The central heating is now fixed... but what a fault to find!
I won't bore the reader with the unquestionable genius involved in finding the fault, as that is well known, I will just keep, more or less, to the details of the fault.
The story begins seven years ago in a boat building yard.
"Just finish that plumbing in the bathroom," the boss shouted through the porthole, "then you can go home."
The young aprentice looked at the scribbled drawing and the fittings in a box. After ten minutes he shouted up.
"What's this cross on the drawing?"
"Oh yes," the boss answered, "That's the by pass valve."
The apprentice shrugged his shoulders having no idea what that meant.
"They used to baffle me when I was your age. The thing is if you pipe it up like you might expect, and then if you turn all your radiators off, there is no flow at all through the coil in the hot water tank, and so you get no hot water. The by pass valve allows a little flow past the radiators so that you get hot water.
"Right," said the apprentice rummaging through the box.
"And that reminds me," the boss said, "we haven't got one."
The apprentice stopped rummaging.
"They missed it off the delivery. But connect everything and leave room for it. That way I can check it all for leaks tomorrow."
Seven years later captain found the plumbing extended slightly beyond the towel radiator to accommodate a valve - but no valve there, just an effective short circuit of the radiators.
There had been a trickle through the radiators on a good day, and a Webasto that went to half load quite quickly as it saw very hot water on the return pipe.
But all piping hot now.