When replacing a D51 with a D61 water heater, the inside diameter of the flue outlet from the water heater must be measured to determine whether or not a new flue kit is required. There are two variations in flue outlet sizes for the D51 heaters. If the water heater flue outlet inside diameter measurement is just over 90mm then the existing flue is suitable for a D61, if it is less than 90mm then a flue kit, FTM038, will be required to replace the D51 with a D61.