Hot water or lack of it

I have a Saunier duval combi boiler and when i turn the hot tap on it comes out cold for about 40 seconds before warming up, I then have to turn down for it to heat up. Have changed thermister to no effect. Any ideas please?

Sounds like a pretty typical combi to me! Isn't that what it's *supposed* to do?

That's more or less how they work. You may find that you have very good mains water pressure and that you will find it helpful to restrict the flow (and so raise the temperature) of the water going into the boiler. The up-market boilers tend to have an adjuster for this.

I understand that they may take time to heat up but this has only just started happening to this extent. The shower will not work unless the tap is turned on full and when it warms up turn the shower on before turning tap off. I wondered whether there may be another explanation. Thanks

How much do you have to turn it down before the water gets hot enough? Once you get it to "hand hot" try measuring the flow rate (bucket of known capacity and a stop watch). That will give us some feeling for the scale of the problem.

Are you in a hard water area? Is the heating working correctly?