Need advice on Vapor-stat selection

Mike_Sheppard

So there is a building in DC I ran a service call to a few weeks back. It has a brand new steam boiler in it, Smith sectional with a PowerFlame on/off J burner. 1,000,000 btu/hr input. It is only used to supply heat.

Without doing a heat load, I am certain this boiler is over-sized. It is a 1,000,000 btu boiler with a 3 inch main in the building. It is two stories and about 100ft long. Maybe the size of a large house. I wouldn't be surprised if this boiler was twice the size it needs to be.

It currently has a Honeywell PA404A1033 pressuretrol on it. Which is not the best for running at very low pressures. But the boiler, being over-sized, makes steam pressure very quickly and short cycles. I know they should likely have a vapor-stat on this boiler, but I don't know how well that would work with the over-sized boiler and short cycling.

What would you guys suggest doing? I could under-fire the burner, but then there might be an issue with condensing flue gasses. Would a 0-4 psi vapor-stat be better than a 1-16oz vaporstat in this situation?

Fred

Does this boiler have a two stage gas valve on it? If so, Is it wired to operate on both stages? Sounds like that boiler may run well on low fire, most, is not all of the time. I would say the 0 - 4 PSI Vaporstat may be better. You can get down to the ranges of the 0 - 16 ounce Vaporstat but also go up to 2 PSI, if necessary. I wouldn't go past 2 PSI Cut-out.

Mike_Sheppard

Fred said:

Does this boiler have a two stage gas valve on it? If so, Is it wired to operate on both stages? Sounds like that boiler may run well on low fire, most, is not all of the time. I would say the 0 - 4 PSI Vaporstat may be better. You can get down to the ranges of the 0 - 16 ounce Vaporstat but also go up to 2 PSI, if necessary. I wouldn't go past 2 PSI Cut-out.

Nope, it's on/off. No staging at all. I'm thinking the 0 - 4 may be better as well. When I arrived there it was cutting in at 8 and out at 12 and had run like that all winter.

Fred

You can try cutting in at 4 and out at 14. That will save one or two short cycles per heating cycle. Hopefully you have a lot of main venting on that system. Getting the air out fast will help even more than the Vaporstat. Eliminates pressure build up during the time the steam is pushing air out of each main. Has anyone calculated the total connected radiation? Maybe the connected EDR is closer to the boiler output than you think. If the boiler is that much over sized, a two stage gas valve will help a lot too.

JUGHNE

Mike, it that 8-12 range PSI?

Mad Dog_2

Good luck. Very disappointed with not just Vapostats, but Honeywell gas valves not doing what they claim. Boy do I miss Bill Wolfe. Is there any Honeywell presence here anymore? Mad Dog

Jamie Hall

I'd use the 0 to 4 psi, and try cutting out at 2 psi.

However, I would also put a low pressure gauge on there -- say 0 to 5 psi -- and watch what it does. If it rises to say half to one psi (hopefully even less!) and then stays there for a while -- at least long enough to get all the mains hot to the ends, before starting to rise again, you may -- may -- have enough main venting. However if it just continues to rise slowly to cutout, with no plateau, then you surely do not have enough main venting -- and more would help the short cycles immensely.

I agree -- that boiler is almost certainly oversized. The main place I care for isn't any smaller than that, and operates very nicely, thank you, at -10 F with a boiler firing at less than 400,000 BTUh.

the_donut

Stinks that there is no staging. Vaporstat won’t help much with short cycling. I had an issue with the boiler short cycling because everyone shut off supply valves to radiators. A few of the apartments with overhead mains were getting cooked while upstairs was living comfortably.

If they are closed, find out why. Fix that and cycling will be better. Went from cycling every 2 minutes to 55 minute cycles after setbacks after opening all supplies.

Mike_Sheppard

Yes, it’s operating currently at 8-12 PSI! That’s how it was when I saw it for the first time. No, the EDR has not been measured. I plan on speaking with the customer about this. But I doubt they will do anything about it. The boiler is only 4-5 months old. The original service call was for “pressure gauge not reading any pressure”. They installed it without a pigtail and ruined the gauge.

Whoever installed it also piped the take offs between the risers.

For comparison, let’s say this building is 100 feet long, 50 feet wide, and two stories. It has a 1,000,000 btu boiler in it. The building I just got done working in today is 100 feet long, 100 feet wide, and six stories, and is running on a 900,000 btu boiler.

Jamie, that’s a good idea. I forgot about the plateau. Whenever I get back out there I will check it out. I’m putting a pigtail with a new 0-30 gauge (required by code here) also installing a vapor stat and a low pressure gauge to read low pressure. Might look into underfiring the burner some. If I can underfire it but not drop the exhaust temp below dew point perhaps that would help some.

Thanks for all the advice.

Fred

@Mike_Sheppard , do you really mean it is running at 8 - 12 PSI or 8 -12 Ounces?

Mike_Sheppard

@Fred, it WAS running at 8-12 PSI. That’s how I found it. When I left, it was turned down as much as possible.

Yes, PSI, not ounces.

Fred

OMG!!! And it was still short cycling??? Any vents on that system are toast. There had to be leaks everywhere, especially at the valves.

Mike_Sheppard

Well on top of the high pressure setting it had, the thing is probably more than double oversized for the building it’s in. So yeah... quite a mess I found. They had steam dumping out of the feedwater tank too. Not sure if they had bad traps or the steam pressure was so high the condensate was flashing to steam after the traps... at least they didn’t have a master trap at the feedwater tank. I have come across “master traps” two days in a row now.

Will be going back soon to install vaporstat, new pressure gauges, and will begin to analyzer the rest of the system.

Fred

Traps have probably been ruined by that pressure.

Mike_Sheppard

Update:

Installed the 0-5 Pressuretrol today and 0-4 gauge. Started the boiler and the steam pressure plateaued around 6oz before rising again. It rose fairly quickly after that due to the oversizing. I set the cutout at 1.5 and cut in at a little under a half psi. System working much better now. Will push for checking traps and air vents next.

the_donut

At some point I will have to convince the team that 6 psi isn’t a good pressure for a 1920’s 2 pipe system. Haven’t seen all the pipework, but I am pretty sure it was a vacuum system back in the day. Also the boiler feed pump in is master trapped. At least they replaced bad traps and new valves this year?

Mike_Sheppard

@the_donut I came across a similar situation early this winter. Probably close to a million dollar job if not more. Three brand new boilers for large apartment building. The system being replaced was a vacuum system. The vacuum system was removed as part of the “upgrade” and replaced by a standard feed unit. Also as part of the “upgrade” they were guaranteed they wouldn’t have to deal with bad radiator traps ever again, because you guessed it: master trap baby! Why did no one think of this in the first place? Genius!

And then winter comes. Half the building is not heating, half of it has open windows, nobody can sleep with the water hammer, and the fuel bills went through the roof.

Million dollar ignorance.

nicholas bonham-carter

The nice thing about the 0-16 ounce pressurestat is the inability of the in house maintenance staff to crank it up.
If you have 6 ounces during the early venting phase, then more main venting is needed. Maybe the main vent after the crossover traps needs enlarging.
If they are using setbacks, then disable those. What sort of control is calling for heat?—NBC