Radiators Cold at the Bottom: Sludge, Not Air
Where a radiator is cold tells you what's wrong with it.
Where a radiator is cold tells you what's wrong with it. Cold at the top and warm at the bottom is trapped air and a bleed key fixes it. Cold at the bottom with a hot top is magnetite sludge lying in the base of the panel, and bleeding will do nothing at all. This page explains the difference, what a proper clean involves, and when a flush is worth paying for.
What sludge actually is
Magnetite is black iron oxide formed when oxygenated water sits against steel radiators. It is heavier than water, so it settles in the lowest, slowest part of the circuit — the bottom of radiator panels and the base of horizontal pipe runs. It insulates the emitting surface, so the room needs longer boiler run time for less heat.
Chemical clean versus power flush
A chemical clean circulates a cleanser through the existing system for a period before draining and refilling with inhibitor; it suits lightly affected systems. A power flush uses a dedicated pump and magnet to push high-volume, low-pressure flow through each radiator in turn, reversing direction to lift settled deposits. It is the right choice where several radiators are cold at the bottom or the system has never been cleaned.
Signs your system needs more than a flush
Pinholed radiators, valves seized solid, or microbore pipework fully blocked won't be recovered by cleaning. We'll tell you plainly when replacement of specific radiators or a section of pipework is the better spend, rather than flushing a system that will disappoint you afterwards.
Keeping it clean afterwards
A flush without a magnetic filter and inhibitor is money half spent. We fit the filter on the return to the boiler where it catches debris before it reaches the heat exchanger, dose the correct inhibitor concentration for the system volume, and check filter and inhibitor level at each annual service.
Common questions
How long does a power flush take?
For a typical three-bedroom house with eight to ten radiators, most of a working day. Larger properties, or systems where valves have to be freed off first, can take longer and we'll say so when we quote.
Will a power flush fix a noisy boiler?
Often, yes. Kettling noise is usually deposits on the heat exchanger surface causing localised boiling. Cleaning the system removes the cause; if the noise persists afterwards, the exchanger itself is scaled and needs treating or replacing.
Is power flushing safe on old pipework?
It uses high flow rate rather than high pressure, so it is safe on most systems. On very old microbore or previously leaking pipework we'll advise a gentler chemical clean instead, because a flush can reveal weaknesses that were already there.
Do I need a flush before a new boiler?
If we're installing a new boiler onto existing pipework, the system must be cleaned first — it protects the new heat exchanger and it is a condition of most manufacturer guarantees, including Worcester Bosch.