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.
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).
I'd suggest from experience that if you can get an acceptable flow of HW at times then this problem is absolutely the diverter valve. The reset cause the system to recalibrate the motion of the diverter valve thus giving a temporary releif to the symptoms.
To fix this fault you will need a new diverter valve (c. £100).
We have one of these in the Scout Hut which appears to have a partially stuck pressure relief valve - drips and the pressure drops, then it is OK for several days, then drips...
I once found the maintenance and installation manuals on line but can't find them anymore - anyone point me at a site??
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