Radiators cold at the bottom: sludge, and what it really means

The two patterns, and what each means

Getting these the right way round saves a great deal of unnecessary work. Air rises; sludge sinks. That is the whole of it.

What magnetite actually is

A sealed heating system holds the same water indefinitely, in contact with steel radiators, copper pipe, brass fittings and often an aluminium heat exchanger. If oxygen is present, the steel corrodes, producing magnetite – a fine, black, magnetic iron oxide.

Oxygen gets into a sealed system by three routes:

1. Fresh water added to replace losses. Every top-up brings dissolved oxygen with it. This is by far the biggest contributor, which is why a system that is repeatedly repressurised is a system that is corroding.
2. Air drawn in on the suction side of the pump, at a weeping joint or a failed seal.
3. Permeation through plastic pipe that lacks an oxygen barrier, which is a real issue on some underfloor heating installations.

The magnetite is heavy and it settles wherever flow is slowest. In a radiator that is the bottom, below the flow path between the two valves at the top. Once a layer has built there, that part of the radiator is effectively out of circuit.

Why it matters beyond the cold radiator

A radiator that is cold at the bottom is the visible symptom. The same material is circulating elsewhere and doing more expensive damage.

• The boiler heat exchanger. Modern condensing boilers have very narrow waterways, because narrow waterways transfer heat well. That makes them exactly the wrong shape for tolerating particulate. A partially blocked exchanger overheats locally, kettles, and eventually fails – and it is the most expensive component in the appliance.
• The pump. Magnetite is abrasive and it destroys bearings. A pump failing every few years on the same system is telling you something.
• Efficiency. A radiator with a third of its surface out of circuit is delivering a fraction of its output, so the system runs longer to heat the house.
• Valves. Sludge causes TRVs and motorised valves to stick.
• Warranty. Manufacturers decline claims on heat exchangers fouled by system debris, and they are entitled to.

Confirming it before spending anything

1. Feel the radiator properlyWith the heating running for a while. Top hot, bottom cold, with a fairly defined horizontal line between them is the classic pattern.
2. Bleed itIf the top is cold rather than the bottom, this is your answer and you are done.
3. Do the magnet testAs above. Black particles on a magnet is definitive.
4. Check the valvesA whole cold radiator is usually a valve rather than sludge. Take the TRV head off and check the pin underneath moves freely – they seize.
5. Check whether it is the far radiators specificallyFar radiators cold with near ones hot is usually balancing, not sludge – see balancing a heating system.
6. Look at the filterIf there is a magnetic filter, what comes out of it at service tells you the system’s condition directly.

Only after those would we recommend a flush. A power flush sold on the basis of one cold radiator that was actually a seized TRV is a bad outcome for everybody.

What fixes it

Chemical clean

A cleaner dosed into the system, the heating run normally for a period – days to weeks – then drained, flushed and refilled with inhibitor. Inexpensive, non-disruptive, and effective on light to moderate fouling. This is where we would start for most systems.

Power flush

A pumping unit circulates water at high velocity with flow reversal and radiator-by-radiator isolation. The velocity is what lifts settled sludge that chemicals alone will not shift. A day’s work, and the right answer for a genuinely blocked system.

Individual radiator removal

Sometimes the practical answer for one or two badly affected radiators: take them off, flush them out outside with a hose, and refit. Messy, effective and cheaper than a full flush if the rest of the system is reasonable.

Replacement

A radiator that has been sludged for years may also be corroded internally to the point where flushing exposes the thinning. If a radiator starts weeping after a flush, the flush found the problem rather than caused it.

More detail in power flush, chemical flush, or neither.

Stopping it coming back

1. Inhibitor, correctly dosed. It depletes over time and it goes down the drain with every drain-down. Test it periodically rather than assuming.
2. A magnetic filter, cleaned at every service. It removes magnetite as it forms rather than letting it settle. A filter nobody empties catches nothing – see magnetic system filters.
3. Fix any leak. A system needing regular repressurising is taking in fresh oxygenated water continuously, and no amount of cleaning keeps ahead of that.
4. Check for air ingress on the pump suction side, and that automatic air vents are working.
5. Use barrier pipe for any new plastic runs.
6. Re-dose after any work that drained part of the system. The most commonly forgotten step in the trade.