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Flushing system, cleaning and inhibitor

Air Source Heat Pumps (ASHPs)

Last Post by Old_Scientist 40 minutes ago

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Old_Scientist

(@old_scientist)

Honorable Member Member

Joined: 2 years ago

Posts: 510

Topic starter 12/08/2026 7:46 am [#3135]

Hi all,

I have some questions about flushing my system, cleaning it and adding inhibitor.

I had a small leak in the system pipework, so drained the system down to fix it (longer story which I will also post about).

Bit of background, the system is 2 years old - everything was new apart from one old retained towel radiator and one small section of 22mm copper piping on the radiator circuit (maybe 5m total). All other pipework and radiators were new.

Whilst the system was fully drained, I took the opportunity to flush the system using the fill and flush loop. I closed all radiator valves and then opened one at a time flushing each radiator in turn until the water ran clear. The water coming out was orange brown in colour. I thought I was doing well, but once the system had settled for a while, if I flushed again more orange brown coloured water would flush out before running clear again. Is this normal, is this colouration from rust? Should I keep flushing? Picture of sample of water after much flushing below (ignore the black floaty bits which were in the container before).

I've bought some Sentinel X400 cleaner and X100 inhibitor. The instructions say the X400 cleaner should be circulated for a minimum of two hours at normal operating temperature and can be left in the system for 3-4 weeks before draining and refilling with the inhibitor. As the heating will not be back on until October, should I add the X400 cleaner now, and just circulate it around the system (radiators) for 10 mins per day as the heating is not on, or should I wait until October before adding it at which point it can work in a heated system? It will get circulated through the primaries and ASHP for 30mins per day during DHW cycles. The heating only ever runs at 32C LWT in October, so maybe not a huge difference to just pumping cool (25C) water round the system now for a few days to clean? I don't expect the system to be full of sludge so it's really a case of just wanting the system to be as clean as possible before refilling with inhibitor and being good to go for the next couple years.

If I don't add it until October, will my system be OK running for two months (DHW only) with just water and no inhibitor?

We are in a soft water area.

This topic was modified 37 minutes ago by Old_Scientist

This topic was modified 36 minutes ago by Old_Scientist

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.

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