Radiator Not Heating Up? Work Out the Cause Before You Bleed It

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A radiator not heating up is telling you something quite specific, and where the cold part is tells you what. Cold at the top with a warm bottom is trapped air, and bleeding it will fix that in about two minutes. Cold at the bottom with a hot top is sludge sitting in the waterway, and bleeding will do absolutely nothing for it.

Those two symptoms get treated as the same problem constantly. Sixteen years running plumbers merchants across the south of England left me with a fairly clear picture of the pattern: the number of people who came to the counter for a bleed key when what they needed was a cleanser, a bottle of inhibitor and a magnetic filter was considerable. Bleeding is the fix everyone has heard of, so it is the one everyone tries, whether or not the symptom points to air.

So this page starts with the symptom rather than the procedure. Find your radiator in the table, then read the section it sends you to. I am not a plumber and have never held Gas Safe registration, so where a job crosses into work that legally requires a registered engineer, that is marked plainly rather than glossed over.

Find your symptom before you touch anything

Turn the heating on and give it twenty minutes, then feel the radiator with the flat of your hand: top corner, middle, and low down near the valves. The pattern of hot and cold is the diagnosis. Feel the pipes at the valves too, because a radiator that is cold while its pipes are hot is a different fault from one where the pipes are cold as well.

Cold at the top, hot at the bottom: trapped air

This is the one symptom where bleeding is genuinely the answer. Air rises, so it collects in the top of the panel and takes up the space hot water would otherwise fill. The bottom stays hot because water is still circulating through the lower waterway.

Turn the heating off and let the radiator cool before you open the bleed valve, because a hot system with the pump running is pressurised and will spit scalding water at you rather than hiss. Open the square bleed screw a quarter turn anti-clockwise with a bleed key, wait for the hiss to become a dribble of water, then close it. The full sequence, including what to do if nothing comes out at all, is in the step by step guide to bleeding a radiator.

Two things worth knowing afterwards. On a sealed system, bleeding lets water out, so the pressure gauge drops and you may need to top the boiler back up before the heating will run properly. And if the same radiator needs bleeding every few weeks, that is not a bleeding problem, it is air getting in somewhere it should not, which usually points to corrosion inside the system or a pump drawing air.

Cold at the bottom, hot at the top: sludge, and bleeding will not touch it

This is the highest volume radiator complaint in the country and the one that gets the wrong fix most often. If the top of the panel is hot and the bottom stays cold, there is no air in it. Air is at the top and the top is working. What is sitting in the bottom is sludge.

That black water is magnetite, iron oxide produced as the steel inside your radiators slowly corrodes in water that carries dissolved oxygen. It is heavy, it is magnetic, and it drops out of suspension wherever the water slows down, which means the flat bottom of a radiator panel. Over years it builds into a silt bed that hot water simply flows over the top of. That is why the top is hot and the bottom is not.

Opening the bleed valve on that radiator releases nothing, or a dribble of dirty water, and changes nothing about the cold bottom. The sludge has to physically leave, and then the conditions that made it have to be dealt with, or it comes straight back.

Route 1 is a realistic afternoon for a competent DIYer, and the practical detail, including how to isolate the valves and cap the tails so you can carry the thing out without emptying black water onto the carpet, is in the guide to removing a radiator. Route 2 is a bigger job and a power flush is not DIY work.

Route 3 is the part people skip, and it is the part that decides whether the problem stays fixed. BS 7593:2019, the British Standard covering water treatment in domestic central heating, sets out that a system should be cleaned, dosed with a corrosion inhibitor, and then have that inhibitor concentration checked every year. Very few UK households do the annual check. Inhibitor depletes, and once it has gone the corrosion restarts.

Fernox F1 is the protector most merchants stock by default, which matters when you want it today rather than in three days. Fernox publish a treatment figure of roughly 130 litres, about 16 single radiators, per 500ml bottle, so check the size of your system before assuming one bottle covers it. How the three main brands compare on dose rate and cost per litre is covered in the central heating inhibitor guide.

The filter is the other half. Inhibitor slows new magnetite forming, a magnetic filter catches what forms anyway before it reaches the boiler heat exchanger and before it settles in the bottom of a radiator. The one constraint that catches people out is height: these sit on the return pipe next to the boiler and need clear space underneath to get the canister off. Sizes and what each one does and does not catch are set out in the magnetic filter guide.

Stone cold all over while the rest of the house is hot: the valves

One radiator completely cold, with the others working, points at that radiator's own valves rather than the system. Feel the pipes at the bottom of it. If the pipes are hot and the panel is cold, water is reaching the radiator and not getting in.

The usual culprit is the pin under a thermostatic radiator valve. Unscrew the plastic head off the valve body and you will find a small spring-loaded pin poking up. It should move down when pressed and spring straight back. After a summer with the heating off, they very often seize down in the closed position, which shuts the radiator off permanently no matter what number the head is set to.

