I have a Myson Apollo boiler and a conventional heating and Ho****er
system.

I have recently replaced the pump (myson 53) with a new one
(circulation pumps 53)after it failed but now have the following
problem.

When the system is running a small amount of water is coming out of
the overflow from the small black plastic tank in the loft. I was told
the pump may be set too low so I increased it to the second setting
but this only increased the flow of water so I put it back.

I have had it suggested that the thermostats in the boiler might have
gone and was wondering if anybody could tell me which. I understand
there is one for the return water temperature one for high and one for
low.

I am assuming it will be the return as the problem is still there
wether the boiler is set to high or low.

If anybody could clear this up for me or has another suggestion then I
would be grateful for any information.