Radiator Not Heating Up? Common Causes and When to Call a Plumber
Quick Checks Before You Diagnose
Before diving into specific causes, rule out these basics. They account for a surprising number of cold radiator calls.
- Is the thermostat set high enough? Try turning it up to 25°C to test
- Is the timer or programmer set correctly? Check it has not reset after a power cut
- Is the boiler firing up? Check for error codes on the display
- Is the boiler pressure between 1 and 1.5 bar? Top up via the filling loop if low
- Are all radiator valves open? Check both the TRV and the lockshield valve at the other end
- Have you bled the radiators recently? Air builds up over time and causes cold spots
All checked and still have a cold radiator? Read on to diagnose the specific cause.
6 Common Causes of a Cold Radiator
Each cause has a different symptom pattern. Match what you are seeing to find the most likely culprit.
Trapped Air (Cold at the Top)
Air naturally builds up inside radiators over time. When it collects at the top, it prevents hot water from filling the radiator completely. The bottom feels warm but the top stays cold. This is the most common radiator problem and the easiest to fix — bleeding the radiator releases the trapped air in a couple of minutes.
Use a radiator key to open the bleed valve at the top corner. Hold a cloth underneath and turn the key anti-clockwise until you hear air hissing out. Close the valve as soon as water starts to flow. Check your boiler pressure afterwards and top up if it has dropped below 1 bar.
Sludge Build-Up (Cold at the Bottom)
Rust, dirt and limescale settle at the bottom of radiators over time, forming a thick sludge that blocks water flow. The top of the radiator feels hot because hot water enters at the top, but the bottom stays cold because sludge prevents circulation. This is especially common in older Exmouth properties with original pipework.
A single radiator can sometimes be removed and flushed out manually. If multiple radiators are affected, the whole system likely needs a power flush — a machine-driven clean that costs £300 to £600 and clears sludge from every radiator and pipe in the circuit.
Stuck Thermostatic Valve (TRV)
The thermostatic radiator valve (the one with numbers on the dial) controls how much hot water enters the radiator. The internal pin can seize in the closed position, stopping water flow entirely even when the dial is turned up. This is a very common cause of a completely cold radiator when others in the house are working fine.
Remove the TRV head (it usually unscrews or unclips). You should see a small metal pin sticking up. If it is stuck down, gently tap it with a hammer or grip it with pliers and ease it up and down until it moves freely. Refit the head. If the pin will not free up, the valve needs replacing — a plumber can do this for around £80 to £150.
System Needs Balancing
If radiators near the boiler get hot quickly but those further away stay lukewarm, the system is unbalanced. Hot water takes the path of least resistance, so nearby radiators get most of the flow while distant ones are starved. This is a common issue after a new radiator is added or a boiler is replaced.
Balancing involves adjusting the lockshield valve on each radiator so that water is distributed evenly across the whole system. A heating engineer measures the temperature difference across each radiator and adjusts the valves accordingly. It typically takes one to two hours and costs £80 to £150.
Faulty Diverter Valve (Combi Boilers)
On a combi boiler, a diverter valve switches between heating your radiators and heating your hot water. If this valve sticks or fails, your boiler may heat your taps but not your radiators — or vice versa. You might notice that radiators only work when no hot taps are running.
A faulty diverter valve needs replacing by a Gas Safe registered engineer. The part itself costs £80 to £200 and labour adds another £100 to £200. Your engineer should also check if the fault has caused other issues with the system while it was malfunctioning.
Failing Central Heating Pump
The central heating pump circulates hot water from the boiler around your radiators. If the pump is failing, water moves too slowly to heat radiators properly — particularly those furthest from the boiler. You might hear humming or clicking from the pump, or notice that all radiators are lukewarm rather than hot.
A heating engineer can test the pump output and replace it if needed. A new pump typically costs £150 to £300 fitted. The engineer should also check for sludge, as a blocked system puts extra strain on the pump and may have caused the failure.
When to Call a Professional
Some radiator fixes are quick DIY jobs. Others need a qualified plumber or heating engineer with the right tools and experience.
You Can Fix
- Bleeding radiators to release trapped air
- Freeing a stuck TRV pin
- Topping up boiler pressure via the filling loop
- Checking the thermostat, timer and valve positions
Call a Professional
- Multiple radiators with cold spots — likely needs a power flush
- System needs balancing — requires measuring and adjusting every radiator
- Faulty diverter valve or pump — boiler component work
- Any work involving gas or the boiler — must be Gas Safe registered
Radiator Problem FAQs
Still Got a Cold Radiator?
If the DIY fixes have not worked, it is time to call in a local professional. Browse our directory of Exmouth plumbers and heating engineers to find someone who can diagnose and fix the problem.
Find Local Heating EngineersNeed a new radiator fitted? Having boiler trouble too? We have guides for those as well.