There seems to be a recurring issue with Willis heaters being installed upside down.  The original MI's that were supplied with these back when they were pretty much only used in NI, had a plumbing diagram that was pretty near identical to this one, when they are used for their intended purpose, as a way of providing an immersion heater for a vented hot water system that didn't have one, and didn't have an immersion boss on the cylinder:     I've seen a couple of photos post