Free tool · One-pipe steam
Radiator vent speed calculator
Put every common radiator vent on the same radiator and see how many minutes each takes to push the air out — the fastest way to choose between Gorton sizes or judge an adjustable Hoffman 1A.
Direct answer
Minutes = (radiator air + runout air) ÷ vent CFM. A 40 EDR tube radiator with 20 ft of 1-1/2" runout holds about 0.80 cu ft: about 3.0 min on a Gorton C, 2.4 min on a D, and about 5.5 min on a Hoffman 1A wide open, at 1 oz (Gill & Pajek).
One radiator
Air in the radiator + its runout (Gill & Pajek worksheet).
| Vent | CFM @ 1 oz | Minutes to vent |
|---|---|---|
| Gorton #4 | 0.025 | 32.1 |
| Gorton #5 | 0.080 | 10.0 |
| Gorton #6 | 0.150 | 5.4 |
| Gorton C | 0.270 | 3.0 |
| Gorton D | 0.330 | 2.4 |
| Hoffman 1A (setting 6, max) | 0.145 | 5.5 |
The judgment call
You are not looking for the fastest vent; you are looking for radiators that all finish at about the same time. Pick the vent whose minutes land close to the rest of the house, faster on far and cold rooms, slower near the boiler and in the thermostat room. If every radiator is slow, the mains are usually the problem: fix them with the Gorton #2 main vent calculator before buying radiator vents.
The Hoffman 1A row is setting 6 (max) only. Lower settings are not published, so none are shown. For a pick with geography built in, use the radiator vent sizer; for the thermostat room, read don't oversize the thermostat vent.
Estimate only
FAQ
How long should a steam radiator take to vent?
There is no single number. Divide the air in the radiator and runout by the vent CFM, then aim for radiators that heat at roughly the same time, with far and cold rooms a little faster and the thermostat room a little slower.
Which Gorton radiator vent is fastest?
Gorton D: 0.33 / 0.54 / 0.70 CFM at 1 / 2 / 3 oz on the Gill & Pajek chart, ahead of C at 0.27 / 0.45 / 0.57.
Is a Hoffman 1A as fast as a Gorton C?
No. At setting 6 (max) the Hoffman 1A measured 0.145 / 0.225 / 0.300 CFM at 1 / 2 / 3 oz, roughly half a Gorton C.
Why does radiator construction matter?
Air per square foot EDR differs by type, from 0.013 cu ft for a circa-1930 tube radiator to 0.025 for column and 0.029 for flue radiators (Gill & Pajek p.9). More air takes longer to vent.
Sources
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Need the full worksheet? Open the Radiator vent sizer.
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