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Air Flow Pressure Sensing Switch

The switch just arrived this afternoon with no instruction mannual, but managed to find the pdf online here: http://unicontrolinc.com/clevelandcontrols/wp-content/uploads/sites/2/2015/11/LT-NS2-11.pdf

By the way, the boiler didn't hiccup the whole weekend at all. Three days in a row with no issue.

Since I got it, I want to go ahead to replace the old (20 years old with the boiler new) next time when the intermittent stall happens. but I am not sure about how the two tubes connect.

What the replacement switch offers:
Air Sampling Method
The High or Positive inlet (P1) is black and
the Low or Negative inlet (P2) is gray. Connect
the sample lines as follows:
Positive Pressure Only: Connect the
sample line to P1; P2 remains open to the
Negative Pressure Only: Connect the
sample line to P2; P1 remains open to the
Two Negative Samples: Connect higher
negative sample to P2; lower sample to
Two Positive Samples: Connect higher
positive sample to P1; lower sample to
One Positive and One Negative: Connect
positive sample to P1; connect negative
sample to P2.

Here's the drawing on the existing Weil Mclain HE II boiler attached.

Any suggestion is greatly appreciated.


    HVACNUT Member Posts: 5,860
    Can you post the boiler wiring diagram?
    The new PS is black +, grey -.
    We'll need to see how the original was wired.
  • 1sttimeposter
    1sttimeposter Member Posts: 39
    Wiring diagram is attached. Hope this is what you asked for, HVACNUT.
    HVACNUT is absolutely right "black" on the new PS is "+" while grey color side is "-" as marked on the PS itself.
    I think I may have found the answer as to where "+" and "-" are connected to the boiler.

    While it's lacking on my WM HE II's manual, it says on their CGI boiler manual that: "The boiler will not operate correctly unless pressure switch hoses are correctly located. The red hose connects from the right side (negative) hose barb to the flue collector. The white hose connects from the left side (positive) hose barb of the switch to the connector box (between flue collector and inducer)"

    So, the "+" on the black side of new PS should hose to the "FLUE COLLECTOR HOOD" while "-" on the grey side should go to the "TRANSITION" as pictured in my last attachment.

    Should I pull the trigger now to replace it or wait till it dies again?

    Let's all pray for Vegas' shooting victims...Very sad..

    HVACNUT Member Posts: 5,860
    2nd paragraph you said,
    Red negative to the flue collector.
    White positive to the transition.

    3rd paragraph you said,
    "+" to the flue collector.
    Grey (negative) to the transition.

    Recheck it.

    Yes, the diagram you posted only shows N.O. switching.
  • 1sttimeposter
    1sttimeposter Member Posts: 39
    Good eye, HVACNUT! Thanks for that.

    Still holding on to this new PS as the boiler has yet to fail from last Friday (the longest lapse since the intermittent problem surfaced three weeks ago).

    Like the intermittent ignitor issue several years ago, it makes the situation very difficult to isolate and solve.

    I mean if the part is truly failed or defective or broken, why would it carry on to work and die and work and die?
  • 1sttimeposter
    1sttimeposter Member Posts: 39
    For those who follow this thread, just a little update.

    Intermittent pre-purge hiccups on our 20-year old Weil Mclain gas HE II boiler has not resurfaced and the new pressure switch is in still unopened in the box .