Heating
Radiator cold at the top? Let the air out
A radiator cold at the top but warm lower down usually has trapped air, and bleeding it once the heating has cooled, as Vaillant and Baxi describe, normally sorts it out.
By the Pipes Up the Stair site team · how pages are checked · Updated
Long pipe runs through old tenement flats, with radiators added room by room over the decades, give air plenty of places to collect. Bleeding a radiator is one of the few heating jobs the boiler makers expect householders to do themselves.
Anything beyond that, from sludge to a pressure that won't settle, goes to a Gas Safe registered engineer or your installer.
Why is the radiator cold at the top?
Air rises, so a pocket of it sits at the top of the panel and stops hot water filling it. Letting it out through the small bleed valve at one top corner lets the water back up. A radiator that is cold at the bottom or in the middle is a different problem, covered below.
How do you bleed a radiator?
Follow the maker's order. Vaillant says turn the heating off and wait about 1 hour, or 2 to be cautious, so the radiators are cold. Hold a cloth and a small bucket or tray under the valve, then turn the radiator key a quarter to half turn anticlockwise. You will hear a hiss; Baxi says that when the hissing stops and water comes out, turn the key clockwise to close it. Finally, check the boiler pressure.
- 1Turn the heating off
- 2Wait an hour or two for radiators to cool
- 3Key a quarter to half turn anticlockwise
- 4Close it when water follows the hiss
- 5Check the boiler pressure gauge
What should the boiler pressure read afterwards?
About 1 to 1.5 bar when the heating is cold, per Ideal and Vaillant, but check your boiler manual. Ideal says the normal cold operating pressure is between 1.0 and 1.5 bar, often shown as a green band on the gauge. Vaillant adds that it should stay below 3 bar when the system is hot, and Baxi expects around 1.5 bar after bleeding.
Can you top the pressure back up yourself?
Only on some boilers. Ideal says topping up is for combi boilers with clearly identifiable filling controls, and warns do not operate unidentified valves underneath the boiler. If you have a system or heat-only boiler with an external filling loop, or you're not sure, call your installer or a Gas Safe engineer.
What if the radiator is cold at the bottom?
That usually points to sludge, and it needs a professional. Baxi says your Gas Safe registered installer can flush and clean the system and add inhibitor. Baxi and Vaillant also both send you to an engineer if the pressure stays high after bleeding.
Why do the radiators keep needing bleeding?
Something in the system is letting air in, and it is worth finding. Vaillant says if you have to repeatedly bleed radiators, discuss it with your installer when the boiler is serviced.
Do better controls help in an old flat?
Often, yes. Home Energy Scotland suggests adding controls if the system lacks a programmer, at least one room thermostat, or TRVs on the radiators, and setting TRVs to their frost setting rather than fully off in empty rooms. Its funding table, when checked on 5 November 2026, put heating controls at a grant of up to £450, with a loan of up to £150; see our page on Home Energy Scotland support.
Does changing radiators need a building warrant?
Not usually, but the work must still meet the standards. Schedule 3 to the Building (Scotland) Regulations 2004 lists pipework, radiators and controls for a combustion appliance installation as type 8, and Schedule 3 work needs no warrant while still having to meet the standards. Any work on the boiler itself is a job for a Gas Safe engineer.
| What you notice | Likely cause | Who sorts it |
|---|---|---|
| Cold at the top | Trapped air | You, by bleeding it |
| Cold at the bottom or middle | Sludge | Gas Safe installer |
| Pressure high after bleeding | Needs checking | Engineer |
| Bleeding needed again and again | Air getting into the system | Installer at the service |
Asked on the stair
Which radiator should you bleed first in a tenement flat?
The makers' advice is the same for every radiator: heating off, radiators cold, then bleed each one that has a cold top. Check the pressure once you have finished.
Can the water from the bleed valve stain the carpet?
It can be dirty, which is why Vaillant suggests a cloth and something to catch it. Black water is worth mentioning to your installer.
Should a TRV be turned right off in a spare room?
Home Energy Scotland suggests the frost setting rather than fully off, so the room still gets some protection in cold weather.
What if you rent the flat?
Bleeding a radiator is usually fine, but tell your landlord or letting agent about cold spots, pressure drops or leaks rather than arranging repairs yourself.
Where this comes from
- Vaillant: How to bleed a radiator — cooling time, key turn, repeated bleeding
- Baxi: How to fix high boiler pressure — closing the valve, sludge and flushing
- Ideal: Topping up boiler pressure — 1.0 to 1.5 bar, unidentified valves
- Vaillant: What pressure should my boiler be? — under 3 bar when hot
- Home Energy Scotland: Thermostats and heating controls — programmer, thermostat, TRVs
- Home Energy Scotland: Grants and loans — heating controls funding
- Building (Scotland) Regulations 2004, schedule 3 — type 8