No hot water to shower after changing thermostatic cartridge

Hi there, it sounds like the non return valve on the hot side is installed the wrong way round, they only open one way. If you have a look on the valve instructions it should give the location, normally just pushed into the inlet. Just needs to be turned around. Or the cartridge itself may be installed the wrong way as if the hot side is lined up with the cold side of the cartridge the safety device will prevent hot water flowing. The cartridge can be rotated 180 degrees to resolve this. It's unlikely to be an airlock if you have a mega flow since this is mains pressure. Best of luck, Rob

Hi there , Try taking the shower head off and lowering the hose down towards the bath/shower tray and turn the shower to cold first then turn the flow on then slowly turn over to hot water and see if this works, if not then go to your kitchen tap , get a cloth wrapped over your hand ( this can be a bit of wet job) then place the palm of your hand over the spout blocking the water from coming out , then open the tap on cold and again slowly turn it over to hot , do this a few times and for about 30 seconds to a minute each time , see if this makes a difference , if not then you will have to trace the hot pipe until it stops getting hot and your airlock is discovered if you do happen to know what to do and find this then try giving it a bang first ! , Last resort , get a wet and dry vacuum and Place the Hoover hose over and around the hot pipe coming out of the wall ( need to take the shower off for this) and this should pull all of the air through 👍👍👍 Hope this helped

could be non return valve,did you use ballfix valve these are cheap and have been known to be defective,also check fillters on shower valve,you should always flush pipework before fitting shower valve,it is doutfull this is an airlock as megaflows are mains pressure.