System mostly new in the last 12-24 months. Latest changes approx. 3 months ago, added a large aluminium radiator (minimal soldering to tails) and one other new steel radiator (straight swap). System fully drained to do that work and refilled with X100 at 1.3% and R700 biocide. System has some copper from the 70s upstairs and one old 70s steel radiator, and a towel rail approx. 10 years old. All other radiators have been replaced in the last 2 years. System water is now pH 8.8. Tap water is pH 7.3 (tested). Why the pH is rising? How can this be controlled? Sentinel test recommended clean with X300, flush and fill. Happy to do that but given system has over £100 of treatment in it want to be sure this will work, or better yet another method? Apparently X100 itself has a pH buffer (controller) in it, however given it's already over 1% I suppose another bottle of it will make no difference either. I also tried draining out 20 litres of water and back filling, this didn't change the measured pH almost at all (allowing 2 hours circulation to mix). System has been filled and drained a few times since the majority of the work was done so I'm a little stumped. Many thanks in advance.
Finally go this sorted... the pH rises due to rust etc not being properly cleaned out. System has been adjusted in lots of stages, so needed proper clean and rinse, Fernox F3 and a TDS meter finally did the job.