18 Litres of Sludge Removed by a Kamco CF90 Powerflush on an Eight-Radiator System

Eighteen litres came out of that system across roughly two hours of circulation. The Kamco CF90 ran at mains pressure through the flow and return, one radiator isolated at a time, with the pump reversed every few minutes to shift debris that had settled behind the baffles. The water started the colour of strong tea and finished nearly clear after the fourth radiator, which is the one downstairs that had a cold bottom third and a warm top. That cold patch was the reason the homeowner called.

Magnetite is heavy. It drops to the lowest point of each radiator and to the belly of the heat exchanger, and once there is enough of it the circulation slows, the boiler short-cycles, and the room that used to reach 21 degrees now stalls at 18. None of that shows up on a flow-temperature readout. It shows up as a gas bill that crept upward over three winters and a lounge that never quite warms.

What the magnet on the CF90 actually caught

The Kamco unit carries a magnetic filter in-line, and the neodymium core came out furred with a grey-black paste after the first radiator alone. That paste is the tell. Iron oxide forms when oxygen gets into the system water and reacts with the steel panels and the cast-iron sections of older pumps. Every top-up through a leaking automatic air vent, every drain-down for a repair that was refilled with raw tap water, feeds the reaction.

On this job the filter was cleared and re-seated three times. The volume recovered on the magnet is not the same as the 18 litres of dirty water pumped to drain, but it is the fraction that would otherwise have jammed the new pump within a season. A Fernox TF1 or Adey MagnaClean fitted permanently on the return would have caught most of it over the years and made this flush a twenty-minute job instead of a two-hour one.

The homeowner had no such filter. The system was installed with a Grundfos pump, a mid-range combi, and no inhibitor dosing that anyone could document. That is the ordinary state of a system that has changed hands twice.

Balancing the eight radiators after the flush

A clean system is not a balanced system. Once the sludge is gone, water takes the path of least resistance, which means the radiators nearest the pump run hot and the far ones lag. Balancing corrects that by throttling the lockshield valve on each radiator so the flow matches the heat the room needs.

The method is unglamorous and works. You open every lockshield fully, run the system to temperature, then use a clip-on pipe thermometer or an infrared reader on the flow and return tails of each radiator. The target is a temperature drop across each one of around 11 to 12 degrees Celsius. The radiator closest to the pump gets its lockshield closed most, sometimes to barely a quarter turn open; the furthest gets left near fully open. On this eight-radiator layout the two downstairs rads and the hall panel needed the tightest lockshields, and the far bedroom needed none.

Get the drop even across all eight and the boiler sees a consistent return temperature, which for a condensing appliance matters enormously. Below about 55 degrees on the return, the flue gases condense and the latent heat is recovered. Above it, the condensing mode barely engages and the efficiency figure on the data plate becomes fiction. A poorly balanced system that returns water at 60-plus degrees is throwing away the very thing the boiler was sold on.

The balance held after the flush because the flow was no longer fighting sludge. Before it, no amount of lockshield adjustment would have fixed the cold bottom on that lounge radiator, because the debris was physically blocking the lower channels.

Weather compensation, if the boiler supports it

The combi on this property had a weather compensation input that was never wired. An outdoor sensor lets the boiler lower its flow temperature as the outside air warms, so instead of firing to 70 degrees on a mild October afternoon it modulates down to 50 and stays in condensing mode far longer. On a freshly balanced and desludged system the effect compounds, because the radiators are now sized correctly against a lower flow temperature.

Wiring it is a job of an hour and a length of two-core cable to a north-facing wall. The payback is quiet and real over a heating season, though it depends heavily on the emitters being generous enough to heat the rooms at 50 to 55 degrees flow. Undersized radiators defeat the whole approach, which is why the sizing survey should precede any control upgrade.

The short version on inhibitor

Dose the system with a corrosion inhibitor immediately after the flush, before the water has a chance to start oxidising the clean steel. Fernox F1 or Sentinel X100 at the correct concentration for the system volume is the whole job.

Condensate frost protection, the overlooked failure

A condensing boiler produces acidic condensate that runs to drain through a plastic pipe, and where that pipe leaves the building it freezes in a cold snap. When it freezes solid the boiler locks out on a condensate fault and the house goes cold on the coldest morning of the year, which is precisely when it is least welcome.

The fix is dull and effective. Any external run should be at least 32mm bore instead of the 21.5mm often used internally, lagged with waterproof foam, and pitched to drain with a continuous fall. Where a long external run is unavoidable, a trace-heating cable on a frost thermostat keeps it above freezing for a few pence a night. On this property the condensate exited through a 21.5mm pipe with no lagging, run externally for nearly two metres to a gully. It had frosted twice the previous winter, and the homeowner had been thawing it with a kettle each time, not realising the pipe bore itself was the fault.

The better practice, where the layout allows, is to terminate the condensate internally into a soil stack or a kitchen waste, keeping the whole run inside the heated envelope. That was not possible here without lifting a floor, so an upsized and lagged external run went in with a short trace-heating element on the exposed section.

This is the kind of detail that never appears in a powerflush quote, yet it determines whether the system the homeowner just paid to clean actually runs through January.

What the flush did not fix

The 18 litres of sludge explained the cold radiator and part of the gas bill. It did not explain why the upstairs rooms took so long to reach temperature, and that turned out to be a loft with barely 100mm of degraded mineral wool laid between the joists. Heat was leaving through the ceiling faster than eight clean radiators could replace it.

A flush restores circulation. It does nothing for a building fabric that leaks heat, and no control strategy, weather compensation included, compensates for insulation that stops at 100mm when current practice runs to 270mm or more. The powerflush was the right call for the symptom the homeowner reported. Whether it was the most cost-effective first pound spent on that house is a separate question, and one the survey never framed.