I have a Vaillant TURBOmax plus combi boiler and I am experiencing an
intermittant problem with the supply of hot water from it. The central
heating function works just fine, but upon trying to draw hot water,
the boiler frequently supplies only lukewarm water. When the fault
manifests, adjusting the hot water temperature control on the boiler
makes no difference. The only thing I have found that seems to get it
working properly again (although not always at the first attempt) is
turning the boiler off and on again at the main switch.
Some additional info that may or may not be relevant: i) the problem
occurs most frequently in cold weather, ii) the boiler is mounted in
an unheated part of the house and iii) the boiler is about 8 years
old.

Any help/suggestions gratefully received.

Thanks in advance, Jim.