I have a Saunier Duval Thelia 623 and although it fires up when i turn on the hot tap, it is refusing to come on to heat the radiators. It was becuase of low pressure initially so i let more in up to about 1.5, but it now seems to be rising of it's own accord and the heating will come on in fits and starts in a random fashion and i don't seem to be able to control it. Any help appreciated. Thanks

Hugh, January 2009