I carried out a mains water flush on our un-vented system today. As some of the pipework is very old, I didn't want to have the system power-flushed. I'll outline the steps that I took (bear with me as I have included as much detail as I can remember):
- Add Sentinel X800 to the system and circulate for a week. You can use Fernox F5 or other similar chemical to loosen sludge/magnetite.
- To allow the chemical to circulate and work: I initially ran the system with all the radiator valves open for 2 days. Then shut all the radiator valves upstairs for 2 days and ran the system. Shut all the radiator valves downstairs and opened the radiator valves previously shut upstairs and ran the system for 2 days. Opened all shut valves and ran the system for a further 2 days. Underfloor heating (in the kitchen) was operated on certain days in between the above radiator valve isolation process.
- Today (8 days later) - switched off boiler and drained system. Initally very black water drained out. Then as per the DIYnot guide, isolated the flow and return to the boiler using the service isolation valves (under boiler).
- With the boiler and system controls off, manually opened central heating and hot water (cylinder) zone valves.
- Connected filling loop and opened both mains cold water and filling isolation valves fully. Monitored pressure gauge on boiler as per DIYnot guide. As expected, no pressure reading (dial on zero) as the flow and return was isolated.
- Initially, water from the drain-off was dark in colour and chemical was still present in the system. Once the water ran clear, all the radiator valves were shut off and each radiator valve was opened individually with cold mains water flushing through until it ran clear from the drain-off.
- After each radiator had been flushed individually, the mains cold water flow through the filling loop was then reduced. The underfloor heating pump was operated to allow chemical to be flushed through that circuit. The mains cold water pressure was reduced to avoid damaging the pump.
- Once the water from the underfloor heating circuit ran clear, the drain-off was closed. With all the radiator valves still closed, the flow and return service isolation valves under the boiler were opened. The system was re-pressurised and the filling loop mains cold water isolation valve closed. The boiler and controls were switched on and the boiler pump operated to allow any remaining chemical in the boiler/pump to be circulated. The drain-off was opened again and the filling loop mains cold water isolation valve was opened slightly to allow the chemical in the boiler to be flushed through. The water from the drain-off was monitored until it ran clear.
- All the radiator valves were individually opened and the low-pressure mains cold water from the filling loop allowed to circulate through the boiler/pump and radiators.
- Once all the radiator valves were opened and the water from the drain-off ran clear, the drain-off was closed and the system was re-pressurised using the filling loop.
- There was a lot of air in the radiators and they had to be bled and the system pressure topped up using the filling loop. Don't forget towel radiators too! There may also be a separate (system) air vent like our's.
- Remember to the close the drain-off, all radiator air-vents and shut the filling loop isolation valves and disconnect the filling loop once you are finished.
The only thing I would say is, allow at least a day to do this. I underestimated the time it was going to take and had to cancel meeting up with a friend later in the afternoon!
If you are able to isolate the pump, then it would make the flushing process much easier.
After carrying out the mains flush, I have noticed that some radiators now seem to be extremely hot at the top but cold at the bottom. I haven't come across this scenario before? I have come across radiators cold at the top which is indicative of air in the radiator. Could the flushing have have caused sludge/magnetite to loosen and accumulate at the bottom of the radiators?
