One-pipe steam · P0
Steam main vent sizing
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.
Direct answer
How many main vents? Schedule 40 main cu ft ÷ target minutes = required CFM. Example: 40 ft of 2" main ≈ 0.93 cu ft ÷ 3 min ≈ 0.31 CFM, so one Gorton #2 (1.75 CFM at 2 oz) covers it. Match to Gorton #2 (1.1 / 1.75 / 2.2 CFM) or #1 (0.33 / 0.54 / 0.70) at your 1 / 2 / 3 oz column—not PSI cut-in—then round up and cap by the open-pipe tapping ceiling.
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.
FAQ
How many main vents do I need on a steam main?
Enter main size and length → Schedule 40 air volume → ÷ target minutes → required CFM. Match to published Gorton #2 (1.1 / 1.75 / 2.2 CFM) or #1 (0.33 / 0.54 / 0.70) at your 1 / 2 / 3 oz column, round up, and cap by open-pipe tapping capacity. Gill & Pajek chart on this page.
How many Gorton #2 main vents do I need?
Required CFM ÷ #2 capacity at your ounce column, rounded up. Cap by the tapping. Gorton’s “#2 = four #1” is relative only — use absolute CFM here. Walkthrough: /guides/gorton-2-how-many.
Why does ounce column matter more than PSI?
Capacity charts list CFM at 1 / 2 / 3 ounces per square inch of boiler pressure during venting. Pressuretrol cut-in in PSI is a different scale — reading the wrong column changes the count. See /guides/vent-oz-and-minutes.
What if I need more vents than the tapping can pass?
No stack of vents beats open-pipe capacity for that tapping size at your ounce column. Enlarge or add tappings, or relax the clear-time target — do not keep stacking past the pipe. Open-pipe table is on this page.
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.