heat exchanger?

Not the kind of thing you should try if you don't know anything about boilers.
It's probably not the heat exchanger anyway.
If you are referring to the item about the halstead quattro gold it's a different boiler to your ace high but the fault is probably the same although the ace has been known to let the heating come on even when the taps are not turned on and no call for heating. If that's the case it may be a fault in the electronics or switches.
Either way i'd get someone in.

T.H.S
June 2007

in the middle of the boiler

hi-spec plumbing
June 2007