Is this a venting issue

Steve Nichols

HI Folks, Steve's back again.

Okay, I have a gas fired steam boiler (IN5).  This is oversized for my connected load (actual EDR = 260; IBR of boiler 358...I know I know..)

I have 1 pipe steam connected to thin tube convectors through entire house.   Two 2"ID mains supply steam to the front and back side of my house.  I have done the volume calcs and have vented accordingly. Front main (30') has 2 Gorton 1's and back main (45') has 4 Gorton 1's.  I cant fit G2's due to clearance between main and "ceiling" of basement.  Mains shut within a minute of each other.  Three minutes to close off both main vents from time boiler produces steam.

 

<strong>That's the background.  Here is the issue:

</strong>Upon initial startup, boiler runs and produces steam at each main vent in a total time of about 13 minutes.  From that time, my pressure gauge barely moves; it's a  0-3 low PSI just hovering above 0. Over the next 6 minutes, as the convector vents start shutting, pressure slowly rises until boiler shuts off on pressuretrol setting (1.5psi).  Total elapsed time from burner fire to pressuretrol stop = 20 minutes.



This is where it gets interesting. The gauge pressure drops down to 0.5 in about a minute and keeps falling.  I don't hear any vents opening on convectors or mains. 

If Tstat still calls for heat, (which is typically does), it takes about 2 minutes for pressure to climbed back to 1.5psi, at which point boiler shuts off again. This cycle repeats until Tstat is satisfied.

I have read a lot about venting but am still unsure of whether the mains should be breathing or not during the time the pressure is dropping.  I get times where the gauge shows negative pressure before the burners re-light.

The only time I hear those G1's open up is if the boiler does not re-fire (Tstat satisfied) and then within a minute, I hear them all opening  with a large whoosh...



Aside from the oversized boiler causing the rapid increase in pressure, is there something I'm missing in terms of how the venting should behave?  I have all Ventrite 31/33's on the convectors.  I can only fit the upright bullet style due to their being right up against the recessed cabinet walls.



I may be missing something here so please feel free to ask away!  You all have been very helpful so far!

BobC

Not much you can do

There has been some discussion about the Gorton vents closing at 145 degrees on other threads. This tells me these vents are not going to open till they fall below 145 or so and all that cast iron holds on to heat very well.



You might be able to downfire the boiler by a bit but I doubt it will be enough to stop the short  cycling.

AlexR

sounds normal

That sounds typical for an oversized boiler.  Mine does the same thing, though it takes longer to start cycling because it takes my cast iron radiators a while to heat up.



The only thing you might do to avoid concerns about the vents opening during the cycling- tweak the differential on your pressuretrol/vaporstat to avoid the vacuum.  It takes my boiler about 30s to relight once the vaporstat trips because of the automatic damper and such, so I set the differential to the lowest value that avoided a vacuum. 

Steve Nichols

Bob and Alex

Thanks for the advice.   I knew the oversized boiler would be an issue.  I may investigate whether it can be downfired a bit but I'm skeptical.

That's a good idea on setting the differential.  Sometimes it pulls vacuum, other times the burner fires off before that happens.  I don't think piping is ideal so I may be getting as much as I can out of this until I can correct some stuff. 

Just trying to get it as good as it can be considering all the factors.



Steve

nicholas bonham-carter

Vents not opening with vacuum

It looks like you have short-cycling after the vents have closed-is that the case?

I am surprised that the vents are not relieving the vacuum, which is job 2 for the main vents.

What is the back-pressure during the venting phase. I usually see 1ounce on my 55 rads. After the vents have closed, then there can be an increase, but the pressure during venting is more important, to verify the capacity of main venting. Is the low-pressure gauge on the same pigtail as the pressuretrol, as it should be? --NBC

Steve Nichols

NBC

Yes, I am experiencing short-cycling after vents close.

I have my low pressure gauge teed off the pigtail directly below my pressuretrol. During venting the gauge barely moves off of the zero mark.  I would guess 0.1psi or so during main venting phase.

The Gortons stay shut well after the boiler has shut off (Tstat satisfied) then they all open at once and I can hear them from upstairs, but as had been noted, the Gortons won't open until below 135.  Perhaps this is the nature of the bimetal and the residual heat in the mains working up through the antler of my vents?

nicholas bonham-carter

Overheated vents

Are those vents up on an antler? That protects them from water-hammer, and enables them to cool down.--nbc

Still check your pigtail, as I would expect some small pressure to show on the gauge.

Steve Nichols

antlers it is!

HI NBC,

Yes, both front and rear main vents are up on antler.  I have 4" piece of 3/4 nipple going to the antler arrangement on both mains.  The gauge shows a little movement but not too much. If I use the marks on the gauge as a reference, I'm probably getting about 1- 2 Oz of pressure during venting phase.

This entire boiler replacment is about 1 month old but I can pull the pigtail and check it.

Thanks for the quick replies.  I'll check back in tomorrow when I'm more awake.

-S

Dave in QCA

My 2 cents

It was stated I think, but to clarify,



When all of your vents have closed and then the boiler shuts off on pressure, as long as there is still a call for heat at the thermostat you do not want the system to breath before it restarts when the pressure drops.  As long as the thermostat is calling for heat you want to maintain a steam condition in the system whether or not the boiler is firing, because in your case, the fire is shutting off to control the pressure.



As for the vacuum, the Gortons may be slow to open, but they will open when they cool down and your system may take a little longer to relieve the vacuum, but that won't matter.  It is important for venting to be fast when the steam is coming up.  In most situtations, the speed of venting to allow air back in is not important, with the key being "in most situations." 

Steve Nichols

That makes sense

Thanks Dave,

That makes sense. Filling the mains with air just means having to push it out again...more fuel, etc.