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I purchased a Kudox steel panel radiator to replace a radiator that developed a leak. There is a warning triangle on first page of the Installation Instructions with the following in red capital letters:

‘AFTER INSTALLATION THE ENTIRE CENTRAL HEATING SYSTEM MUST BE THOROUGHLY FLUSHED WITH A ‘DOMESTIC WATER TREATMENT ASSOCIATION’ (DWTA) APPROVED CENTRAL HEATING CLEANSER.'​

The second page of the Installation Instructions includes:

‘9. Flush the entire system with a DWTA approved central heating cleanser and then refill using a DWTA approved central heating protector.’​

Is a central heating system flush necessary and why?

I do not intend to change the piping. I plan to re-use the old radiator tails with some PTFE tape.

I recently fitted a City Plumbing Halcyon (Stelrad Accord) radiator. The Stelrad Accord Installation Instructions do not mention a flush.

For reference, this is the third radiator that developed a leak recently and the second since having a new boiler and unvented hot water cylinder installed two months ago by a Gas Safe registered company. The radiators were previously gravity fed and are now on a sealed system.

Thanks
 
1. The system should have been flushed (chemical or power flush) when the new boiler was installed. If this was done I see no reason to do it again just because you have changed a radiator.
2. I would suspect that changing to a sealed (pressurised) system will have exposed existing weaknesses in the leaking radiators. A flush might have made the situation more obvious by removing a layer of protective "sludge", but if I'm right, the underlying problem was already present.
3. As exedon2 says, you may be in for further problems if the existing radiators were already weakened while the gravity system was in place.
4. I would suggest you remove and water pressure test all existing radiators one at a time to 3 bar, and replace any which fail. Could be done one at a time. Don't test with air, its not safe.
5. If you do 4. It would be worth repeating a chemical clean of the whole system to remove any residual sludge from the pipe work and satisfy any warranty issues.
6. I trust a magnetic filter was fitted to the boiler return pipe by the installers.
 
Technically the new radiator may contain oils from the manufacturing process (and to protect it in storage?) which will foul the system and may, hypothetically, cause problems with any EPDM rubber seals. So best practice would be to circulate some new system cleanser eg X300 or F3 and then drain and refill the system with inhibitor.

Most customers will not pay for this level of attention to detail, so it rarely actually happens and I certainly would not bother with an old system with an old-fashioned indestructible boiler or on any system of a fair age. The fact that you have a recent boiler which is under warranty inclines in favour of making the effort, however.
 

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