Why would a Megaflow ‘boiler’ be lukewarm one day and hot the next?

[michellesooty87]

I am currently renting a room in a house. The water in the shower runs lukewarm. When I have mentioned this issue the water is then hot for a few days on the run. It then returns to being lukewarm. Can anyone explain to me why this is happening?
I am trying to understand how these particular systems work? I understand it is an unvented system and it isn’t a boiler I’m it’s own right.

Thanks for any advice

 

[SJB060685]

It’s an unvented hot water cylinder or mains pressure cylinder.
Does a boiler heat the cylinder or immersion heater?