The flat that we rent out has a Baxi Mermuda (601?) back boiler that's
making "kettling" sounds. The system is pumped with a 3 way valve (Y plan?).
I guess the boiler's getting a bit ancient but it passes the safety tests
OK.

I'll try turning up the pump speed but is the problem likely to be a scale
build up? If so, what's the best additive to try to descale it? (Because the
rads have vent keys at the back (!!!) I'll need to add anything via the
header tank).

Dave S