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pressuretrol as vaporstat backup?

regarding the shoddy controls in the new vaporstats, I see it is wise to add a pressuretrol as a backup.  However, if I have an additive pressuretrol, this will not work, correct?

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for example:

Vaporstat -> switch makes at 6oz (.4psi), and breaks at 8oz(.5 psi).

Additive Pressuretrol -> makes at .5 psi (8oz, lowest possible setpoint) and breaks at additive of 1.5psi which is 2psi, (as a safety)

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This will not work.  obviously.

So therefore, I need a subtractive pressuretrol:

Vaporstat -> switch makes at 6oz, breaks at 8oz

Subtractive Pressuretrol -> makes at differential (say 1psi) and breaks at setpoint (say 2psi),  

This combination will work.

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So am I correct in the statement that I need a subtractive pressuretrol to be a safety backup if I am installing a vaporstat? 



Thanks!
beautiful Conshohocken PA

Comments

  • MrDvorakMrDvorak Posts: 63
    edited November 2011
    Confusing

    I think that you are a bit confused (edit - or I am confused by your terminology), it is much simpler than that. What you need to do is wire them in a sequence so if either one cuts out, the burner shuts down. Normally, only the Vaporstat should be breaking the circuit and the Pressuretrol should be set to higher pressure and stay on all the time. Only in a (rare) case of Vaporstat's failure to break the circuit, the Pressuretrol will cut out when the higher pressure (i.e. 1.5 PSI) is reached. Everything else (additive/subtractive pressuretrol) is irrelevant IMHO.
  • Long Beach EdLong Beach Ed Posts: 688Member
    Manual Reset

    Safety would be served best with a pressuretrol having a manual reset piped on its own steam trap (pigtail).   That's what many codes require for commercial applications. 



    In this manner, your attention would be drawn to a boiler malfunction and unsafe condition which would otherwise go unnoticed. 
  • Jamie HallJamie Hall Posts: 10,668Member
    Manual reset

    should, in my humble opinion, be required on residential boilers as well.  I see (says he, sounding fractious) why a device which is supposed to function only when something isn't working right should be so automatic that it doesn't warn that there is a malfunction.  Therefore... the backup pressuretrol (which can be set to cut out at any reasonable pressure between the normal control and about half the safety valve) should be manual reset.  And on it's own pigtail, to further reduce the common failure problem.



    IMHO, the second LWCO should also be a manual reset, too...
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • TomMTomM Posts: 233
    edited November 2011
    yes but

    *edit* ok i was thinking about it wrong.  I was thinking that if they were wired in series, then the cut-in would be from the pressuretrol, and the cut-out would be from the vaporstat, an undesirable condition.  I was wrong.  Either an additive or subtractive pressuretrol will work. 
    beautiful Conshohocken PA
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