One-pipe steam · P0
Radiator vent sizer
Add the air in the radiator to the air in its runout, divide by how long you will give it, and match that rate to a Gorton vent. Then adjust for where the radiator actually sits — the math gets you close, geography finishes the job.
The radiator
Equivalent direct radiation — count the sections and look them up, or measure.
Sets the air volume per square foot EDR. Cast-iron tube (circa 1930) at 0.013 cu ft/EDR is the common case.
The runout
The runout has to be filled with steam too, and it tries to steal some on the way.
Conditions
Higher pressure pushes more air through the same vent, so the same radiator takes a smaller vent at 3 oz than at 1 oz.
Three minutes is the working default in the source method.
Use the math answer as-is.
Air to move
| Source | Rate | cu ft |
|---|---|---|
| Radiator | 0.013 /EDR | 0.520 |
| Runout (1-1/2") | 0.0141 /ft | 0.283 |
Vent recommendation
Capacities read from the 1 oz column.
Install
Gorton C
Straight from the math, with no geography adjustment.
“Install on radiators farthest from the boiler, and in coldest rooms (for example, third floor radiators).” — Gorton
| Gorton vent | CFM @ 1 oz | Verdict |
|---|---|---|
| Gorton #4 | 0.025 | Too small |
| Gorton #5 | 0.080 | Too small |
| Gorton #6 | 0.150 | Too small |
| Gorton C | 0.270 | Recommended |
| Gorton D | 0.330 | Enough capacity |
Radiator vent capacities
CFM at boiler pressure, as published.
| Vent | 1 oz | 2 oz | 3 oz | Gorton's own rating |
|---|---|---|---|---|
| Gorton #4 | 0.025 | 0.040 | 0.055 | — |
| Gorton #5 | 0.080 | 0.130 | 0.160 | Equal to 4 ordinary type air valves. |
| Gorton #6 | 0.150 | 0.235 | 0.300 | Equal to 8 ordinary type air valves. |
| Gorton C | 0.270 | 0.450 | 0.570 | Equal to 15 ordinary type air valves. |
| Gorton D | 0.330 | 0.540 | 0.700 | Equal to 27 ordinary type air valves. |
The CFM columns are measured data from the HeatingHelp capacity chart. The right-hand column is Gorton's own wording — “equal to N ordinary type air valves” — reproduced as published. It is not the basis of any calculation here, because an “ordinary type air valve” has no defined flow rate.
Air per square foot EDR
By radiator construction.
| Radiator type | cu ft / EDR |
|---|---|
| Cast-iron tube (circa 1930) | 0.013 |
| Cast-iron flue (antique radiators) | 0.029 |
| Cast-iron wall- or ceiling-mounted | 0.028 |
| Cast-iron column (circa 1900) | 0.025 |
| Cast-iron thin tube (as made today) | 0.009 |
| Cast-iron radiant radiator (5" deep) | 0.008 |
| Cast-iron baseboard (10"-high) | 0.07 |
| Cast-iron convector (in cabinet) | 0.0003 |
| Unit heater | 0.0002 |
Worked example from the source
40 sq ft EDR cast-iron tube radiator, 20 ft of 1-1/2" runout, 1 oz, three minutes:
- runout 20 × 0.014 = 0.28 cu ft
- radiator 40 × 0.013 = 0.52 cu ft
- total 0.80 cu ft ÷ 3 min = 0.26 CFM
A Gorton C passes 0.270 CFM at 1 oz, so it covers it — but only just, and the radiator is not close to the main. The source blends in geography and raises it to a Gorton D. Set the calculator to those inputs and you will see the same path.
Method and sources
- Radiator air = EDR × cubic feet per EDR for its construction.
- Runout air = length × cubic feet per linear foot (Schedule 40).
- Required rate (CFM) = (radiator + runout) ÷ target minutes.
- Pick the smallest Gorton vent that meets the rate at the working pressure, then adjust up or down for geography.
One vent that should stay small