Radiator Not Heating Up? Find the Cold Spot, Find the Cause

First, check it properly

When the heating first comes on, a radiator is hotter at the bottom near the inlet, and the heat spreads up over the next few minutes. So give it 15 to 20 minutes, then put your palm flat at the bottom in the middle and slide it up to the top. A working radiator feels about the same temperature all over.

Cold at the top: trapped air

Air collects at the top of the radiator, where hot water can't reach. The fix is to bleed it:

1. Turn the heating off and let the radiators cool: the water inside can scald.
2. Find the bleed valve at the top: on the end, at the back, or on top of a towel rail. Most need a radiator key; some take a flat-head screwdriver.
3. Hold a cloth and a small container under it, and open it slightly anticlockwise. You'll hear a hiss as the air escapes.
4. Close it as soon as water comes out. Gently: it doesn't need to be tight. Don't open it all the way.
5. Check the boiler pressure. British Gas says it should be about 1 to 1.5 bar with the heating off; if it's dropped below that, top it up (see our boiler pressure guide).

If radiators need bleeding again and again, air is getting in or being made inside the system. Corrosion inside it gives off gas as well as sludge, which is one reason an inhibitor in the water matters.

Cold at the bottom: sludge

A radiator that's warm at the top and cold at the bottom has hot water flowing across the top but not through the bottom, almost always because of sludge. It's iron oxide from the inside of steel radiators and pipes, and it settles in the middle and bottom of the radiator. Bleeding won't help: there's no air in the way.

• Just one radiator: it can be taken off, flushed through with a hose outside until the water runs clean, and refitted. Ideal Heating says that's fine for an accomplished DIYer; British Gas advises against flushing radiators yourself. We'd say one radiator is a reasonable DIY job if you're confident isolating the valves and topping up the boiler pressure afterwards (taking a radiator off nearly always drops it). If not, it's a job for a heating engineer.
• Several radiators: the sludge is through the whole system. A power flush forces water and cleaning chemicals through every radiator; a chemical clean, left in the system for a while then drained and refilled, is the other option. Both are jobs for a heating engineer.

Stopping it coming back: ask for an inhibitor in the system water, which slows the corrosion that makes sludge, and a magnetic filter on the boiler pipework to catch what does form (Ideal Heating).

One radiator cold all over

If one radiator stays cold while the rest heat up, start with its valves:

1. Turn the thermostatic valve right up. It may simply be set too low.
2. Bleed it, in case it's full of air.
3. Check for a stuck pin. Thermostatic valve pins can stick. With the heating off, unscrew the valve head and press the pin with the flat end of an Allen key: Danfoss, which makes them, says it should move in 2 to 3mm and spring back. If it's stuck, tap the side of the valve gently and work the pin in and out until it frees, then refit the head and check for leaks.
4. Check the lockshield valve at the other end, under its plastic cap. If it's been closed, the radiator gets no water; it opens anticlockwise with a lockshield key or adjustable spanner.

If it's still cold, the valve may be blocked with sludge or limescale, or the radiator itself may be failing: that's for a heating engineer.

Some rooms always colder: balancing

If every radiator works but the ones furthest from the boiler are always slow or cooler, the system needs balancing. Without it, the radiators nearest the boiler take most of the hot water. Balancing means adjusting each lockshield valve so they all heat evenly. Screwfix aims for a 12 degree drop between the pipe in and the pipe out of each radiator, measured with a thermometer. It's also worth doing after a new radiator goes in. If a new radiator stays cold, check both its valves are open and bleed it first.

Every radiator cold, but there's hot water

When all the radiators are cold, the problem is the boiler or its controls, not the radiators. Glow-worm's checklist, in order:

1. The boiler settings: is the heating switched on, not just hot water, and not held off by a timer?
2. The flow temperature: if it's been turned right down, radiators only get lukewarm. Glow-worm suggests 60 to 70°C.
3. The room thermostat: turned up (18 to 21°C is comfortable), out of direct sun and draughts, and, if it's wireless, with working batteries.
4. The pressure gauge: top it up if it's low (see our boiler pressure guide).
5. Any fault code on the display: look it up in the manual or our boiler error codes guide.

On a combi boiler, if all that's fine and every hot tap is off but the heating still won't come on, it could be a stuck or faulty diverter valve, the part that switches the hot water between the taps and the radiators.

Frequently asked questions