Radiator Cold at the Top - One Section Will Not Heat Up

A radiator that is cold at the top has trapped air in the system, not a circulation problem. Bleeding the radiator releases the air and restores full heat.

Category:HVAC

Difficulty:Easy

Time:10 min per radiator

Success:50%

Updated:May 27, 2026

quick_referenceQuick Answer

For Radiator Cold at the Top - One Section Will Not Heat Up, start with "Turn on the heating system and wait 10 minutes until the pump starts": Turn your thermostat up so the boiler fires. Wait for the circulating pump to start (you will hear a low hum). The air inside the radiator expands when heated, which makes bleeding easier. Never bleed a cold radiator - the pressure differential is not enough to push the air out. Stop DIY if water is shooting out of the valve under pressure - close it fully immediately and call a plumber. This is listed as a easy recovery and usually takes about 10 min per radiator.

verifiedGuide Snapshot

Repair areaHVAC

Estimated time10 min per radiator

DifficultyEasy

Stop conditions4

Last updated May 27, 2026. Review the stop conditions before continuing.

account_treeRecovery State

DeviceRadiator

Current stateCold At Top

Specific stateAir Trapped In Top Section

Failed stepHeat Output Inspection

Likely failure typeBlocked Path

DIY boundaryDIY recovery first

paymentsCost decision

help1. Understand the Problem

Air gets trapped inside radiators naturally over time - dissolved air comes out of the water and collects at high points in the system. When air is trapped, hot water cannot enter the top of the radiator, so only the bottom half heats. The fix is bleeding the radiator with a key or screwdriver.

build_circle2. Try This First

Best First Step

Turn on the heating system and wait 10 minutes until the pump starts

Turn your thermostat up so the boiler fires. Wait for the circulating pump to start (you will hear a low hum). The air inside the radiator expands when heated, which makes bleeding easier. Never bleed a cold radiator - the pressure differential is not enough to push the air out.

visibility3. Visual Guidance

See what's happening and how to try the first recovery step.

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Locate the bleed valve at the top corner of the radiatorThe bleed valve is a small square or round nut at the top end of the radiator - sometimes hidden behind a cap. Use a radiator bleed key (available at any hardware store for a few dollars) or a flathead screwdriver. Place a cloth or small container under it to catch water.

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Turn the key counterclockwise slowly until you hear a hissSlowly turn the key left. You will hear air escape with a hissing sound. When water starts dripping steadily - not spurting - close the valve by turning it clockwise. Do not open it fully, just enough for the air to escape. Tighten until dripping stops.

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Check the pressure gauge on the boiler after bleedingAfter bleeding, check the system pressure gauge on your boiler. It should be between 1 and 1.5 bar. If it has dropped below 1 bar, you need to re-pressurize the system by opening the filling loop valve. Running low on pressure causes this problem repeatedly.

autorenew4. If That Doesn't Work

Try the next recovery options.

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Check all radiators, not just the cold oneAir travels through the system. If one radiator is cold at the top, bleed all of them - especially the ones highest in the house. Air naturally rises to the highest point.

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Install automatic air vents at high points in the systemIf you bleed radiators frequently, your system needs automatic air vents at the highest pipes. These release air automatically without manual bleeding. A plumber can add them to your pipes.

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Frequently Asked Questions

How often should I bleed radiators?expand_more

Once a year, typically at the start of the heating season. If you notice cold patches in winter, bleed again. Frequent bleeding means your system needs automatic air vents.

Is it normal for radiators to need bleeding every few weeks?expand_more

No. If you are bleeding monthly, the system has an air leak or the automatic vent is blocked. Call a heating engineer - there is a problem with the system seal.

Will bleeding help if the radiator is cold at the bottom?expand_more

No. Bottom-cold radiators have sludge in the bottom channels - bleeding will not fix that. You need power flushing or manual cleaning of the radiator.