One-pipe steam · P0
Steam main vent sizer
Work out how much air is sitting in the main, divide by how fast you want it gone, and match that rate to vents whose capacity has actually been measured. The tapping size is included because it sets a ceiling no amount of venting can beat.
The main
One row per run. Include the vertical main up to the top floor if you want it vented at the same rate.
Conditions
The pressure column the capacity data was measured at. Vents pass more air at higher pressure, so the answer moves with it.
Three minutes is the working default in the source method. Time an open tapping first if you can — no number of vents beats that.
The tapping is a hard ceiling on the manifold. Set to “Not considered” to see the raw vent count.
Air to move
| Run | cu ft / ft | Length | cu ft |
|---|---|---|---|
| 2" | 0.0233 | 40 ft | 0.932 |
Vent recommendation
Capacities read from the 2 oz column.
1 × Gorton #2
1.75 CFM
Gorton #2 only
1 × Gorton #1
0.54 CFM
Gorton #1 only
Capacity chart needed
No published CFM measurement exists for Gorton Big Mouth, so it is left out of the counts above rather than guessed at.
It is a large-capacity main vent and may well be the right answer here — but picking a count requires a rated figure. See the sources below, or ask the manufacturer for a measured CFM at 1, 2, and 3 oz.
Main vent capacities
CFM at boiler pressure, as published.
| Vent | 1 oz | 2 oz | 3 oz |
|---|---|---|---|
| Gorton #1 | 0.330 | 0.540 | 0.700 |
| Gorton #2 | 1.100 | 1.750 | 2.200 |
| Gorton Big Mouth | chart needed | chart needed | chart needed |
Gorton publishes no CFM ratings of its own — only relative comparisons such as “venting capacity equal to that of 4 #1 Gorton Air Eliminators.” Those are quoted in the code but never converted into a flow rate.
Open-pipe capacity
What the tapping itself will pass, wide open.
| Tapping | 1 oz | 2 oz | 3 oz |
|---|---|---|---|
| 1/8" | 1.20 | 2.00 | 2.50 |
| 1/2" | 2.60 | 3.40 | 4.80 |
| 3/4" | not measured | not measured | 9.50 |
| 1" | not measured | not measured | 11.00 |
Blank cells are genuine gaps in the test data: the source notes that “due to the size of the flowmeter and pipe, at least 3 ounces was required to procure a reading.”
Schedule 40 pipe volume
Computed from ASME B36.10M internal diameters.
| Nominal | ID (in) | Area (sq in) | cu ft / ft |
|---|---|---|---|
| 1-1/4" | 1.380 | 1.50 | 0.0104 |
| 1-1/2" | 1.610 | 2.04 | 0.0141 |
| 2" | 2.067 | 3.36 | 0.0233 |
| 2-1/2" | 2.469 | 4.79 | 0.0332 |
| 3" | 3.068 | 7.39 | 0.0513 |
| 3-1/2" | 3.548 | 9.89 | 0.0687 |
| 4" | 4.026 | 12.73 | 0.0884 |
| 5" | 5.047 | 20.01 | 0.1389 |
| 6" | 6.065 | 28.89 | 0.2006 |
| 8" | 7.981 | 50.03 | 0.3474 |
Method and sources
- Air volume = Σ (run length × cubic feet per linear foot), from Schedule 40 internal area.
- Required rate (CFM) = volume ÷ target minutes.
- Vent count = required rate ÷ published capacity at the working boiler pressure, capped by the open-pipe capacity of the tapping.
The source method treats this as a starting point, then verifies by timing: pull the vent, fire the boiler, and time steam to the open pipe. Reinstall vents until you get close to that time.