Dirty system water but mag filter clean?

[RayPlumb]

Hi I have a client with a boiler I fitter a few years ago (about 4 years ). I replaced all radiators and half the pipework. System was treated with a X400 cleaner and then flushed clear, and inhibitor added. All good.
Few annual services in I noticed in the mag filter water was going brown, but the mag filter had no magnetite in the magnet at all. I drained some out until running a lot clearer and added another 1.5 bottles of inhibitor (11 panel rads and 1 towel rail) .
This years service it was a lot worse, very brown but mag filter still nothing on the magnet or any debris.
I was told that a few months earlier the kitchen fitter had drained the system so presume didn't add inhibitor.
How do you get brown water without any magnetite on the magnaclean magnet?
I thought I had flushed it well when I first installed with all new rads and half new pipework.
There is a vertical section of about 1.2mtr of the return pipe in old 28mm that was unable to be changed due to it buried in the listed building wall.. Could this larger pipe section have not been cleaned properly due to less flow rate through the larger pipe and only now cleaning off. ? If so why nothing on the magnet. ?
I'm going this week to re drain the whole system down and add more cleaner then back next week to re-flush system though. Any thoughts. ..

 

[Chuck]

Is this a vented or unvented CH system? I'm guessing it's vented.

TL;DR: brown sludge that is not trapped by a magnet is usually a problem in systems with low water temperature and is due to bacteria/biofilms, which love water at around 40°C. The F/E tank of unvented systems and UFH systems are particularly prone. Brown sludge can also be caused by a leak in an unvented system causing continuous filling of the F/E tank with fresh water. Under these conditions brown iron oxide, which is not as magnetic magnetite, forms.

More details here:

What's The Difference Between Black & Brown Central Heating Sludge

Got black or brown central heating sludge or want to know what the difference is, here we explain the process and how it harms your central heating system

www.emergencyplumber.uk.com

Also check the pH of the system water. If the boiler has an aluminium HX the pH needs to be within a fairly narrow range or you'll get corrosion, which can look brown.

Sentinel / Adey / Fernox all do biocide additives and have technical briefings / instructions on their website. They also offer a lab-testing service for samples of water if you want a definitive answer but they aren't free (ca £50 IIRC)

 

[RayPlumb]

So I flushed it mostly out today . The system is a sealed system worcester oil combi boiler. Water was pretty black but mag still clean. I let it run and boiler pump circulating round rads while filling and draining . I've added 2 bottles of sentinal x400 and told them to run it at least an hour every day and Ill come back in a week to flush out.
Just thinking now I think they might have had a water softener fitted 2 years ago with the new kitchen ... How much of an impact will this have on the x400 if its filled with softened water. If so ill stick it in bypass when flushing and filling

 

[Chuck]

Just thinking now I think they might have had a water softener fitted 2 years ago with the new kitchen ... How much of an impact will this have on the x400 if its filled with softened water. If so ill stick it in bypass when flushing and filling

Back when heat exchangers were cast iron and systems were vented it was common to fill central heating systems with softened water.

Then, IIRC, boiler manufacturers started using aluminium heat exchanges and the MIs said don't use softened water, which got added into the British Standard at some point.

Water-softener manufacturers say that there is no evidence that their product damages heating systems but they should be fitted with a by-pass in any case. So I would always flush and fill with unsoftened water. I might not bother for the occasional top-up however if bypassing and flushing through to unsoftened water was going to be painful.