This was an intermittant fault. Usually occured when Heating came on. Before Heating failure, Hot water worked. Heating would stop this operating. When unit was 1st installed it had same problem. This was 4 years ago though and was fixed under warranty. Engineer said this was quite common fault with unit leaving factory calibrated incorrectly. Parts were not required to fix. It took him couple minutes to adjust. I just don't know what he adjusted.

Rob S., January 2006

probably the circuit board

mike, January 2006