Brian Gaff (Sofa) laid this down on his screen :
It may well be, but I recall somebody asking it on another group only
recently, seems a very common problem. One might have thought with today's
boilers and the servicing that goes on such things would be consigned to the
bin of history, like the original post.
I guess that some people do not see the harm in perpetuating a thread
rather than create a new one, unaware of the way Usenet outside works.
Brian

With the answer given several times over in the thread already, even an
answer from me from several years ago.