Two pipe vapor system with vapor stat: to cycle or not to cycle?

motordiscord628 · 4:34AM

My two pipe vapor system now has a new main vent (the saga of the missing main vent can be seen here) and a vaporstat in in parallel with a pressuretrol. The ancient honeywell 77 was replaced in favor of a TH6220WF2006 to have control over cycles per hour (and wifi connectivity ain't bad either).

So the question now is about pressure cut out and if I should be shooting for higher pressures with no cut out or a lower pressure with cycling from the vaporstat.

Current setting is about 22" h2o (~12in/oz) resulting in no cycling from the vapor stat. Setting it lower will of course resulting in some cycling. All of the radiators get hot quick with the 12in/oz setting but the interesting point for me is the main vent. At the 12oz setting the vent vents and then kinda stops/slowly oozes air - steam never reaches it. If I set it any lower, the system cycles when the boiler in an out (the gauge never goes to zero and the main vent doesn't seem to "inhale") the main vent seems much more "active".

That said, I did a little test at 8oz, 10oz, 11oz, and 12 and the results weren't pretty.

8oz = 2:19 from boil on to shut down - 30 seconds off before restart - 1:30 to shut down

10oz = almost identical except it was 1:45 on vs 30 seconds off

11oz = 3:18on

12oz = on until call ends.

It strikes me as too much cycling but wanted an opinion. Also we just got our first full month's gas bill and it was eye watering… we've had a few sub 10º days and this house has stood without any insulation for 120 years… but $650 a month for gas… such pain. I'm curious what the previous owners were paying back when there wasn't a main vent, 3 of the radiators didn't work, and the pressure was set at 3psi! Hudson valley NY, BTW.

delcrossv · 4:47AM

Wonder what the restriction is that you'd need 12 oz. to not cycle on pressure.

EBEBRATT-Ed · 5:11AM

Doesn't make sense to me that at 11oz you get a 3 :18min run time and at 12oz it will not cycle on pressure.

motordiscord628 · 2:29PM

https://forum.heatinghelp.com/discussion/comment/1835537#Comment_1835537

Likely the boiler is just too big. We have 2 rooms closed off too which eliminates a bunch of EDR but yeah… pretty sure boiler is too big by about 25%

Jamie Hall · 3:45PM

2:19 from start of boil to shutdown? No. Something is seriously wrong with the venting. Or, possibly, the piping to the main vent.

You say this is a vapour steam system. How, exactly, are the mains and dry returns vented? I've looked at the previous thread, but it isn't clear to me what was done about main and dry return venting.

The main vent on the dry returns, whether there are crossover traps from the mains to the dry returns or not, should NEVER close — never see steam at all. Once the boiler fires, you can feel the progress of steam along the mains — 20 feet per minute is not a bad sort of ball park figure — and that should be steady progress. If there are vents on the mains, they should close when steam hits them — likewise crossover traps.

I can't help but think that somewhere in there there is a significant restriction — but what it is I couldn't say without really looking at the system.

There is another possibility, however, which has occurred to me. Where are the vapourstat and pressure gauge connected? If they are connected directly to the steam chest — which is the usual connection, through a nipple or through the sight glass — is the vapourstat responding to real system pressure, or is it responding to pressure oscillations in the steam chest? If the pressure gauge is on the same pigtail, is it bouncing around as steam is rising, or is it rising steadily? Note that this is not related to surging water level! If the pressure gauge is bouncing around, it is likely responding to steam pressure variations as bubbles form and burst, not to the steady pressure found in the header — and the mains. This could be the cause of the quick shutdown, as vapourstats are quite sensitive to pressure spikes of this sort. There are ways to cure this problem. The best is to connect the vapourstat and low pressure gauge to the header, not to the boiler. Another is to use a snubber on the pigtail, but that is problematic as snubbers have a way of clogging.

Whatever, it ain't right.

delcrossv · 7:04PM

If I had to guess, missing or bad crossovers. As @Jamie Hall says, something is restrictive and definitely not right. Absent a restriction elsewhere, steam chest oscillations shouldn't normally trip a vaporstat because the steam chest has a huge outlet to the header and mains.

Tripping because the boiler is too big happens when the traps are hot and closed because there's more input than the rads can condense. That's a lot more then 3 minutes.

Two pipe vapor system with vapor stat: to cycle or not to cycle?

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