Free it by gripping the pin with pliers and working it gently up and down until it springs back on its own. Tap it sideways rather than hammering it, because the pin operates against a seal and a bent pin means a new valve. A squirt of penetrating oil helps, kept off the paintwork. If it will not free, or it weeps once it moves, the valve body needs replacing and that means draining down. What the valve is actually doing and how the numbers map to temperature is explained in the guide to thermostatic radiator valves.

The other valve is the lockshield at the opposite end, usually under a push-on plastic cap. It sets how fast water can leave the radiator and it does get closed off, most often by someone decorating or taking the radiator off and never reopening it. Lift the cap, turn the spindle anti-clockwise, and see whether the radiator comes to life. The mechanics of both valves are covered in how to turn a radiator off, which is the same operation in reverse.

Every radiator cold: the problem is not the radiators

If nothing in the house is getting hot, stop looking at radiators. Nothing you do to a panel will help when no hot water is being sent to it. Work through the cheap checks in order.

1. Is the heating actually calling? Room thermostat above current room temperature, programmer in a heating period, and any smart control not sitting in a holiday or away mode it was left in months ago. This is a more common answer than it sounds.
2. Check the pressure gauge. On a sealed system a gauge reading near zero means the pump cannot circulate. Repressurising is a two minute job with the filling loop, set out in how to repressurise a boiler. If it drops again within days, you have a leak somewhere and that needs finding.
3. Listen at the pump. A circulating pump that is silent and cold is not running. One that is humming but where the pipes either side stay the same temperature has probably seized.
4. Hot water but no heating. On a combi that pattern points at the diverter valve or a motorised valve on a system boiler, both of which are engineer jobs rather than DIY. Work through the full routing, including the free checks worth doing before you call anyone, in no heating but hot water.

Age matters here. If the boiler is old enough that parts are getting hard to source, spending several hundred pounds on a component can be the wrong call, and the honest version of that arithmetic is in how long a boiler actually lasts. If the heating is going to be off for more than a day or two, a plug-in heater in the one room you are using is a reasonable stopgap, and the running cost arithmetic for that is set out in the guide to the best oil filled radiators.

Cold in the middle, hot at both ends

This one is genuinely unusual and it almost always means heavy sludging. Water entering at one end is taking the shortest route across the top of the panel and out the other side, because the middle of the lower waterway is packed solid enough that nothing wants to go through it. You get warmth at both ends where the flow enters and leaves, and a dead cold band across the centre.

Bleeding achieves nothing, and a chemical cleanser circulating through the system will struggle to reach a blockage that solid. Realistically the radiator comes off the wall and gets flushed outside with a hose, and if that does not clear it the panel is finished and wants replacing. Either way, treat the system afterwards, because a radiator that bad means the rest of them are on the same road.

Noisy but still heating up

Gurgling, trickling and light banging in a radiator that is otherwise heating properly is usually air moving through it, or water being pushed through faster than that radiator needs. Neither is a fault in the radiator itself.

Bleed it first, since that is quick and rules air out. If the noise carries on, it is generally a balancing issue, which is the next section. Kettling, a hard rattling boil that comes from the boiler rather than the radiator, is a different matter and is a sign of scale or sludge on the heat exchanger. That needs an engineer looking at it rather than a bleed key.

Downstairs cold, upstairs roasting: balancing

If whole floors or whole ends of the house behave differently, and no individual radiator has a cold patch, the system is out of balance. Water takes the easiest route, so radiators nearest the boiler and on shorter pipe runs get more flow than the ones at the far end.

Balancing means partly closing the lockshield valves on the greedy radiators so more flow is forced round to the starved ones, working towards a similar temperature drop across every radiator. It is a slow job rather than a difficult one, usually done with two thermometers and a lot of patience, and it is worth doing after any system clean because the flow pattern changes once the sludge has gone. The full sequence, the temperature you are aiming for, and how to confirm the system really is out of balance rather than faulty, are all in how to balance radiators.

What each fix actually does, and what it cannot do

When none of this will fix it

There is a point at which a radiator stops being worth repairing. If the panel is pitted or weeping rust through the paint, it is corroding from the inside and no amount of flushing puts metal back. If you have flushed it and the bottom is still cold, the silt has bonded and it is not coming out. If the tails or the valve threads are so corroded that they will not seal on reassembly, the cost of the labour to keep fighting it exceeds the cost of a new panel.

Replacement is also the sensible answer where the radiator was undersized for the room from the outset, which you can spot when it runs hot along its whole length and the room still never gets warm. That is a heat output problem, not a fault, and cleaning it will not add a single watt.

One warning before you order. If several radiators are silted and you replace just one, the new clean panel becomes the path of least resistance in a dirty system, and it will collect magnetite faster than the one you took out. Clean the system, dose it and fit a filter, then change the radiator.

