Short Cycling?

cdm

Background: 1200 sq ft. house, one pipe steam system. 236 EDR with a 78,000 BTU boiler, Mains vented @ 165% over required venting @ 1oz. System runs well at 70 degrees, and maintains temperature with no indication of pressure on a 0-5 psi gauge.

Problem: When I raise the temperature from a set-back of 65 degrees to 70 degrees, the pressure rises to 2.25 psi on my 0 to 5 psi gauge (pressuretrol) - then the burner shuts down until it reaches 0.5 psi, where it restarts. This sequence will repeat 4 cycles, until the 70 degree temperature is reached. I also see the same sequencing when I raise the temperature from 70 to 72 degrees, but with only 3 cycles, but the temperature has reached 70 degrees PRIOR to the first cycle rise in pressure to 2,25 psi

Am I missing something? If I install a vaporstat in the loop - I know I can reduce the operating pressure - but won't the short cycling increase?



thanks -cd mulford-

Jamie Hall

What you are seeing

is perfectly normal for a slightly -- but not badly -- oversize boiler.  In theory, if your boiler were perfectly sized to your load, it wouldn't cycle on pressure at all.  But it is better -- for pickup, if nothing else, for it to be slightly over.



If it is, when you are coming out of a deep setback (and 5 degrees is deep for steam; to deep in my humble opinion, but that's just me) it will get to the point where all your radiation is filled, and is condensing steam just as fast as it possibly can.  But your boiler will be producing steam a little faster, so the pressure will rise.  At which point the pressuretrol will shut it off until the radiators can catch up, then it will fire again and so on until the thermostat is happy.



So it's normal.



You could install a vapourstat, but it wouldn't change things much (although it would be nice if you could dial that pressurestat back a tad) because once the radiation is full, it doesn't take long -- a minute or two -- for the pressure to rise from running pressure (a few ounces or so) to half a pound, and then only another minute or so to get up to 2 pounds.  So nothing much would change.  The "short cycling" (it isn't really) would change a little, but not enough to make any difference.

cdm

Short Cycling?

Jamie - Thank you for your response - are you implying I should not invest in a vaporstat, and just leave well-enough-alone?



thanks -cd mulford-

Jamie Hall

Basically...

Much as I like vapourstats, and much as they do give better control, if you can dial that pressurestat back just a half a pound (which you might well be able to) that's all that could be expected or, for that matter, really needed.  And vapourstats aren't all that cheap.

cdm

Short Cycling?

Jamie - thanks for the advice.



-cd mulford-