Field guide · Radiator vents
Cold third-floor radiator on one-pipe steam
One upstairs or third-floor radiator is late or cold while nearer rooms cook, and somebody says “put a bigger vent on it.” Often the bottleneck is still air trapped in the basement main — every upstairs branch waits until that main clears. Fix the mains first; then step the late radiator orifice.
Direct answer
Cold third-floor rads usually mean starved mains, not tiny upstairs orifices. Clear mains first — Gorton #2 is 1.10 / 1.75 / 2.20 CFM at 1 / 2 / 3 oz (Gill & Pajek). Then step the late rad: 1A max 0.145 / 0.225 / 0.300 → C 0.27 / 0.45 / 0.57 → careful D; never fatten the thermostat rad.
Estimate only
What the symptom usually means
One upstairs rad late — and the wrong first fix.
The classic complaint: one upstairs or third-floor radiator is late or stays cold while rooms nearer the boiler cook. The counter advice is almost always “hang a bigger vent on it.” Sometimes that helps — but only after steam can actually reach the branch.
On one-pipe steam, upstairs branches wait until the basement main dumps its air. If the end-of-main vents are undersized or dead, every radiator on that main is starved; the farthest and highest ones show it first. Frame it this way: mains dump air fast so steam arrives together; radiator orifices only fine-tune after that.
10-minute field checklist
Do this before you buy vents.
- Supply valve fully open — not cracked, not painted stuck. A half-closed valve masquerades as a cold radiator all winter.
- Pitch and water — radiator pitched slightly toward the valve; no waterlogged spit at the vent. Pitch back toward the valve holds water and blocks steam.
- Vent itself — vertical, hole clear, not painted shut. Shake or vinegar is only a dead-vent check — not a sizing fix.
- Time the main vs the rad — fire from cold. Clock when steam reaches the end-of-main tapping versus when the upstairs rad goes hot end-to-end. If the main is still cold or hissing when upstairs should already be heating, stop upsizing radiator letters — fix the mains.
Mains first
Starved mains make every upstairs orifice look too small.
Measure each main size × length → cu ft → ÷ target minutes → required CFM. Match published Gorton #2 / #1 at the ounce column (not pressuretrol PSI). Gorton #2 is 1.10 / 1.75 / 2.20 CFM at 1 / 2 / 3 oz. Gorton places #2 “at the end of the long main running to the cold side of the building.”
Do not upsize every upstairs orifice first. A D on a starved main is a louder version of the same problem — and fast venting into a wet main invites hammer.
Work the numbers in the main vent sizer and check the count against how many Gorton #2s you actually need.
Vent ladder after mains OK
Published radiator capacities only — from the cited chart.
Once the end-of-main vents clear on time, step the late upstairs radiator on the published ladder. Gorton places C on “farthest from the boiler, and in coldest rooms (for example, third floor radiators)” and D “where a large amount of air must be vented.”
| Vent | 1 oz | 2 oz | 3 oz | Note |
|---|---|---|---|---|
| Hoffman 1A (setting 6, max) | 0.145 | 0.225 | 0.300 | Adjustable; figures are the maximum setting |
| Gorton C | 0.270 | 0.450 | 0.570 | Fixed orifice · farthest / coldest (e.g. third floor) |
| Gorton D | 0.330 | 0.540 | 0.700 | Fixed orifice · large air dump only |
Usual step from a maxed Hoffman 1A: 1A → Gorton C (~2× at the chart columns). Jump to D only when required CFM or geography needs the large dump — see Gorton C or D. Keep the thermostat-room rad small (don't oversize the thermostat vent).
When NOT to put D on every upstairs rad
Gorton D is 0.33 / 0.54 / 0.70 CFM — faster than C (0.27 / 0.45 / 0.57). That is a specific condition, not a third-floor default. Skip the blanket D when:
- C already meets the required rate. Extra capacity you do not need just moves the imbalance.
- Mains are still the bottleneck. A D on a starved main is louder, not hotter upstairs.
- Hammer, noise, or short-cycle neighbors. Fast orifices on every third-floor rad can race the thermostat room and cut the boiler early.
- It is the thermostat radiator. Never. That rad is the building's sensor — keep it small.
Dual path — mains, then the late rad
Next step
(1) Put each main's size, length, ounce column, and target minutes into the main vent sizer — confirm #2 / #1 count before you shop radiator letters. (2) Then size the late upstairs rad from EDR + runout in the radiator vent sizer.
Related: How many Gorton #2 · Hoffman 1A vs Gorton C · Gorton C or D · Don't oversize the thermostat vent · All guides
FAQ
Why is my third-floor radiator cold on one-pipe steam?
Most often undersized or dead end-of-main vents: upstairs branches wait until the main clears. Time steam to the main tapping before upsizing radiator vents.
Should I put a Gorton D on every upstairs radiator?
No. After mains are OK, size from EDR + runout; C (0.27 / 0.45 / 0.57) is the usual third-floor step from a maxed 1A; D (0.33 / 0.54 / 0.70) only when required CFM or geography needs the large dump — never on the thermostat rad.
Mains or radiator vents first?
Mains first. Use the main vent sizer and published Gorton #2 capacities (1.10 / 1.75 / 2.20 CFM at 1 / 2 / 3 oz), then tune the late rad.
Is a Hoffman 1A on 6 enough upstairs?
Setting 6 is still modest (0.145 / 0.225 / 0.300 CFM). With mains proven and the rad still late, Gorton C (0.27 / 0.45 / 0.57) is the usual next published step (~2× at chart columns).
What if the supply valve is hot but the radiator stays cold?
Check pitch, a failed/clogged vent, and whether the main itself ever cleared; a hot valve with a cold body often means air or water still blocking the section — not “buy every letter larger.”
Sources
Every CFM figure on this page — Gorton #2 main vents and the Hoffman 1A / Gorton C / Gorton D radiator ladder — is pulled from lib/vent-capacity, transcribed from the Gill & Pajek MAIN VENTS and RADIATOR VENTS tables at 1 / 2 / 3 oz. Placement wording (third floor / farthest rooms / large air dump) is Gorton's own. No invented CFM, and no Big Mouth capacity claimed.