Where this stops being a DIY job

Bleeding, freeing a TRV pin, opening a lockshield, topping up pressure and taking a radiator off the wall are all householder jobs. Some things are not, and two of them are a matter of law rather than caution.

• Anything on a gas appliance or its flue must be done by a Gas Safe registered engineer. Under the Gas Safety (Installation and Use) Regulations 1998 it is illegal for anyone not on the Gas Safe Register to carry out work on gas fittings and appliances. That covers opening up the boiler, diverter valves, gas valves and flue work. It is not a recommendation.
• Unvented hot water cylinders require a competent person holding the relevant qualification. Work on an unvented system is notifiable under the Building Regulations and the safety devices on a cylinder are not something to take apart on a Saturday.
• Power flushing is not illegal to attempt, but it involves a machine pushing water through a system at pressure and it finds every weak joint you have. Paying someone whose insurance covers the outcome is the better call.
• Repeated pressure loss means water is escaping somewhere. Topping it up every week hides a leak and adds fresh oxygenated water to the system every time, which accelerates the corrosion that caused your sludge in the first place.

Frequently asked questions

Why are my radiators not heating up even though I have hot water?

Hot taps working while radiators stay cold tells you the boiler is firing, so the fault is in whatever routes that heat to the heating circuit rather than the hot water. On a combi that usually means the diverter valve is stuck on the hot water setting. On a system or regular boiler it is more often a motorised valve or its actuator. Both are engineer jobs. Before calling anyone, rule out the free answers: a programmer set to hot water only, and a room thermostat set below the current room temperature.

Why is one radiator not getting hot after bleeding?

Because bleeding was almost certainly the wrong fix. If water came out immediately with no hiss, there was no air in it to release. Go back to the pattern: cold at the bottom is sludge, cold all over with hot pipes at the valves is a seized TRV pin or a closed lockshield, and cold pipes as well as a cold panel means nothing is reaching that radiator at all. Bleeding only ever cures a cold top with a warm bottom.

Why is my radiator cold at the bottom but hot at the top?

Sludge. Magnetite formed by internal corrosion is heavier than water and settles across the flat bottom of the panel, so hot water flows over the top of it. Bleeding does nothing because there is no air involved. A single affected radiator can be taken off and flushed through outside. Several affected radiators means the system needs cleaning, and either way it should be dosed with inhibitor and given a magnetic filter afterwards.

Why is my radiator cold at the bottom when there is no sludge in it?

If the water you drained out ran clear, look at flow instead. A lockshield closed too far, or a system badly out of balance, can starve a radiator enough that the water gives up most of its heat in the top of the panel and never properly fills the bottom. Newly fitted radiators that are cold at the bottom are nearly always this rather than sludge, since they have not been in long enough to silt up. A weak or failing circulating pump produces the same effect across several radiators at once.

What happens if you bleed a radiator with the heating on?

The system is hot and pressurised with the pump running, so instead of a hiss of air you can get a jet of scalding water out of the bleed valve, and on a sealed system you will pull the pressure down faster than you expect. Turn the heating off and let the radiators cool first. It costs you twenty minutes and removes the risk entirely.

Why are my radiators lukewarm even when the heating is on?

Warm everywhere but never properly hot is a system-wide symptom rather than a radiator fault. The most common cause is a boiler flow temperature set low, which many modern condensing boilers are deliberately set to, since running cooler makes them more efficient but means radiators feel tepid rather than scalding. Beyond that, look at a system thick with sludge slowing circulation, a tired pump, or a boiler cycling off early. Check the flow temperature first, because it is free.

How do I know when a radiator needs replacing rather than fixing?

Replace it when the metal has gone rather than when it is dirty. Rust weeping through the paint, pinholes, pitting, a persistent cold bottom after a proper flush, or valve threads too corroded to reseal are all end of life. A steel panel radiator has no published service life, because what kills it is the condition of the water inside it rather than its age. Panels in a system that was cleaned and kept dosed with inhibitor outlast panels in a neglected one by a very wide margin.

How much does it cost to replace a radiator?

There is no single honest figure, and anyone quoting one without seeing the job is guessing. What moves the price, roughly in order of impact: whether the pipework has to be altered or just reconnected to the existing tails, whether the system has to be drained down or can be isolated at the valves, the size and type of the panel, whether new valves are needed, and local labour rates. Get two quotes from people who have looked at it, and ask whether draining down is included, because that is where the surprises live.

Related guides

• How to bleed a radiator: the full procedure, and what it means when no air comes out.
• Central heating inhibitor: how much your system needs, how the brands compare, and when to top it up.
• Magnetic filters: what they catch, what they miss, and whether one will physically fit your boiler cupboard.
• Thermostatic radiator valves explained: what the numbers actually mean and why the pin seizes.
• How to remove a radiator: isolating, draining and getting it off the wall without flooding the room.
• How to balance radiators: for radiators that all warm up, but at very different rates.