Sealed system losing pressure with a cold-bottomed radiator and kettling

Start with the expansion vessel, because a 0.4 bar pre-charge on a system with 1.5 bar cold fill is too low.

With the system at zero pressure, the air charge should be roughly the static head at the vessel plus 0.3 to 0.5 bar. In a two-storey house with the vessel near a ground-floor boiler, static head is about 0.5 bar, so the pre-charge should be around 0.8 to 1.0 bar. At 0.4 bar the vessel is waterlogged almost immediately: as the water heats and expands there is no gas cushion, pressure rises to the 3 bar relief setting and the valve discharges. Over a few heating cycles that easily accounts for a fall from 1.5 to 0.9 bar. The relief pipe may show no staining because each discharge is small and evaporates, and the tundish is dry between events.

Test sequence:

1. Confirm the gauge reads zero when cold; drain to zero if not.
2. Check the Schrader valve again; a hiss of water means a failed diaphragm and the vessel must be replaced.
3. Check the relief valve by isolating it, raising pressure and watching the tundish.
4. Isolate boiler and circuit sections and pressure test to exclude a hidden leak; inspect radiator valves, the automatic air vent and the boiler internals.
5. Check the gauge itself against a known pressure.

If the vessel holds air, recharge to 1.0 bar with the system drained, then refill to 1.5 bar cold. If it does not hold, replace with a correctly sized vessel; 12 L for 130 L is about 9%, which is workable but check the acceptance factor for the actual fill pressure. Stop the fortnightly top-ups; fresh oxygenated water drives corrosion and the cause must be fixed first.

The radiator and the kettling point to the same underlying problem: poor water quality and restricted flow.

Cold bottom third of the radiator. Hot at the top and cold at the bottom is the classic signature of sludge (iron oxide and biofilm) settled in the bottom of the panel where flow is slowest. It is not air, which collects at the top and gives a cold top, and not a closed valve, which would leave the whole radiator cold. Confirm by measuring the temperature across the flow and return tails with the heating on; a large drop with a cold base supports it. Then isolate, remove the radiator and flush it outdoors; the colour of the water shows the severity.

Kettling at full output. Kettling is local boiling, scale or debris in the boiler primary heat exchanger. Check in order: pump setting or a failing pump, a partly closed bypass, and scale or corrosion products on the heat exchanger. A sealed system should not scale if it has never been topped up with hard water, but repeated top-ups introduce fresh water and oxygen, and the corrosion products then insulate surfaces and create hot spots.

Plan:

1. Fix the pressure loss first, or the flush will be undone.
2. Powerflush or chemically clean to a recognised standard, then flush until the water runs clear.
3. Fit a magnetic filter on the return and dose inhibitor to the manufacturer's concentration, recording the dose.
4. If kettling persists after flushing and correct pump operation, the heat exchanger may need descaling or replacement; follow the manufacturer's procedure before using acid descaler.
5. Re-balance the system and re-check radiator drops, aiming for 10 to 12 °C on each.

Tell the customer that topping up every two weeks is not a repair; each top-up adds oxygen and dilutes the inhibitor.

Answer directly: no, topping up is not a repair, and adding inhibitor without fixing the cause wastes money.

Reasoning to give the customer:

• Every top-up adds fresh water containing dissolved oxygen and carbonate hardness. Oxygen drives corrosion of steel radiators, producing the sludge already blocking the lounge radiator. Hardness deposits on the hottest surfaces, which is one route to kettling.
• Each top-up also dilutes whatever inhibitor remains, so the protection falls further with each event.
• The pressure loss has a cause. On the evidence it is most likely the undercharged expansion vessel discharging through the relief valve, but a hidden leak has not yet been excluded. Adding chemicals to a system that is still losing water simply repeats the cycle.

Sequence to agree: find and fix the pressure loss, recharge or replace the vessel, clean the system, fit a magnetic filter, dose inhibitor at the correct concentration, then refill to 1.5 bar cold and monitor for four weeks with no top-ups. Only a stable pressure justifies the chemical treatment.

Record on the paperwork: the initial and final pressures, the vessel pre-charge before and after, the relief valve test result, the volume of water drained and its condition, the flush method and water clarity, the filter fitted, the inhibitor product and dose, the balance settings and the radiator temperature drops, plus a statement that the system is now leak-free to the best of your knowledge and that the customer has been advised not to use the filling loop except for maintenance.