Cold top vs cold bottom
- Cold at the top: air — bleed it.
- Cold at the bottom: sludge.
- Cold in the middle or patchy: sludge or a blockage inside.
- Cold all over: valve closed, TRV pin stuck or system balance.
What is sludge?
Inside steel radiators, water slowly rusts the metal. The rust turns into a black mud called magnetite. It is heavy, so it settles at the bottom of radiators and in pipes, slowing the flow and making the system less efficient.
Flushing a radiator (overview)
- Heating off and cool.
- Close both radiator valves — count the turns on the lockshield so you can reset it.
- Put towels and a tray down, undo the valve unions and drain.
- Lift the radiator off and take it outside.
- Flush with a hose until the water runs clear.
- Refit, open valves, bleed, and check the pressure.
Pro tip: Radiator water is black and stains carpets badly. Use lots of towels and keep the radiator upright when carrying.
Prevent sludge
- Keep the right amount of inhibitor in the system.
- Fit a magnetic filter.
- Fix leaks so fresh water (and oxygen) is not constantly added.
