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# honeywell pressuretrol

Member Posts: 4
I want to ask you guys,,,how does this honeywell pressuretrol 404 f

1367  setup works,, ,where it says that the DIFF “Differential is

Subtractive”  what do it means,,,,that if  you have MAIN,,,,lets say at

3 psi,,,you have to set the DIFF at 1.5,,,or if you have Main at

2,,,them DIFF  will be 1,,,or if you put Main at 1  them DIFF  be

.5,,,is this how is when the say “Differential is Subtractive” is thats

the way or I am wrong???

• Member Posts: 514
Diff subtractive

Example Diff set at .5

set point 3

means switch on at 2.5 and off at 3.5
• Member Posts: 4

Diff subtractive

Example Diff set at .5

set point 3

means switch on at 2.5 and off at 3.5

on that example that you gave me ,,, If I put that Diff at .5 like you say,,,set point 3,,,,what's the set point,,,the Main?,,,,and how do you get to those numbers  switch on at 2.5 and of at 3.5,,,that's what I what  to  learn ,,,I really appreciate if you could explain  to me in a easier way and I thank you for your time,,thanks
• Member Posts: 4
thank you furnacefigher15

Diff subtractive

Example Diff set at .5

set point 3

means switch on at 2.5 and off at 3.5

is this what you mean,,,by if Diff is at .5  and

set point at 3,

is like subtracting 5  at switch on

and it adds 5 at off ?

I think I starting to understand ,,,what you saying
• Member Posts: 604

If the differential is subtractive, it does not add to the main setpoint. For example, if the main is set to 3 PSI and the differential is 1, then the control will cut out at 3 PSI and cut back in at 2 PSI.  ( 3 -1 = 2 ) The differential does not affect the main setting, only the pressure at which it cuts back in.
• Member Posts: 514
My mistake

Mike is correct

Subtractive differential only effects cut in point.

set point is all ways the cut out point and the cut in is the cut out minus the differential.

so set point 5 psi is the cut out point

differential of 1 would cause a cut in at 4 psi.

I need to stop giving answers this late at night.

Tony, the example I gave before is for standard differential (say for a residential thermostat)
• Member Posts: 4,482
If using a standard

P 404 pressuretrol we would want it to shut off at 1.5 to 2 lbs and back on at .5 that is typical for a residential steam system using that pressuretrol.
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