Energy

Designing homes to conserve energy and use it efficiently, from sources that cause least environmental harm.

Wetbacks

Wetbacks generally provide a boost to the water heating system, particularly in the winter when most required. They are most suitable for cold climates where a lot of space heating is used.

The combustion process of the solid fuel burner heats a water jacket installed in the firebox. Options are available for 1.54 kW water jackets. Water circulates through the water jacket, then back to the storage heater.

Wetbacks must meet a minimum thermal efficiency of 65%. Burners that meet this requirement can be found on the list of authorised burners on the website of the Ministry for the Environment.

Most wetbacks circulate the water by a thermo-siphon effect (i.e. warmer water rises and cooler water falls, creating a natural flow through the water jacket). The hot water cylinder should be close to the heat source to minimise heat loss. The compromise may be that longer pipe runs are required to hot water outlets than would otherwise be needed. Large diameter wetback water pipes (25 mm) are recommended to improve flow.

Some systems incorporate an electric pump to increase pressure and allow the storage cylinder to be located further away. In this situation, open venting is required.

Key factors when incorporating a wetback system:

  • Wetbacks generally only make a substantial contribution to water heating in cold climates where a lot of space heating is used.
  • Some burners produce a higher proportion of their output as hot water these are suited to houses where space heating demand is lower.
  • The input from the wetback must be supplemented by other means of heating when the solid fuel burner is not being used.
  • The emissions rating for a solid fuel burner incorporating the water jacket must be determined as the water jacket cools the combustion process, the particulates emissions level will increase.
  • The installation of water jackets within the firebox reduces the heat output to the adjacent space.
  • The circuit between the water jacket and the storage cylinder must be open-vented for safety. This means that either a low pressure open-vented cylinder is required or that an isolated circuit separate from the water in the cylinder is open-vented and heat is supplied to the water in the cylinder by a heat exchanger coil. (This option allows the cylinder to be run at mains pressure.)
  • The isolated open-vented supply line must incorporate a valve to allow the water to be replenished if overheating occurs. Overheating will cause a drop in pressure in the line, which will activate the valve to replace the water lost from the top of the vent pipe.
  • Only copper piping must be used.

Updated: 18 June 2013