Where a radiator is cold tells you what is wrong with it. Cold across the top with a hot bottom is trapped air, and bleeding fixes it. Cold across the bottom with a hot top is sediment — magnetite sludge that has settled where flow is slowest. Bleeding does nothing for that, and running the system harder just moves the debris into the pump and heat exchanger.
The third pattern, cold radiators only at the far end of the run, is usually a balancing problem rather than a cleanliness one.
Reading the cold pattern
- Cold top, hot bottom — air. Bleed, then check why air is entering.
- Cold bottom, hot top — magnetite sludge settled in the base of the panel.
- Cold at one end only — poor balancing or a partially closed lockshield.
- One whole radiator cold — a seized valve, a stuck TRV pin or an airlock at the branch.
- Whole system sluggish, boiler noisy — circulation restriction, often a blocked exchanger.
Why sludge forms in the first place
Magnetite is iron oxide from the inside of steel radiators. It forms wherever oxygen meets steel and water: fresh top-ups after a leak, an ageing or absent corrosion inhibitor, or a system that has been drained several times for radiator changes. Once formed, it settles in the lowest, slowest part of the circuit — the bottom of the panels — and gradually starves the boiler of flow.
Power flush, chemical clean or neither
A chemical clean with a circulating cleanser suits lightly affected systems and can be done alongside a service. A full power flush — high-velocity flow with magnetic capture, radiator by radiator — is what heavily sludged systems need, and it is the correct step before fitting a new boiler onto old pipework.
Neither is worth paying for on a system that is simply out of balance. If the panels are clean and the pattern is end-of-run coldness, balancing the lockshields costs far less and fixes it properly.
Protecting the system afterwards
- Fit a magnetic filter on the return so future debris is captured, not circulated.
- Dose to the correct inhibitor concentration and test it at each annual service.
- Rebalance the radiators after any flush — flow rates change once the system is clean.
- Keep top-ups rare: every top-up reintroduces oxygen.
Frequently asked questions
- Will bleeding help a radiator that is cold at the bottom?
- No. Bleeding only removes air from the top of the panel. Cold at the bottom is sediment, and it needs flushing out.
- How long does a power flush take?
- A typical three-bedroom house takes most of a working day. Larger properties, or systems with heavy magnetite, take longer because each radiator is treated individually.
- Do I need a power flush before a new boiler?
- If the new boiler is going onto existing pipework and radiators, a proper clean is normally required to protect the warranty. Manufacturers are specific about system cleanliness, and a new heat exchanger clogs quickly on a dirty system.
- Can sludge come back?
- Yes, if the causes stay. A magnetic filter, correct inhibitor and fewer top-ups are what keep a flushed system clean.
Find out whether you need a flush or just balancing
Tell us which radiators are cold and where — we can usually tell you which of the two it is before we visit.