Trying to get Hot Water in my shower

[Sir Bagalot]

As it's getting worse it's time to sort it.

Loft bathroom. Wash basin and thermostatic control shower. Similar setup on first floor and it's all fine. Megaflow hot water system

Loft. Turn on basin tap (monoblock) and move to hot and all you get is cold water. Turn on shower cold and you then get hot water at basin. You can then turn off the shower and hot water will continue at the basin.

Until recently the only way to get hot water in the Shower was to turn basin cold on fully. Water would be warm but not hot but this didn't worry me as don't like too hot.

Recently however it changed, you now also have to flush toilet to get hot water in shower and as soon as cistern fills the water in the shower goes cold.

I was previously told the problem was the one way valve in the shower. Someone else tells me the thermostatic cartridge in the shower is knackered.

One way valve means removing tiles. Thermostatic cartridge is fully accessible.

Anyone offer advice on possible cause?

[Sir Bagalot]

Saleen836 said:

How would a faulty thermo cartridge in the shower cause the problems? it's sole job is to alter the temperature of the water before it exits the shower head.

Exactly. If it's faulty then it won't do that.