The system works except for the weather compensator showing an outside temperature of minus 50 centigrade 5 or 6 times a day ( usually for about 15 to 20 seconds each time). When this happens the boiler shuts down until the outside temp shows the real outside temp again then continues as normal.The Vaillant engineer changed many of the parts when it was under guarantee but the problem has continued.I have searched but never found anyone with the same problem.