Wetback research: Thermodynamic flow characteristics of passive thermosyphon energy transfer from independent heat source to remote storage using both direct and indirect systems

Abstract

The method of heating water by means of a water jacket installed in the firebox of a solid-fuel burner, connected by pipes to a hot-water cylinder (HWC), was once commonplace in New Zealand, but has become less so in recent years. These systems, known as wetbacks, are covered by Building Code Clause G12, but considerable confusion has been caused by conflicting rules specified in the two main compliance documents, G12/AS1 and AS/NZS 3500.4 (hot water). In both compliance documents, wetback systems are defined as ‘uncontrolled heat sources’ with specific rules and limitations that stipulate how they are to be installed, and to keep them safe. However, other than common safety rules, each has entirely different ‘mandatory’ installation rules that affect the efficiency of the systems.

They couldn’t both be right, but they could both be wrong.

This is further confused by another standard, NZS 4603:1985, being mentioned in G12 as ‘another acceptable solution’ and differing from and contradicting both of the above. This standard is often quoted by manufacturers in their instructions.

One driver of this research, from a regulatory view, was to determine which of the two main compliance documents was correct, if either.

By building a wetback heater with measurable and constant inputs, and measuring the results using a state-of-the-art data logger, we were able to measure and record the efficacy of different pipe configurations and different systems.

A series of practical tests was conducted over a two-year period, to determine the effect different pipe configurations (pipe diameter, length, gradient, etc.) had on water circulation, and thus the efficacy of different installations.