Radiator cold at the top: air in the system or something else

Why a radiator stays cold at the top

Radiators heat by circulating hot water through them. Water is heavier than air, so when air gets into the system it rises and collects at the highest point it can reach, which is usually the top of the radiator, sometimes right along the top edge. That pocket of air stops water reaching that part of the panel, so the top stays cold while the bottom, still full of water, heats normally.

Air gets in for ordinary reasons: the system was drained and refilled over summer, a radiator was removed and reinstalled, or a small amount simply works its way out of the water as it heats and cools with the seasons. It is more common at the start of the heating season, which is why this complaint clusters in September and October across Tallinn.

One cold radiator among several warm ones almost always means air in that radiator specifically, not a system-wide fault. If every radiator in the flat is behaving the same way, the cause is more likely upstream, which the next sections cover.

How to bleed a radiator yourself

Bleeding a radiator means releasing the trapped air through the small valve at one end, usually the top corner opposite the thermostatic valve. You need a radiator bleed key (a few euros at any hardware shop), a cloth, and a small container or bowl to catch drips. Ten minutes is enough for a flat with several radiators.

Turn the key slowly anticlockwise, just a quarter to half turn. You will hear a hiss as air escapes. Keep the cloth pressed near the valve, because water follows the air. As soon as the hiss stops and water comes out in a steady stream with no spitting or bubbling, close the valve again — clockwise, snug but not forced.

After bleeding, check the boiler or check the pressure gauge if your building has one visible, since letting air and water out can drop system pressure slightly. In a flat on district heating this is rarely something you need to top up yourself; in a house with its own boiler, low pressure after bleeding several radiators may need topping up at the filling loop, and the boiler manual will show how.

• Radiator bleed key and a cloth
• Turn anticlockwise a quarter to half turn, no more
• Hold the cloth ready — water follows the air within seconds
• Close as soon as the flow is steady with no hissing
• Check pressure afterwards if the system has a visible gauge

When the problem is not air but something else

If you bleed a radiator and the top warms up but goes cold again within a day or two, air is coming from somewhere continuously, which is worth mentioning to a plumber rather than bleeding it weekly indefinitely. A slow drip at a joint, a fitting, or the radiator itself can be feeding air in as it feeds water out, and it is easier to find early.

If several or all radiators in the flat are cold at the top, or cold altogether, bleeding one by one rarely fixes it. In a block on district heating, this points more often to the building's substation or the shared heating loop than to any single radiator, and is worth raising with the korteriühistu or property manager, since the fix is usually outside your flat.

A radiator that is cold along the bottom, or cold in patches rather than a clean band at the top, is a different problem: that pattern usually means sludge or scale sitting inside the radiator rather than air at the top, and bleeding will not clear it. It often needs flushing or, if the radiator is old, replacing.

What is dangerous and what can wait

A cold-topped radiator on its own is not dangerous. It is a comfort and efficiency issue — the radiator is working harder than it needs to for the heat it gives out — and it can wait a day or a week without harm. Bleeding it is safe to do yourself with an ordinary key; there is no gas involved and nothing to switch off first.

Take it more seriously if you notice water pooling under a radiator, a valve that will not close properly once you have opened it, or a hissing that does not stop even after water starts coming through. A valve that will not seal is worth calling in rather than leaving, since a slow leak inside a wall cavity or under flooring is far more costly once it has been running unnoticed for weeks.

We do not touch gas appliances, so if your heat comes from a gas boiler and the fault seems to sit at the boiler itself rather than the radiators, that is a job for a gas-qualified engineer, not us.

When to call for help and what it costs

Bleeding a radiator is squarely a do-it-yourself job and there is no need to call anyone for it. Call if bleeding does not hold, if a valve is stuck or leaking, if several radiators are affected at once, or if you would rather not deal with an old, seized bleed screw that will not budge with hand pressure.

Plumbing call-outs for this kind of fault start from 45 € per hour, with the first hour usually enough to bleed and check several radiators, test valves, and rule out a leak. If a radiator itself turns out to be the problem — corroded through, or too old to hold pressure reliably — a full radiator installation is priced from 85 € per job, quoted once we know the radiator and the room.

If the issue turns out to be the building's heating loop rather than your flat, we will say so rather than keep working on a radiator that was never the cause, and point you to raising it with the property manager instead.