Goto 'www.idealboilers.com/docs/pdf/Isar_he_users_Guide.pdf' and download the 4 page user guide.
Then goto: 'www.fixed4free.com/answers.php?id=30793' there you will find the possible answer to your question i.e. low system pressure.
There is more if you enter Isar HE30 in your Google search box
Cledlyn
July 2008