Start with the pattern
Feel the radiator all over, because the cold spots tell you a lot. Where it is cold tells you a lot. A cold top suggests air, while a cold bottom suggests sludge.
Check if one radiator is cold or several, because that narrows the search. One cold radiator often has a local fault. Several point to a system fault.
Fault type 1: Cold at the top (air in the radiator)
Air collects at the top of a radiator and stops hot water filling it, so bleeding helps. Bleeding lets the air out. Use a radiator key and a cloth, and close the valve when water appears.
Check the boiler pressure afterwards. Our guide to how to bleed a radiator shows each step.
Fault type 2: Cold at the bottom (sludge)
Sludge is a mix of rust and dirt that settles at the bottom, and it blocks the flow. It blocks the flow and leaves a cold strip. Bleeding will not fix it.
A heating engineer can clean the system. A powerflush may help if the sludge is heavy.
Fault type 3: Completely cold (no circulation)
A radiator that stays cold may have a stuck valve, while a weak pump can cause the same fault. The pump may also be weak. Check that both valves are open and that the thermostat head is not stuck.
If several radiators are cold, the pump or a system valve may have failed. Ask an engineer to check, because these parts need testing.
Fault type 4: Radiators cold when hot water works (combi boiler)
If hot taps work but the radiators stay cold, a diverter valve may be stuck. The boiler may also be set wrongly. Check the heating controls and thermostat.
This is a job for a Gas Safe registered engineer, since the fault is inside the boiler. Ask to see their ID card.
Fault type 5: Last radiators on the circuit cooler
Radiators far from the boiler can run cooler, since the hot water reaches them last. The system may need balancing. Balancing adjusts the valves so each radiator gets a fair share.
An engineer can do this with a thermometer. Meanwhile, check for sludge, because it can cause the same problem.
Fault type 6: Radiator cold after being removed
When a radiator is taken off, air and dirt can enter, so bleed it after refitting. After refitting, bleed it and check the valves. Make sure both are fully open.
Check the boiler pressure and top it up if needed, since air loss can drop it. If the radiator stays cold, call a plumber.
When to book a powerflush rather than a repair
Book a powerflush when sludge affects many radiators. Signs include black water, cold bottoms and a noisy boiler. Read our guide on what a powerflush is and the powerflush cost guide.
If only one radiator is faulty, a simple repair is usually enough. Ask an engineer to explain the choice.
Related guides and local help
If you want to know more, read 7 Signs You Need a Powerflush, Not Just a Boiler Service. You can also read Powerflush Cost Guide: What Affects the Price You Pay and Underfloor Heating Problems: A Diagnosis and Repair Guide, because they cover closely related problems.
These guides help too: New Boiler Guide: How to Choose the Right Boiler for You, What Is a Powerflush and Does Your Boiler Need One Now?, How to Drain and Flush a Central Heating System at Home, Annual Plumbing and Drainage Maintenance Checklist and How to Bleed a Radiator: A Simple Step-by-Step UK Guide. Each one explains a related topic in plain English.
Need help near you? Read our powerflush page first, or see Powerflush in Aviemore and Powerflush in Awsworth. You can also see Plumbing Repairs in Carlton, since every service page has a free quote form. We pass each request to a local drainage engineer.
