think don?

[condenseddave]

Cut in and cutout need to be set to the evaporator temperature, not the box temp.

This means that you need to know the delta t of the system.

I get the impression that you're confusing the PT relationship somewhere along the line.

[condenseddave]

Originally posted by lennoxman
I have found that if possible add a pump down solenoid and penn stat. Pressure controls can be a real headache. A stat is usually an easier way to contol temp.

Thermostats are much better than Penn stats, and work in all states, including those SOUTH of the Mason-Dixon line. True story.

[lennoxman]

Yeah I get it. Not a phillips screw driver or a crescent wrench. It's a cross head screw driver and ajustable wrench. Thanks.

[lennoxman]

I gotta give props to the old timers. It seems like they had to learn how to really work on things. Older refrigeration really kicks my butt.