Search with Wattson

Notifications

Clear all

Joining the Renewable Heating Hub forums is completely free and only takes a minute. By registering you’ll be able to ask questions, join discussions, follow topics you’re interested in, bookmark useful threads and receive notifications when someone replies. Non-registered members also do not have access to our AI features. When choosing your username, please note that it cannot be changed later, so we recommend avoiding brand or product names. Before registering, please take a moment to read the Forum Rules & Terms of Use so we can keep the community helpful, respectful and informative for everyone. Thanks for joining!

Flushing system, cleaning and inhibitor

Old_Scientist · 2026-08-12T06:46:31Z

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.

Page 2 / 2 Prev

Air Source Heat Pumps (ASHPs)

Last Post by Papahuhu 57 minutes ago

13 Posts

4 Users

3 Reactions

72 Views

RSS

Papahuhu

(@papahuhu)

Reputable Member Member

Joined: 1 year ago

Posts: 380

13/08/2026 3:08 pm

@old_scientist

That sounds like text book biofilm (I used to work on very high purity water systems for IV use and experienced it several times). If you still have the pipework you cut out, can you swab it out to see if it’s slimy. If it is, then that’s the cause of you colouration and corrosion.

You are better finding someone who knows more than me about glycol, but I believe that concentration and the right mix of additives (including biocides I’d guess) is critical. I’d have thought your installer owns at least part of the remediation.

This post was modified 49 minutes ago by Papahuhu

ReplyQuote

Page 2 / 2 Prev