A radiator that’s hot in one place but cold in another is one of the most common central heating complaints in the UK — and also one of the most fixable. The location of the cold patch tells you almost everything you need to know about the cause.
Quick Diagnosis
Before touching anything, feel the radiator across its full height and width with your hand:
| Pattern | Cause | Fix |
|---|---|---|
| Cold at the top, warm/hot at the bottom | Trapped air | Bleed the radiator |
| Cold at the bottom, warm/hot at the top | Sludge (magnetite) build-up | Power flush or chemical treatment |
| Cold in the middle (or diagonal cold strip) | Sludge patch or corroded section | Power flush |
| Cold all over but pipework warm | Stuck thermostatic radiator valve (TRV) | Free or replace the TRV pin |
| Cold all over and pipework cold | No flow — pump fault, air lock in system, valve closed | Check valves, system pressure, pump |
| Some radiators hot, others cold | Poor system balancing | Balance the system |
Fix 1: Bleeding a Radiator (Cold at Top)
Air gets into a central heating system through normal operation, filling, and minor leaks. Being lighter than water, it rises to the top of each radiator and prevents the top section from heating.
What you need: radiator bleed key (£1–£3 from any hardware shop or DIY store), a cloth or small container.
- Turn the heating on and let the system heat up for 15 minutes.
- Feel each radiator for cold spots at the top.
- Turn the heating off before bleeding — this prevents drawing more air into the system.
- Locate the bleed valve at the top corner of the radiator (a square or slotted nut recessed into the end cap).
- Hold the cloth under the valve and insert the bleed key.
- Turn the key slowly anticlockwise (half a turn maximum). You should hear air hissing out.
- As soon as water appears (a dribble or small jet), close the valve immediately. Don’t let it run — the system water may contain corrosion inhibitor and isn’t pleasant.
- Check the system pressure gauge on the boiler. Bleeding releases water pressure — top up via the filling loop if the pressure has dropped below 1 bar (typically needs to be at 1–1.5 bar when cold).
- Turn the heating back on and check the radiator heats uniformly.
Repeat bleeding needed? If air returns within days or weeks, it suggests an ongoing issue — either a micro-leak drawing air in, or hydrogen gas being generated by internal corrosion (a sign the system needs inhibitor treatment or a power flush).
Fix 2: Power Flushing for Sludge (Cold at Bottom)
Magnetite sludge is the dark, fine powder produced when mild steel radiator panels corrode in the presence of oxygen. It settles at the lowest point of each radiator — the bottom — creating cool or cold patches. Over years it can block heat exchangers in combi boilers, reduce pump efficiency, and cause complete radiator failure.
Signs sludge is the problem:
- Multiple radiators cold at the bottom
- Boiler making a kettling noise (sludge accumulating in the heat exchanger)
- Heating system slow to warm up
- Dirty, dark brown water when you bleed a radiator
Chemical treatment (mild sludge): Add a central heating cleaner (e.g., X-400 or equivalent) through the feed and expansion tank, or via a radiator bleed point. Run the system for 1–2 weeks with the cleaner circulating, then drain, flush, and refill with clean water plus a corrosion inhibitor (e.g., X-100). A dose of inhibitor costs £15–£30; a cleaner £15–£25.
Power flushing (moderate to severe sludge): A heating engineer connects a power flush machine to the system. It forces water at high pressure and velocity through each circuit to dislodge sludge deposits, then flushes to drain. A magnetic filter is usually fitted at the same time (£50–£150) to capture future sludge before it can settle in radiators.
Power flushing costs £300–£700 for a typical 3–4 bedroom house (£400–£600 is the most common range in 2026). It should be carried out by a Gas Safe registered heating engineer.
Fix 3: Freeing a Stuck TRV Pin
Thermostatic radiator valves (TRVs) have a pin inside the valve head that rises and falls with the thermostat to control flow. If a radiator sits unused all summer, the pin often corrodes in the lowered position and refuses to open in autumn. This leaves the radiator cold all over even though the system is running and pipework is warm.
To check: remove the TRV head (usually a click-off or screw-off cap) and look for a small brass pin in the valve body. It should move freely when pressed. If it’s stuck down, it’s blocking flow.
To free: grip the pin with pliers and waggle gently up and down. In mild cases this frees it. If stuck fast, spray with a small amount of penetrating oil, wait 15 minutes, and try again. Worse cases require a new TRV valve head or the full valve (£15–£40 for a quality thermostatic head; £25–£80 for a full valve replacement).
Fix 4: Balancing the System
In a central heating system, radiators closest to the boiler and pump get the most flow and heat up fastest. Those at the far end of the circuit can be sluggish. Balancing adjusts each radiator’s lockshield valve to restrict flow to near radiators and increase it to far ones, distributing heat evenly.
To balance a system:
- Fully open all radiator lockshield valves (turn anticlockwise fully, then back slightly). Close all TRVs except on the radiator you’re starting with.
- Turn heating on. Use a clip-on thermometer or infrared thermometer to measure the flow and return pipe temperatures at each radiator — you’re aiming for a 12°C differential between flow and return.
- Starting at the radiator closest to the boiler, gradually close the lockshield valve until the differential reads 10–15°C.
- Move to the next radiator and repeat, working around the system away from the boiler.
This is fiddly but free to do yourself. Some heating engineers offer balancing as a standalone service for £80–£150.
When to Call a Heating Engineer
Contact a Gas Safe registered heating engineer if:
- Bleeding doesn’t resolve the cold-top radiator within a couple of cycles
- Multiple radiators are cold at the bottom and chemical treatment hasn’t improved it within two weeks
- The boiler is making loud banging or kettling noises
- System pressure drops regularly without obvious external leak
- You’re losing significant water and can’t locate the source
Preventing Future Sludge Build-Up
Once a system has been flushed and inhibitor added, maintain it by:
- Fitting a magnetic filter (inline on the return pipe before the boiler) — captures magnetite particles before they settle. Clean annually.
- Topping up inhibitor annually or checking the existing concentration with a test strip (£5–£10) — inhibitor depletes over time.
- Avoid constant draining and refilling — every new charge of water introduces fresh oxygen that accelerates corrosion.
A well-maintained system with good inhibitor concentration can run for 20–30 years without significant sludge issues. Ignoring it risks a blocked heat exchanger (£500–£1,200 to replace on a combi) or premature boiler failure.
