Radiator cold at the top: how to bleed it

Cold at the top, warm at the bottom: that's usually trapped air.

The answer in 2 lines. First, know which system you have. A hot-water (hydronic) radiator or baseboard that's warm at the bottom and cold at the top usually has trapped air: turn the heat off, let it cool, open the small bleed valve until water comes out, then close it and check the boiler pressure. A steam radiator is different: it has no bleed valve but an air vent on the side, and you don't bleed it. If a radiator is cold at the bottom or cold all over, the cause is elsewhere.

The first 3 steps

With the heat running, feel the radiator: warm at the bottom and cold at the top means air; cold all over or cold at the bottom, see "Likely causes".
Check which kind you have: a small square-headed valve near the top corner means hot water; a bullet-shaped vent halfway up the side means steam.
For hot water: turn the heat off at the thermostat, wait until the radiator is cool, and get a radiator key (or a flat screwdriver, depending on the valve), a cup and a rag.

Stop: danger

Never bleed a hot radiator: the water can scald. Heat off and radiator cool first.
Steam radiator: never unscrew the air vent or anything else while the system is running; steam burns. See the steam section below.
Electric radiator or oil-filled heater (no pipes): there's nothing to bleed. Don't open it.
Smell of gas (rotten eggs): don't flip switches, light anything or start the car. Leave the house, leave the door open, and once outside call 911 or your gas utility's emergency line (propane: your propane supplier).

Bleeding a hot-water radiator, step by step

Find the bleed valve: a small valve at the top corner, usually on the side opposite the pipe that feeds it. On baseboards, it's often under a small cover at one end.
Hold the cup and rag under it, then turn the key a quarter turn counterclockwise: you'll hear air hiss.
When water comes out steadily with no bubbles, close the valve. Snug, not forced.
Repeat on any other radiators that heat poorly.
Check the boiler gauge: bleeding lowers the pressure. Many systems refill automatically; if yours has a manual fill and it's low, see our guide on boiler pressure.

This Old House shows the steps: How to bleed a radiator. A radiator key costs a few dollars at hardware stores; take the old one or a photo of the valve to get the right size.

Steam radiators: what's different

No bleeding: the air vent on the side lets air out automatically as steam comes in.
Cold radiator: the vent may be clogged or painted over. Once the system is off and cool, a vent can be unscrewed and replaced; vents are sold in hardware stores. If you're not sure, ask a steam heating pro.
The valve at the bottom on one-pipe steam must be fully open or fully closed, never halfway.
Banging or spitting water: often water trapped in the radiator; a slight tilt toward the supply pipe can help (This Old House — quiet a banging steam radiator).
Apartment building: steam heat is usually run by the building. Tell the super or landlord rather than touching anything.

Likely causes

"Cold at the top, warm at the bottom", gurgling in the pipes: air in a hot-water system. Bleed it.
"Cold at the bottom, warm at the top": sludge settled in the radiator; a heating contractor may need to flush the system.
"One radiator cold all over": its valve may be closed or stuck, or (steam) its air vent is blocked.
"Every radiator is cold": the boiler is off, locked out or low on pressure. See our furnace and boiler guides.
"Upstairs radiators heat poorly": air, or pressure too low to push water up.

What not to do

Unscrewing the bleed valve all the way: a quarter to half a turn is enough.
Bleeding every week without asking why: frequent air means a leak or a fault for a pro to find.
Painting over steam vents or bleed valves.

When to call a pro, and who pays

Heating contractor: a broken or leaking bleed valve, a stuck radiator valve, sludge, pressure that keeps dropping, any steam boiler problem.
Renters: tell your landlord in writing. Heat is generally the landlord's responsibility; exact rules vary by state and city (see our guide on who pays for repairs).
Water leaked into the unit below: tell the neighbor and building manager, take photos, and call your insurer.

Good to know

Bleed hot-water radiators once a year when you first turn the heat on in the fall.
Check the boiler gauge once a month during heating season.
No official statistics on radiator air problems were found as of November 1, 2026.

In Boston, Priya turns the heat on in October: the living room radiator is warm at the bottom and cold at the top. She turns the thermostat down, waits an hour, and opens the bleed valve a quarter turn. Air hisses, then water drips, and she closes it. The boiler gauge reads a little lower but refills on its own. By evening, the radiator is warm all over.

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on November 1, 2026, for the United States. Confidence: high for the bleeding steps and safety warnings; medium for causes, which a heating contractor confirms on site. This guide describes common cases, not a diagnosis.