I have a very annoying problem with our 6 year old Gloworm Swiftflow
combi boiler. When using the shower (just a bathtap mixer type one)
the water gets very hot and then adding extra cold water to cool it
down results in the water turning completely cold. The shower water
temperature then oscillates between very hot and very cold.

When washing the dishes downstairs (the boiler is in upstairs spare
room) the water becomes boiling hot and then goes cold and repeats
this continuously.

I've had the installer come and look at it and he said it's fine and
is just something that happens in summer so live with it!

Anyone got any ideas what the problem might be?

Thanks - Art