We own a Chausson Welcome 35 with a Webasto Dualtop heating sysyem fitted. Whilst filling up prior to a trip weekend away, we noticed water discharging from the drain valve of the boiler. No amount of playing with the manual drain selector would make any difference and the water continued to flow. We contacted our local dealer who then referred us to Webasto technical who were brilliant! They called us back at 6pm on a Friday night and talked us through the problem! After half an hour on the phone, after running several checks, the technician from Webasto diagnosed that our unit was probably furred up and would require descaling. We thanked him profusely (no charge for all his help!) and headed to Tesco for some vinegar! The newer owner's manuals for the dualtop detail a descaling proceedure using white wine vinegar. We used malt, but what the hell! It worked! The valve, however, was still sticking. Like everything else that sticks, I reckoned it needed working to free it off. I must emphasise the following advice is not from Webasto! If you hold the selector dial on the control panel to the left and hold it in the manual drain position, the valve will operate every five seconds. Clip a pair of molegrips lightly on the switch, holding it in that position and go and watch Eastenders! During your break, the valve would have been open and shut 180 times, thus freeing it off and allowing it to seal once more! We live in a hard water area and wild camp a lot. Therefore we always fill our water tank before we go. A 6 monthly descale looks like a small trade-off for having an otherwise brilliant heating system! A bit shout out also for Webasto for the best customer care we have ever experienced in the field of motorhoming!