Problem with Vaillant boiler

Hi
I have a Vaillant TurboMAX plus 837 combi boiler which is mis-behaving. The heating seems fine (although I may not have noticed any problems with it I guess) but the hot water is mostly distinctly coolish (although not actually cold). The actual temperature seems to vary every time I switch it on - sometimes it seems completely fine but this is getting much rarer. One confusing thing is that the temperature display on the boiler panel claims that the temp is getting up to around 55C as it should be.
Any ideas? Looking at the schematic on the Vaillant site, it looks like a sticking divertor valve could be the problem though I don't really understand the termperature display bit. Is this likely on this boiler? If so, is it any easy part to replace?
Many thanks Mark
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In scribed:

Mark, are your boiler burners cycling on/off every 20-30 secs or so whilst the Domestic Hot Water is on full draw?
Nigel
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It could be the diverter valve. The rubber ball tends to swell and get stuck inside the valve on a Turbomax Plus due to an adverse reaction with water additives (inhibitor etc). The new valves do not do this.
Dead easy to change. Isolate the water to the boiler using the valves at the back of the boiler (make sure mains water into the boiler is off - check by turning a hot tap on, you should get nothing out). Turn off power. Drain down the boiler using the drain-off cock under the boiler. Remove the diverter valve by pulling out the retaining clip, undoing the nut and removing the plug. Replace with new valve remembering to put new fibre washer and o'ring in. Turn water and power back on & recharge pressure to around 1.2 bar. Done.
NB: when removing the valve, put a cloth under it to protect the PCB underneath.
Angus
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On Tue, 17 Jan 2006 06:40:14 -0800, fearonm wrote:

Temperature display is the not helpful in DHW mode, no doubt it relates to something.
The most probably causes are: 1) diverter valve is not going to DHW mode fully. so with the flow pipe coolish, run a hot tap and see if the flow pipe heats up , if it does then that's a problem. (after running the tap it may well get hot).
2) The modulator has gone belly up. (PCB, gas valve or wiring) this moves the rate from ignition setting say 15kW to 37kW DHW mode.
3) DHW mode temperature sensors (an NTC thermistor) which say that the DHW is hot enough and so cuts (or cuts-back) the gas.
HTH
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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Thanks for all your help - as life would have it, it seems to be working fine this evening. I'll try your suggestions when it next plays up. Thanks again Mark
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