Because I am a Webasto user I read through all the comments and agree that you have had some helpful suggestions. However,I think
you will save a great deal of time (not money!) by asking a Webasto expert to test your unit and system. It does not appear to be a standard
set-up which is essential for these heaters to work consistently well. If you are located anywhere near the North West I can recommend an
engineer who has sorted my system,but there are very few of these people about.