"pauly" wrote in message
...
Not really a DIY question however this is the most appropriate group i can
find.

I just had a new combi boiler fitted to replace an old system with boiler
and immersion. Problem is the water temperature varies quite a lot -
sometimes its blazing hot and give it a minute and it goes cold before
getting too hot again.

The guy who fitted it is really good (reputation) and am trusting him to
figure it out however i've got curious as to what it could be.

What could be the cause?

various things but as a starter can you quote the commissioning figures he
has filled in for model number, water temperatures, flow rates, gas rate etc
on your little blue benchmark book that came with the boiler