Why not a cut in of zero?

Clank Clank

With a vaporstat installed, why not set the differential to a value that has the pressure drop down to something close to zero as it goes thru its cycles? What is the minimum it should be set to and how is that determined?

ChrisJ

Why not zero

My opinion is you want the system to maintain enough pressure to keep the furthest radiators from sucking air back in. If the pressure drops too much, including the time it takes the burner to relight and get things cooking again then radiators far away will suck air in and you'll have to push it back out.



I would say it all has to do with your specific system. For example on mine as long as it didn't drop below 1 ounce I would be golden.

Charlie from wmass

accuracy

the setting  marks lack it so we need to make it close to zero yet as Chris said, not so low as to allow the system to breath.

Jamie Hall

And furthermore...

What you don't want is to get into a situation where the pressure does NOT drop to what the vapourstat thinks is zero.  It could happen, and that would effectively lock out the boiler.

Pughie1

"Cut- in" Setting

I've just been reading my 2nd copy of  the Lost Art, for at least the 3rd time. On pages 13 & 14 "The Man" discusses setting the pressuretrol "for good system operation, the cut-in setting of the pressuretrol should never be less than twice the system pressure drop. This is a safety factor & it means that in our example building we only have to have 1/2psi (or 8 ounces) at the boiler to get steam to the furthest radiator".

John Pughe

Clank Clank

Hmmm

Not understanding the safety factor involved in having a lower cut-in ... What's the risk that is introduced?

Fred

Not understanding the example given

I'm not understand how the quoted excerpt from Dan's book allows Pughie to conclude he only needs .5psi to get steam to his furthest radiator ? All I took away from that excerpt is that the Cut-in should never be less than twice the pressure drop, whatever that might be. 

Schvenzlerman

vaporstat setting at zero

What does the expression that you used, "lock out the boiler" mean?

Fred

Correction needed?

Actually, the more I think about it the more it seems that that excerpt should say " The Cut-in pressure should never be less that HALF the pressure drop"

It currently say "twice" which doesn't make sense unless I am misunderstanding something???

Mark N

Cut In Pressure

No it actually states twice the system pressure drop. Do you have a copy of TLAOSH. If you do read chapter 2. If not I would suggest you buy the book and read it. In chapter 2 it is explained how system pressure drop is determined.

Fred

Got it

I have the book and re-read that chapter. I got it now. Thanks

Jamie Hall

Schvenzlerman asked

what I meant by the possibility of a vapourstat being set too low and locking out the boiler.



What it is is this.  As the pressure drops, the diaphragm in the vapourstat relaxes.  At a certain point, it relaxes enough so that the switch flips from off to on, allowing the boiler to fire if the other controls say "go".



If the cutin is set too low, it is possible for the pressure being sensed by the diaphragm to never drop low enough for the diaphragm to relax enough to flip the switch -- and if that happens, the boiler can't run.  Hence the term "lockout".



Problem is, it won't do it every time.



If  vapourstats were perfectly repeatable, then this would not be a problem -- but they aren't.  There are small mechanical friction losses involved, as well as other sources of small variations in the pressure both at which the vapourstat will cut out and the pressure at which it will cut in (and, yes, it is possible to build them to minimize that problem; the old mercury switch ones were much better in that regard.  You don't want to pay what it would cost to build one with significantly better precision today, though).



End result?  Don't set that cut in too low.  If you really want to skate at the edge, you probably will be alright with a cutin of an ounce or two.  If you want to be more certain, best not to have the cutin much less than half the cutout.