Signs your radiators need bleeding, and what you need
Air gets into the system when it is topped up, after repairs or slowly with use. It collects at the top of the radiators (radiadores) and stops hot water filling them.
- Typical signs: the radiator is hot at the bottom and cold at the top, you hear gurgling or bubbling in the pipes, or the flat takes much longer to warm up than last winter.
- You need a radiator key (llave de purgar), a small square-ended key sold in any ferretería for very little. Some bleed screws open with a flat screwdriver. Also a cloth and a small cup.
- The best time is at the start of the heating season, before it gets properly cold.
How to bleed radiators step by step
Advice on the order varies. The most common recommendation is to start with the radiator furthest from the boiler (caldera) and finish with the nearest; in a duplex, do the lower floor first, then the upper one, since air rises. In a single-floor flat, what really matters is doing all of them.
- Turn the heating on for a while so the radiators warm up and you can feel which ones are cold at the top.
- Turn the heating off and let the radiators cool down a bit. With the pump stopped, the air rises to the top, and you avoid a spray of very hot water.
- Find the bleed valve (purgador): a small screw at one of the top corners, usually on the opposite side from the main valve.
- Hold the cloth and cup underneath and open it a quarter to half a turn anticlockwise with the key. Don’t take it right out.
- You will hear air hissing. When water comes out steadily with no bubbles, close it again without overtightening.
- Repeat on every radiator, then check the boiler pressure before switching the heating back on.
Boiler pressure, and buildings with communal heating
Bleeding lets a little water out, so the pressure drops. On the boiler’s gauge (manómetro), with the heating cold, a flat is typically between 1 and 1.5 bar, but follow your boiler’s manual. If it is low, open the filling tap (llave de llenado), usually under the boiler, slowly while watching the needle, and close it firmly when you reach the right figure. If you overshoot, bleed a radiator briefly to bring it down.
Many older Madrid buildings have communal heating (calefacción central): there is no boiler in your flat, and the building’s maintenance company manages the pressure. Ask the porter (portero) or the building administrator (administrador de fincas) before bleeding. Often the maintenance company does it when the heating starts up, and bleeding while the system runs can release very hot water or drain part of the circuit. Never touch the building’s main valves.
For the rest of your winter checks (boiler service, mandatory gas inspections, water heater), see Get your heating ready for winter.
Aluminium, cast iron and automatic bleed valves
- Aluminium (aluminio): the most common type in flats with their own boiler. Each radiator usually has its own manual bleed valve. If one keeps filling with air, there may be corrosion in the system, and it is worth having it checked.
- Cast iron (hierro fundido): common in older buildings, often with communal heating. They are slow to heat up and cool down. Some have a bleed valve and some don’t; if you can’t see one, the air is released elsewhere in the system.
- Automatic bleed valve (purgador automático): a small cylinder with a cap that lets air out by itself. You don’t need to open it. If it drips or leaks, it needs replacing.
If a radiator is still cold after bleeding
- Make sure the radiator valve (llave del radiador) is open. If it has a thermostatic head, turn it to maximum; the pin underneath sometimes sticks after summer and needs freeing.
- Check the lockshield valve (detentor) at the other end, usually hidden under a plastic cap. If someone closed it, no water gets through.
- If the furthest radiators are much cooler than the nearest ones, the system may need balancing. That means adjusting the lockshield valves one radiator at a time.
- If it is cold at the bottom and warm at the top, the problem is sludge (lodos), not air. The radiator has to come off the wall to be flushed, or the whole circuit needs cleaning.
When to call someone, and who pays in a rental
Call a professional if the boiler pressure keeps dropping within days (usually a leak or a faulty expansion vessel), if a bleed valve or radiator valve drips, or if a radiator stays cold after bleeding and checking the valves. Any work inside a gas boiler must be done by its official service team or an authorised gas company.
Replacing bleed valves, radiator valves, lockshields or thermostatic heads, balancing the system or taking a radiator off to flush it are jobs we do in one visit. Send us photos of the radiator, its valves and the boiler gauge and we’ll quote in English with no obligation. If you rent, tell your landlord in writing first; our guide Who pays for repairs? explains how costs are usually split.