Pressuretrol Settings

Poppi

We recently replaced our home boiler. The new system includes a Honeywell pressuretrol with two settings: Main and Diff. Small print on the pressuretrol says “Diff is subtractive.”
Initially, Main was set to app. 1psi and Diff to app. 1.5. Everything seemed to work fine at those settings. My understanding is that the Main (cut-out) setting of 1 is a good setting for a domestic heating system. My understanding is also that the boiler will cut back in when the pressure reaches the point determined by subtracting the 1.5 Diff setting from the 1.0 Main setting. This calculation gets me to -.5psi which makes me wonder how that works as -.5 would indicate a vacuum in the system.
After a few weeks, I decided to drop the Diff setting to .5 to see what would happen and the system continues to work just fine.
So I have two questions:
1. How does the system work at all with Main set to 1 and Diff set to 1.5?
2. With Main at 1, which is the better Diff setting, .5 or 1.5 (or some other combination)? Why?
Much thanks.

Fred

The main should be set to 1.5 which becomes the max cut out pressure. The subtractive Differential should be set to "1" which means, should the boiler pressure get to 1.5PSI, the boiler will shut down and it will not come back on until the pressure drops to .5 PSI (1.5 - 1= .5). It is currently set backwards. Keep in mind the Pressuretrol is suppose to be a high limit safety device and should not be the control that typically turns your boiler off. That should happen because the Thermostat has been satisfied. On days when it's really cold or on set-backs when the boiler runs for extended periods, or if you don't have adequate venting on your mains, causing the boiler to run longer, the Pressuretrol may come into play but typically not as a normal course of day to day heat cycles.

Poppi

Fred,
Thank you for the information. Your explanation of the role of the pressuretrol was especially helpful. I think I had erroneously been thinking of the pressuretrol as a control device rather than as a safety device. As you indicated, I don’t think the pressuretrol has come into play in normal daily operations.