"Peter Hawkins" wrote
| The immersion heater in my house is of a type I haven't seen
| anywhere else. There are no elements in the main tank. Instead
| the element is mounted inside a small vertical cylinder plumbed
| into the side if the tank but seperate from the main tank.
| Its mounted vertically with the element upside down and fitted
| from the bottom (which makes it hard to see the stat setting
| without a mirror). The secondary cylinder is 3 inches diameter
| and about 15 inches long.

Yes. It's called an external circulator and was often used to convert an
existing cylinder with an indirect heating coil that didn't have an
immersion heater boss. Because the position of the coil wasn't known, using
an external circulator avoided dealing with the obstruction inside the
cylinder.

It is possible the bottom connection to the cylinder is blocked which
prevents water circulating. A small quantity of extremely hot water is being
created at the top of the circulator and passing into the cylinder, but the
cooler water from the bottom of the cyclinder is not passing into the
circulator, allowing the heat to be dispersed through the whole of the
cylinder. This would also mean the thermistat at the bottom never has hot
water flowing over it. Bottom mounting of the heater/stat is normal.

Probably installed 1940s or 50s.

Owain