Combi Boiler Hot & Cold Shower

[BigRooster]

Morning everyone today had a British Gas boiler engineer out to find out why my thermostick shower keeps going hot and cold .
I’m no expert but I ruled out the mixer as the hot water pipe was going cold before it entered the mixer .
Engineer did his vacuum test which was “ok”then ran the hot water for 10mins which was “ok” then told us there’s nothing wrong with the boiler it’s are shower

[AKMild]

We had a similar problem with our old boiler. It was 'cycling' (i.e. heating, cutting out, heating, cutting out etc.) The problem was finally identified as a furred-up heat exchanger in the boiler. The furring meant that the heat exchanger was getting too hot too soon and the cut-out operated for safety reasons. A new heat exchanger was half the cost of a new boiler so we bit the bullet and bought a new boiler with a stainless steel heat exchanger.

[bookbinder]

It could be the water company's fault. I speak from experience. If your water company is tinkering with the pressure (e.g. digging up a road somewhere), even though they might be doing it a long way from your house, it could affect your shower, and the shower is where fluctuating pressure will show up. See if you can find out if your water company is responsible.