My advice to get it working.
(1) Be careful, if it boils or you get an air-lock, then it might burn out.
(2) You must have the heater control on *full heat* or it blocks off the flow
This is what happened to mine, luckily a fuse went rather than the
whole thing.
(3) Consider getting a small pump. A 12v one would do, together with a low-power 12v transformer. Get an impeller type pump that doesn't impede normal water flow. Use this to circulate the water around the engine for a much better coolant heating effect.
Without (3) above, it will not be very good at all, as, the water flow to/from the heater matrix is horizontal so convection will not work very well.
Finally, even my 3kw engine heater (currently needing to be serviced so my MPG is dropping) doesn't get my engine up to temp, but about half-way (which is well over the amount to avoid enrichment, and makes the engine run very smooth from the start).
Edit - maybe the reason for the temperature readings are as follows:
(1) It heats up due to convection
(2) It only heats up a bit more, as convection is inefficient
(3) Thermal cutout as the heater temperature gets too hot