Don't oversize the thermostat radiator

One-pipe steam fills each radiator only after the air inside it (and its runout) leaves through the vent. A larger vent clears that air faster, so steam arrives sooner. The thermostat does not know about remote wings — it only feels the room it sits in. When that room reaches setpoint, the call ends for the whole building.

That is why balancing language keeps the thermostat-room vent on the small side even when EDR and runout math alone would allow a bigger letter. You want other radiators to catch up before that room declares victory.

• Short cycles — thermostat room gets hot quickly; boiler drops out while far or upstairs radiators are still lukewarm.
• Cold remote rooms — complaints cluster in the wing farthest from the boiler or on long runouts, even when those vents look "right" on paper.
• Geography bump applied in the wrong room — someone treated the thermostat radiator like a cold far room and stepped C up to D (or hung an oversized adjustable) "just to be safe."

The fix is rarely "more boiler." It is usually slower steam into the thermostat room and adequate venting everywhere else.

Size ordinary radiators from EDR plus runout volume, divided by your clear-time minutes, then pick the smallest published vent that meets the rate — with far / cold rooms stepped up when the job needs it. Walkthroughs: Gorton C or D and vent ounces and minutes.

For the radiator in the thermostat's room, deliberately stay on the small side of what the math alone suggests. Do not apply the far-room / cold-room geography bump there. If that room still lags after everything else is balanced, fix thermostat location or differential before upsizing its vent.

Work the numbers in the radiator vent sizer, then override the pick downward for the thermostat radiator when the calculator's geography nudge would push it larger.

Fire the boiler and watch order of heat: remote cold rooms should not still be air-bound when the thermostat room is already shutting the plant down. If the thermostat room wins every race, step its vent down (or restore a smaller letter) and re-time.

Published capacities and clear-time targets are starting points. Dirt, pitch, and valve condition all change the clock — adjust from what the building actually does.