I'm interested in seeing your "NZBC approved" solution, I haven't seen any changes to the compliance documents showing a new way. Who gave it NZBC approval?

Sorry its taken me a while to get back to you on this..

Where a wetback position is separated from the storage cylinder by a doorway as an example, it is impossible to install the return pipe in the normal manner. In such a case the pipe must pass under the floor and therefore below the boiler, (I have attached an image called Wetback installation). This set up is known as an "Over and under system".

The installation of a return pipe below a boiler introduces a problem. Ideally, cooling water should be falling towards the boiler. In the diagram attached, the water between points A and B, the coldest water in the circulation system, is rising.
All circulating pipes should be insulated apart from an un-insulated loop in the flow line (heat-trap) into the water heater which assists in preventing back circulation.

Under certain conditions the circulating pressure may be insufficient to overcome the frictional resistances within the system and therefore there is no circulation. It is generally recommended that there is at least 300 mm above the boiler for each 25 mm (a ratio of 1 in 12) of the total dip in the return pipe below the boiler. If a cylinder is below the minimum differential height it is possible to create a flow of heated water to the „cylinder using a "pulse valve" (image attached) A pulse valve is used where a natural thermosiphon or pumped system is not being used. Flow is obtained by using a special pulse flow valve fitted to the wetback. The valve is mounted on the cold water inlet (bottom connection). The valve traps the cold water in the fire element until it is heated. Once heated by natural expansion by 3%-the water causes a powerful surge in the hot outlet pipe. The valve then opens to allow a fresh charge of cooler water into the element. The cycle then repeats. By this means the cylinder can be located many metres away from the fire making wetback heating possible without requiring a circulation pump.

It is important to note that the valve must only be used: on an open vented pulse pumping system
on the cold inlet side of the heat source with the valve installed in the correct direction in the up position
in an approved circuit configuration.

The customer should be advised, before installation, that the pulse valve makes a noise when operating.

Low-pressure electric storage water heaters may be self-contained with a cold feed tank attached at their top. These are suitable for single- and multi-storeyed flats, factories, offices. The image attached shows a low pressure system with a Wetback attached, the same would apply when using a mains pressure tank incorporating a Wetback designed coil. Keep in mind these systems must remain open vented under any circumstances and only a qualified plumber must attempt this configuration, if installed incorrectly you can end up with a life threatening situation.  I hope this info helps.