Jim Walsh wrote:
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.

Interestingly, I had the same intermittent symptoms with my GloWorm
Micron of about the same vintage, although it affected all hot water
temperatures being a vented non-condensing boiler system.

Not being confident of doing a technical diagnosis competently myself, I
called in a local heating engineer (I know, it's not DIY!) who tried to
guess what was wrong and proceeded to replace parts in the hope that the
problem would solve itself. I had to knock that cavalier and expensive
approach on the head quickly.

I then called in British Gas for a fixed repair fee. They replaced the
PCB and re-adjusted the misconfigured gas valve pressures left by the
previous "engineer" but that didn't solve the problem. It ended up
costing them a lot more than the fee I paid, but they did solve it
eventually.

On the last but one visit, the fitter was accompanied by a supervisor
who eventually suggested that they try measuring the resistance of the
thermistor (water temperature sensor) and discovered that it was way out
of specification (2Mohms instead of a couple of Kohms). They fitted a
new thermistor on the last visit and that solved the problem. It must
be said, however, that the old PCB was in a poor condition with brown
marks (overheating or blown capacitor) so that might have contributed to
the problem as well.

The obvious first point, for those with multi-meters and access to
*full* technical specifications, is to check the system carefully for
actual errors to diagnose the problem *before* fitting any new parts.

The second point is that British Gas are nowhere near as bad as they are
sometimes painted; they do keep trying until they find the answer and
their fixed fee repair scheme can be good value for money, especially if
you have an unusual problem. I would be happy to use them again (I do
not have shares in British Gas).

--
Dave N