Hydronic uses pressurized hot water (~180°F). Steam uses literal steam (boiling, ~215°F). Procedures are different. Don't bleed a steam radiator with the steam still on — burn risk. If your radiators have a single pipe coming up from the floor (no return line) and large round Hoffman or Gorton air vents on the side, you have steam — different procedure (see "Steam radiators" section below).
The 7-step hydronic bleed
- Turn on the heat + let it run 15 min. Heat up the radiators so trapped air rises to the bleed valve at the top corner.
- Identify the bleed valve. At the top corner of each radiator — a small square or hex nut, sometimes a slotted screw. If yours is a square nut, you need a radiator bleed key ($3 hardware store) — same key fits 95% of bleed valves in North America.
- Hold towel + bowl under the valve. Water will spit out at the end — be ready.
- Open the valve 1/4 turn counter-clockwise. You'll hear hissing — that's the trapped air escaping.
- Wait for steady water stream. When the hissing stops and water flows steadily for 5 seconds, all air is out.
- Close the valve. Clockwise, hand-tight plus 1/8 turn. Don't over-tighten — the brass stem strips.
- Check boiler pressure. Hydronic system: gauge should read 12-15 psi cold, 20-25 psi hot. If lower after bleeding, top up via the automatic fill valve or manual feed (boiler-room procedure).
Bleed order — start at the highest radiator
Air rises. The highest radiator in the house collects the most. Start at the top floor, work down. Multi-story houses: top floor first, then second, then ground. In commercial buildings I'd start at the top of the highest riser. Same logic at home.
The no-pressure trap
You open the bleed valve. Nothing happens. No air, no water. Two possibilities:
- Entire radiator is dry. System pressure too low. Walk to the boiler. Pressure gauge should read 12-15 psi cold. If it's at 5 psi, the system is low on water. Top up at the auto-fill valve (PRV — usually a small valve near the boiler with a lever) or open the manual feed valve for 10 seconds until pressure climbs back to 15 psi. Then re-try the bleed.
- Supply valve at the bottom is closed. Someone turned off this specific radiator. Open the supply valve (clockwise full open, then back off 1/4 turn — never leave a valve at full-open under pressure, the seat seizes). Re-try the bleed.
Close the bleed valve before troubleshooting either.
Steam radiators — different procedure
Steam systems use one-pipe parallel-flow design (typically) with cast iron radiators and large air vents on the side of each unit. The air vent is the bleed mechanism — it auto-vents air when steam arrives, then closes when steam contacts the vent (the bimetallic element). You don't manually bleed steam radiators. If a steam radiator runs cold:
- Air vent stuck closed (most common) — replace with a Hoffman 40 or Gorton #4 ($15-$25). Standard 1/8" NPT thread.
- Pitched wrong — radiator should slope back toward the supply pipe by ~1/4" so condensate drains. Shim the far foot.
- Steam main pressuretrol set too low — boiler doesn't make enough steam to push past the air-vent on the far radiator. Boiler service call.
FAQ
Why is the top of my radiator cold but bottom warm?
Air trapped in the radiator. Water heats the bottom but can't circulate to the top. Bleed the valve at the top corner. 5-min fix.
Hydronic vs steam radiators?
Hydronic = pressurized hot water (~180°F), small bleed key. Steam = literal steam (~215°F), Hoffman/Gorton air vent (auto, replace if stuck). Different procedures.
Do I need to bleed all radiators?
Bleed the cold ones. Annual preventive maintenance: start with highest radiator (most air accumulation) and work down.
What if no water comes out when I open the valve?
Two causes: (1) entire radiator is dry — system pressure too low, refill at boiler auto-fill; (2) supply valve at bottom is closed — open it. Close the bleed valve first.
How often should I bleed?
Once at the start of every heating season as preventive maintenance. Also anytime a specific radiator runs cold.
Related guides
- Boiler knocking water hammer — related hydronic-system diagnostic
- Best smart thermostats 2026 — compatible with hydronic systems (verify R/W terminals)
Editorial standards: Reviewed by Al, Building Doctor — IUOE Local 39 Stationary Engineer with commercial boiler + hydronic + steam experience.