Not necessarily - it might just mean the shower flow is still exceeding the (increased) minimum flow meter rate whereas the new tap isn't. Put the shower on hot only and measure that rate too.
Ok, i installed the new tap, opened the isolators and this one does actually provide hot water. Once this one was installed i tried the problematic one and that one works too. I have even tried it first thing in the morning before anyting else was on and it actually heated up. The only thing that i can think was an airlock. But all good now
Thanks for all your help
Yup, if all you have done is add a new tap and now everything is ok then it probably wouldn't be an airlock with a mains pressurised, open ended pipe. We use mains pressure water to flush airlocks out of gravity systems.
As suggested, chances are something wasn't open properly and there wasn't enough flow through the boiler to activate the switch.
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