friendley formate 80 E water tempurture problem?

It sounds like you have a partially blocked plate heat-exchanger, the only way to cure it would be to remove it and flush it out or better still replace it.
Magnatite from the system blocks these so have the system flushed at the same time as you change the plate heat-exchanger and it should stop it happening again.

Kevin, a boiler engineer
February 2006