I have had the same problem with this same boiler. I have noticed that if I turn on the hot tap for a while then reduce the flow and keep the water running.(or if I put both the dishwasher and the washing machine on at once, the radiators get hotter. Apparently it is to do with the boiler diverting from heating to hot water. If you read the other queries, (by putting Vaillant turbomax 242E in the search box), it could give you some possible solutions. Read "how do I change a diverter valve on a vaillant turbomax 242E" Also the diaphram keeps coming up as the possible cause for hot water problems.
I am assuming that your radiators have been bled and don't contain air. You can tell if this is the problem as they tend to be cold at the bottom and warmer at the top. If so use radiator key to gently let out air on each one. Use a flannel to collect any water( if done slowly this should be minimal, and make sure that you re-tighten sufficiently). If all else fails Check out www.vaillant.com, e-mail: info@vaillant.co.uk,Tel: 01634 292393 or 0870 6060 777. To call out a Vaillant engineer is expensive but they are experts. Some plumbers whilst corgi registered just don't have the same expertise and it can be false economy in the long run. Good luck!
Frank
February 2007
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