Radiator not heating up? 9 reasons and how to fix each

A radiator that stays cold with the heating on has one of nine faults, and where the cold sits tells you which. Cold at the top is air. Cold at the bottom is sludge. Every radiator cold points back to the boiler. We match each symptom to its fix, give the steps for the jobs you can do yourself, and show where a heating engineer takes over and what that costs.

Why your radiator is not heating up: match the symptom

Before you touch a valve, put your hand on the radiator and on its neighbours. Where it is cold, and how many radiators are cold, narrows nine causes down to one or two.

Work down the table in order. The first three cost nothing and need only a radiator key, a spanner or a pair of pliers, so rule them out before paying anyone to look.

One radiator not heating up

When one radiator is cold and the rest are hot, the boiler is doing its job and the fault is at that radiator: air in it, a valve shut, or a valve stuck. Do not leave it for the winter. A room with a cold radiator has cold walls, and cold rooms and condensation go together, because moisture in the air settles on the coldest surface it finds.

Trapped air: cold at the top, warm at the bottom

Air collects at the top of a radiator and stops hot water filling it, so the top stays cold while the bottom warms up. You may hear gurgling too. Bleeding lets the air out:

1. Turn the heating off and wait until the radiator has cooled.
2. Put a cloth under the bleed screw, the small square valve at one top corner, and fit a radiator key.
3. Turn the key slowly anti-clockwise, about a quarter turn. Air hisses out.
4. Close the screw as soon as a steady stream of water comes out, and check it for drips.
5. Check the boiler pressure gauge, top it up if it has dropped, and turn the heating back on.

A valve turned down or closed

Every radiator has two valves. The thermostatic radiator valve (TRV) carries numbers, usually 1 to 5 or a snowflake symbol for frost protection, and lets in less hot water as the room warms up. The lockshield valve at the other end has no numbers and hides under a plastic cap.

Turn the TRV to 5. Then pull the cap off the lockshield and check it is open. If it has been shut, turn it anti-clockwise a quarter to one full turn with an adjustable spanner or pliers.

A stuck TRV pin

If the TRV is on 5 and the radiator is still cold all over, the pin under the TRV head has probably stuck down, holding the valve shut. To free it:

1. Turn the heating off.
2. Unscrew the TRV head. Underneath is a small metal pin.
3. Press the pin down with your thumb or the handle of a spoon. It should spring back up.
4. If it stays down, ease it up and down gently with your fingers or pliers until it moves freely. A little silicone lubricant helps.
5. Refit the head, turn it to 5 and switch the heating back on.

Radiator still cold after bleeding

Bleeding fixes air and nothing else, so a radiator that stays cold afterwards has a second fault. Check these in order:

1. Boiler pressure. Bleeding lets water out of the system. Below 1.0 bar the boiler cannot circulate water properly, so top it up to 1.2 to 1.5 bar.
2. Air still inside. Air moves round the system as it refills. Run the heating, let it cool and bleed the radiator a second time.
3. The valves. Check the TRV is on 5, its pin moves, and the lockshield is open.
4. Sludge. If the top now heats but the bottom stays cold, the radiator is partly blocked.
5. A leak. A system that keeps losing pressure is losing water somewhere, so look for drips at valves and joints.

Radiator cold at the bottom: sludge

A radiator that is warm at the top and cold at the bottom is holding sludge: rust, dirt and debris from inside the system that sinks and blocks the lower part of the radiator. Bleeding will not shift it. Other signs are dirty or discoloured water when you bleed, heating that takes longer to warm the house, and a boiler that bangs, gurgles or kettles.

The fix is a flush. A power flush pushes a high-pressure flow of water through the radiators and pipes to clear the sludge out, and finishes with a fresh dose of corrosion inhibitor. Where the sludge is light, an engineer may suggest a lighter chemical clean instead.

A power flush costs £300 to £700 in 2026, and the number of radiators sets the price. A flat runs £300 to £400, a terraced house £350 to £500, a semi-detached house £400 to £600 and a detached house £550 to £700 or more.

Once the system is clean, a magnetic filter on the return pipe to the boiler catches new sludge before it settles in the radiators. A proper boiler installation includes a chemical flush and a filter, so a system flushed as part of a boiler swap should already have one.

Some radiators hot, others cold: balancing the system

When the radiators nearest the boiler get hot fast and the ones furthest away stay lukewarm, the system is out of balance. Hot water takes the easiest route, so the near radiators take most of it. Balancing turns their lockshield valves down a little so more water reaches the far ones.

