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# Main vent sizing confusion

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Member Posts: 8
Reading through the Gerry Gill main vent sizing, I wanted to do some calculations for myself. Assuming 1 oz pressure table, I realized that my three mains are undervented, and two of them with radiator vents no less...

Current state:

Main #1: 1.05 ft^3 Vent-Rite 35 (.110 ft^3/min @1oz)
Main #2: .543 ft^3 Flair #51 (.116 ft^3/min @1oz)
Main #3: .427 ft^3 Flair #51 (.116 ft^3/min @1oz)

The vent sizing technique suggests using 3 min to vent the air in the pipes, but I figure sizing for ~2 minutes will only help.

My new plan:

Main #1: Two Gorton #1 (0.33*2=.66 ft^3/min, ~2 min to vent Main #1)
Main #2: One Gorton #1 (.33ft^3/min, ~2 min to vent Main #2)
Main #3: One Gorton #1 (.33ft^3/min, ~<2 min to vent Main #3)

Here's the confusion. I've read probably 10 threads saying the rule of thumb is Gorton #2 for every 20 feet of 2 inch pipe.

2 inch pipe = .023 ft^3 volume per linear foot of pipe

.023*20=.46ft^3

Assuming the standard suggested 3 minute purge, I need a vent that can handle .46ft^3/3min=.15ft^3/min.

By the 3 minute logic, Gorton #2 @ 1.1 ft^3 / min is way oversized.

I cannot figure what I'm doing wrong, but the math doesn't work. Is this rule of thumb inconsistent with the Garry Gill method, or am I missing something?

• Member Posts: 201
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You could try the lower number then time it and adjust if needed. Vents are pricey these days.

I added vents based on the more is better philosophy a long time ago.  My longest main has 6 Gorton 1's and a Hoffman 75 on an antler.  Also a Gorton D tapped on a nipple in the middle.
• Member Posts: 5,737
edited January 2022
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You aren’t starting at 1 ounce, and if your doing things right you won’t get to 1 ounce until vents on the radiators start closing.  They had to measure vent speed with some measurable pressure due to limitations of the gauges.

The chart is just a guide for relative speed from one vent to another.  Running the calculations as you have gives you a starting point, but isn’t the end.

I have a 25’ main with 3 Gorton #1 vents and it takes 3 minutes from burner on to vent the entire main.
2014 Weil Mclain EG-40
Boiler pictures updated 2/21/15
• Member Posts: 8
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All great explanations, makes sense to me.. I guess the book was a little misleading in how they walked through the calculations, though it does also describe the "common sense" test of testing the system reaction time with no main vent.

I'll stick with my plan for now, and may end up introducing a Gorton 2 into the mix eventually.
• Member Posts: 8
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Looking for some help -- I timed my system from a cold start (system resting for 2-3 hours), main vents removed, to see how long it takes to vent.

I was surprised to see that all three main vents were around 9-12 minutes, quite a long time.

The main vents are located at the end of the return line, next to the boiler, before it drops vertically down. My pipes are not insulated, which probably is contributing.

Is this normal? I was expecting closer to 3-5 minutes, based on my air volume. Is there something else I should be looking at in my system before I test main vents?
• Member Posts: 5,737
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The time is to get steam to the last radiator on the main, not necessarily to the vent location.

Cold starts will be a little slower due to heating up the mains.
2014 Weil Mclain EG-40
Boiler pictures updated 2/21/15
• Member Posts: 201
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Can't help,  I insulated my mains and even insulated my returns when I replaced them
• Member Posts: 5,737
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NHOwner said:

KC_Jones said:

You aren’t starting at 1 ounce, and if your doing things right you won’t get to 1 ounce until vents on the radiators start closing.  They had to measure vent speed with some measurable pressure due to limitations of the gauges.

The chart is just a guide for relative speed from one vent to another.  Running the calculations as you have gives you a starting point, but isn’t the end.

I have a 25’ main with 3 Gorton #1 vents and it takes 3 minutes from burner on to vent the entire main.

That's not very impressive. My main is about 45 feet long. Took about 3 minutes from the riser and header being cold to the touch to the air leaving the main air vent at the end of the line. I have only 1 maid o mist main air vent. Maybe you have an undersized boiler? I have a Burnham IN5.
My understanding of your comment is that it took 3 minutes after the boiler fired for air to start coming out. If that's the case, to compare to my comment. I had steam at the main vent 3 minutes after the boiler fired, not just air coming out. That's the time I get from the end of main sensor, measured by the PLC controller.

My boiler is not undersized.
2014 Weil Mclain EG-40
Boiler pictures updated 2/21/15
• Member Posts: 5,702
edited January 2022
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Don't time the cold system if you're trying to figure out if the main vents are worthy. Time it hot. A cold system is going to take extra time to get steam to the vents regardless of insulation. In the winter when you are doing lots of calls for heat, and quick response time is more important for comfort, the system is staying pretty hot.

A more realistic test would be to wait 30 minutes since the last firing--that would approximate a 2 calls per hour scenario that is common for steam.

If you have a cold drafty basement especially, then yes go ahead and insulate but in my basement, insulation has not changed very much about my system.
NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
• Member Posts: 201
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If you have a masonry house with solid brick walls you will probably average 1 call every 2 hours or do just because of thermal mass.

In that case insulation is important
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Can anyone answer if the ptrol/vaporstat in the photo has a mercury switch? If so, I believe the pigtail should be rotated by 90 degrees.
• Member Posts: 201
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What bothers me
Size of those mains relative to the size of the header.  Looks like 2" header feeding two 3 or 4 inch mains.  Those mains have a lot of air that needs to be evacuated

Only one of the two supply taps is used
• Member Posts: 23,303
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That's a newer one with a microswitch, from what I can see, @SteamingatMohawk
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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@Jamie Hall I knew you would have the eagle eyes to figure it out.
• Member Posts: 8
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What bothers me

Size of those mains relative to the size of the header.  Looks like 2" header feeding two 3 or 4 inch mains.  Those mains have a lot of air that needs to be evacuated

Only one of the two supply taps is used
The picture can be misleading -- it's a 2 inch header feeding two 2-inch mains, confirmed by measured circumference. I do think using two supply taps would have been ideal.
NHOwner said:

This is a burnham.

Do you have a "dry return?" How is your one pipe system set up?

If I understand the question correctly, three main vents are located at the three return lines where it is a dry return. The main vent does produce dry steam (tested with main vents removed) before it drops into the wet return below the boiler water line and into the Hartford loop.

(as an FYI, I've since changed out the vents to Gorton configuration as discussed in the first post)

As for the overall system, it's a single 2 inch header, splits in two directions. Facing the boiler, right side is Main #3. Left side splits into Main #1 and Main #2. 3 dry returns lines are plumbed for each main. What may not be ideal is that there are some radiator lines that continue onwards past the dry return U-turn plumbing.

I can take more pictures if anything I've mentioned sounds suspects and should be examined further. Thanks for all your help!

• Member Posts: 8
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NHOwner said:
Ok, so i measured my riser and header. They are both 2 1/ 4 iches wide. The pipe that goes from header to main line is 2 3/8 inches wide. Could that be the problem? Small pipes?
Convention with steel pipe is nominal size. If those dimensions you're measuring are outer diameter, then you mostly likely have 1.5" nominal pipe and 2" nominal pipe for the two dimensions you shared.

https://www.pdblowers.com/tech-talk/schedule-40-pipe-data/