How do you bleed a radiator?
Short answer: Turn the heating off and let the radiator cool. Hold a cloth under the bleed valve at the top, open it about half a turn with a radiator key, and close it when water flows steadily instead of air. Then check the boiler gauge against the range on your boiler's label or manual. Bob Vila puts a typical two-story home at about 12-15 psi.
How do I know if my radiator needs bleeding?
Bleeding applies to hot-water radiators and baseboards, not steam. Common signs of trapped air:
- Cold at the top. With the heat on, the top of the radiator is cool or much colder than the bottom.
- One cold radiator. A single unit stays cold while the rest of the system heats normally.
- Noise. Gurgling is often blamed on air, but treat it as a hint, not proof.
What tools do you need?
| Item | Why |
|---|---|
| Radiator bleed key | Opens most bleed valves |
| Flathead screwdriver | Works on slotted valves instead of a key |
| Rag or towel | Catches drips and protects the floor |
| Small cup or bowl | Holds the water that comes out |
| Work gloves | Protects hands from hot metal and water |
Bleed valves vary, so check that the key fits before you start.
How to bleed a hot-water radiator, step by step
- Turn the heat off. Bob Vila notes that bleeding with the system running can pull more air in.
- Let it cool. Which?, a UK consumer group, advises waiting 20-30 minutes. Radiator water can be hot enough to burn.
- Find the bleed valve. It is usually a small fitting at the top of one end of the radiator.
- Get ready to catch water. Hold the rag and cup under the valve.
- Open slowly. Turn the key counterclockwise, no more than about half a turn. You should hear air hiss out.
- Close when water flows. Once a steady stream of water replaces sputtering air, turn the key clockwise to close. Don’t overtighten.
- Repeat on other radiators. Bob Vila suggests doing every radiator, including ones that seem fine.
- Turn the heat back on and check that each radiator warms evenly.
Which radiator should you bleed first?
Which? (UK) suggests starting with the ground-floor radiator farthest from the boiler. It gives no order for upstairs units. If your boiler maker’s manual gives an order, follow it.
Check the boiler pressure afterward
Bleeding releases water, so pressure often drops a little. Check the pressure gauge on the boiler once you finish.
| Item | Value | Source |
|---|---|---|
| Rule of thumb | About 1 psi per 2 feet of rise | Bob Vila |
| Typical two-story home | About 12-15 psi | Bob Vila |
| Your system | Whatever your boiler’s label or manual says | Manufacturer |
The rule is simple arithmetic. A system about 24-30 feet tall works out to 24 / 2 = 12 psi and 30 / 2 = 15 psi. These are Bob Vila’s rule-of-thumb figures, not a spec. Your boiler’s label or manual sets the correct number, so it wins over any rule of thumb.
If the pressure is low, refill only as your boiler manufacturer’s instructions describe. If you’re unsure, or the pressure keeps dropping, call a licensed heating technician. Repeated pressure loss may point to a leak.
Can you bleed a steam radiator?
No. Steam radiators don’t have a bleed screw. The air vent on the side releases air automatically. A working vent hisses as air leaves, then closes with a click when steam arrives.
| Vent symptom | Likely problem |
|---|---|
| Never hisses, radiator stays cold | Vent stuck shut, air blocks the steam |
| Constant hiss | Vent stuck open, wasting steam |
Don’t try to open or unscrew a steam vent the way you would a bleed valve. A faulty vent gets replaced, ideally with the system off and cool. Never open fittings on a hot steam radiator. Live steam can scald almost instantly.
Safety notes
- Hot water and steam burn. Work only on a cooled, shut-off system.
- If you smell gas or oil, see smoke, or find a wet floor near the boiler, leave the house. Call 911 and your utility from outside.
- Fuel-burning boilers can produce carbon monoxide. See our guide on carbon monoxide detector placement.
Quick answers
How long does it take to bleed a radiator? The bleeding itself takes seconds per radiator once air stops. Allow 20-30 minutes beforehand for the radiator to cool.
Should you bleed radiators with the heat on or off? Off. Bob Vila warns that running the heat while bleeding can pull more air into the system.
How far do you open a radiator bleed valve? About half a turn at most. Close it as soon as a steady stream of water appears.
What should boiler pressure be after bleeding? Bob Vila puts a typical two-story home at about 12-15 psi. Check your boiler’s label or manual for its specific range.
Can I bleed a radiator without a key? Slotted valves take a flathead screwdriver. Otherwise, use a radiator key that fits; Bob Vila also mentions needle-nose pliers as a fallback.
Do steam radiators need bleeding? No. Their side air vent releases air automatically. A vent that never hisses or hisses constantly likely needs replacement.