It takes a few hours, mostly waiting for radiators to warm up. Bleed every radiator and check the boiler pressure before you start.

1. Turn the heating off and let the system cool.
2. Turn every TRV to its highest setting and open every lockshield valve fully.
3. Switch the heating on and note which radiators heat up first. These are usually the ones closest to the boiler.
4. On the fastest radiators, turn the lockshield slightly clockwise with a spanner to cut their flow.
5. Let the system run, feel which radiators are still slow, and repeat in small adjustments until every radiator heats evenly.
6. Set the TRVs back to the room temperatures you want.

All radiators cold

When every radiator is cold, the fault is not in the radiators. Look at the boiler: its pressure, the controls that tell it to heat, and the pump that sends the heat round.

Low boiler pressure

The pressure gauge is usually on the front of the boiler. With the system cold, the needle should sit between 1.0 and 1.5 bar. Below 1.0 bar the boiler cannot push water round the radiators.

Most boilers top up through a filling loop, a braided hose under the boiler with a valve at each end:

1. Turn the heating off and let the system cool.
2. Open both filling loop valves slowly. You will hear water entering the system.
3. Watch the gauge and close both valves firmly when it reaches 1.2 to 1.5 bar.
4. Restart the boiler.

Fill slowly and stop in time, because going over 2 bar can trigger a fault code of its own. If the pressure falls again within days, water is escaping through a leak or a fault in the boiler, and topping it up only hides the problem.

Thermostat and timer settings

A boiler only heats the radiators when the thermostat and timer ask it to. Turn the thermostat up to 21°C or above and listen for a click. Check the timer is not set to off for that time of day, and bear in mind that a power cut can reset some controls to their factory settings.

Switching the controls to manual or constant for a few minutes shows whether the boiler fires for heating at all. If it does not, the fault is in the controls or the boiler, not the radiators.

A failed pump

The pump pushes hot water from the boiler round the radiators. If the boiler fires, the pressure is right and the controls are calling for heat, but every radiator stays cold, the pump is the next suspect. On a combi it sits inside the boiler casing, so replacing it is an engineer's job.

Hot water but no heating

On a combi boiler, hot water at the taps with stone-cold radiators points to the diverter valve. It switches the boiler's heat between the taps and the radiators, and when it sticks on the hot water side, the radiators get nothing. Other signs are a boiler that fires when you run a tap but not when the thermostat calls for heat, or a boiler you can hear running while no heat reaches the radiators.

Rule out the thermostat and timer first, because a control fault looks the same from the radiator. The diverter valve is inside the boiler, and replacing it is not a DIY job.

When to call a heating engineer

Stop and call a heating engineer when:

• The boiler keeps losing pressure after you top it up.
• You can see a leak, or corrosion around a radiator or valve.
• Radiators knock or gurgle loudly and bleeding has not stopped it.
• You have bled and balanced, and the same radiators are still cold.
• The radiators are cold at the bottom and need flushing.
• You have hot water but no heating, or the boiler fires and nothing warms up.

Work inside the boiler needs an engineer on the Gas Safe Register, because anyone employed to work on gas appliances in a home has to be Gas Safe registered and competent in that type of gas work. The back of the engineer's Gas Safe ID card lists the work they are qualified to do.

A heating engineer in London charges £260 to £460 to replace a pump and £240 to £420 for a diverter valve, parts and VAT included. Labour is the part of the bill that moves with the postcode, so the same jobs usually cost less outside the capital.

Expect a call-out charge on top. In London a weekday daytime call-out costs £0 to £65 and is often waived if the work goes ahead, and an evening call-out costs £45 to £95.

Repair the heating or replace the boiler

Most cold-radiator faults sit in the system: air, sludge, valves and balance. The boiler becomes the question when it is over 15 years old and the fault is in the boiler itself, such as pressure you have to top up again and again, or a pump or diverter valve failing on an old unit.

A gas boiler usually lasts 10 to 15 years. Past that point, weigh a £240 to £460 repair against a new boiler: a like-for-like combi swap costs £1,800 to £3,500 fitted.

If one room stays cold with every fault fixed, the radiator may be too small for the room, and a bigger radiator or an extra one is the answer. In England, adding extra radiators needs building regulations approval, which an installer on a competent person scheme can self-certify. Wales has its own Approved Documents, Scotland issues building warrants and Northern Ireland runs separate building control.