The system is about 35 years old, so I won't be spending about £1500 on it before selling the house next year.
The leak problem originally occurred last summer, so I removed the boiler, partially dismantled it, and attempted to get the crack in the heat exchanger core welded up. After three attempts to get it watertight, it seemed OK until recently, when the problem recurred.
The welding job I had done was messy, since it was not possible to preheat the affected casting with the unit only partially dismantled.
The proper procedure is apparently to cut a V-groove along the crack, preheat the casting to about 500 C, and to seam-weld into the V-groove using nickel welding rod. It was not done this way. It was done with the casting cold, and in places the weld did not amalgamate with the cast iron effectively. There was also a lot of unnecessary welding material each side of the crack.
While the system was drained last year, I removed the hot water cylinder, which is of the Primatic type, as the immersion heater was found to be difficult to remove with the cylinder in situ. The immersion heater had largely decomposed, and the resulting leakage currents via the on/off switch had resulted in pinholes appearing around the pipe entries due to galvanic action. These I repaired with solder. I also removed a total of 22lbs of lime scale from the cylinder via the immersion heater aperture, using a tyre lever to break up the large chunks. The cylinder is fortunately still OK!
The heat exchanger core is actually in two halves, linked by a manifold top and bottom. If I can't get a replacement casting, I will strip the unit down fully so that the affected casting can be preheated and the welding job done properly. This will entail grinding away the excess welding material and cutting the V-groove before presenting the casting for re-welding. The crack extends into the machined area where the manifold connects, so that this will have to be machined or ground to the correct profile after welding. That's why I would prefer to fit a replacement casting if possible.
BTW - I'm an engineer, not an MP!
Ciao!
