I have a vaillant VCW GB 242EH. It's about 9 years old and has been serviced every other year. The problem I have with it is that the hot water temperature is now only tepid. The central heating works fine. The servo moves when hot water is drawn, the boiler fires up to full as normal, the servo filter is clear, the servo diaphragm looks sound, the diverter valve seems ok and the diaphragm seems sound, I think the divertrer valve is ok because the radiators don't heat up when running a hot tap for half an hour and the gas pressure is within parameters. Enough information? The corgi guys I've had don't seem to have a clue what's wrong and 1 of them just wants to change the parts 1 by 1 till it works (the bit's that were ok would be non returnable though). Unfortunately the parts all seem to upwards of £100+vat so it would only take a few parts to come to the same cost as a new condensing boiler (cheap one obviously). Could it be the DHW heat exchanger or something as simple as the DHW thermostat (my 2 corgi plumbers think not). Any ideas would be welcome.