Gorton C or D?

Before steam can heat the iron, the air inside the radiator and its supply runout has to leave through the vent. A larger EDR and a longer or fatter runout both raise the required venting rate. Gorton C and D are the common “far / cold room” picks in Gorton's placement language; smaller letters (4, 5, 6) cover nearer radiators when the math says so.

Read the radiator's square-foot EDR from the nameplate or a takeoff. Note the construction (tube, column, etc.) — the calculator applies published cubic-feet-of-air per square foot EDR for that type, following the Gill & Pajek worksheet approach.

Measure the runout length and nominal size from the main to the radiator valve. Convert with Schedule 40 volume per foot, same as on the main. Total air to vent is radiator air plus runout air.

Compare that rate to the published Gorton radiator-vent capacities at your working ounce column. Pick the smallest vent that meets the rate, then step up or down for geography: far from the boiler and cold rooms usually want more venting; near-boiler warm rooms usually want less.

Gorton's own chart describes C as for radiators farthest from the boiler and coldest rooms, and D where a large amount of air must be vented (with a hand-fired caution). Those are placement hints — the absolute CFM figures the calculator uses come from the Gill & Pajek test chart, not from converting “equal to N ordinary valves” into flow.

Work the numbers in the radiator vent sizer rather than memorizing capacities. The page also shows the source's worked example path from a modest EDR and runout into a C that the geography bump can raise to a D.