Field guide · Vent charts
Vent ounces and minutes
Capacity charts for one-pipe steam vents are not a single CFM number. They are columns at 1, 2, and 3 ounces of boiler pressure, read against the minutes you will give the pipe or radiator to clear. This page explains those two knobs — not the CFM figures themselves.
Estimate only
Two inputs, one rate
Volume ÷ minutes, then pick a chart column.
Both the main and radiator methods in Gill & Pajek follow the same shape:
The required rate is then compared to published vent capacities at the ounce column that matches how the boiler is actually set. Wrong column or unrealistic minutes will push the pick high or low even when the volume math is right.
The 1 / 2 / 3 oz columns
The Gill & Pajek venting-capacity chart lists CFM at 1 oz, 2 oz, and 3 oz boiler pressure. Those are ounces per square inch — the low working pressures typical of one-pipe steam — not pounds. Pick the column that matches the pressure the boiler runs at while the vents are doing their job.
- Match the cut-in / working set — if the boiler is held near 2 oz during warmup, read the 2 oz column. Do not size from a higher column the plant never reaches.
- Higher ounces, higher published CFM — the same vent passes more air at 3 oz than at 1 oz in the measured tables. That is why the column choice changes the count.
- Some cells are blank on purpose — the source notes that certain open-pipe rows needed at least 3 ounces for the flowmeter to read. Where a cell is unpublished, the calculators say so instead of filling a guess.
Gorton's own literature does not publish CFM by ounce. It gives relative ratings and placement language. Absolute CFM in the calculators comes from the Gill & Pajek instrumented chart.
About a three-minute clear time
The calculators default target minutes to three because that is the working default used with the source balancing method: give the main (or radiator plus runout) a few minutes to dump its air, then match vents to the resulting rate. Three minutes is a starting clock, not a code requirement.
Shorter targets raise required CFM and push toward more or larger vents. Longer targets lower the rate and can under-vent a cold wing if the building needs steam there sooner. When you can, time an open tapping or a pulled vent on a fire-up and set the minutes from what the job actually does.
No stack of vents can pass more air than the open tapping itself. If the calculated rate sits above the published open-pipe ceiling for that tapping at your ounce column, fix the tapping or the target time — do not keep adding vents past the pipe.
Where to apply it
Same ounce column and clear-time idea on both steam tools:
- Main vent sizer — main air volume from pipe size and length, then rate and #2 / #1 count. Volume walkthrough: steam main air volume; count walkthrough: how many Gorton #2.
- Radiator vent sizer — radiator plus runout volume, then C vs D (and smaller letters) from the same ounce columns. Pick walkthrough: Gorton C or D; thermostat-room caution: don't oversize the thermostat vent.
Next step
Set the ounce column to the boiler's working pressure and keep target minutes near three unless you have timed the job otherwise. Read capacities from the calculator tables — not from memory.
Related: Steam main air volume · How many Gorton #2 · Gorton C or D · Don't oversize the thermostat vent · All guides
Sources
Ounce columns, open-pipe notes, and the balancing procedure come from the public Gill & Pajek PDF. Relative ratings and placement language come from Gorton product materials and the air-valve chart. This guide explains the inputs only — it does not invent CFM figures.