How a wetback works
Pipework runs through the firebox. When the fire is lit, the water in those pipes heats up, rises to the hot water cylinder and warms the stored water, so the electric element doesn't have to run. That's Rheem's description, and it's the basic idea on every wetback.
Open vented low pressure systems have their own installation standard, NZS 4603, which covers wetbacks feeding the cylinder to supplement the electric element. A wetback adds to the cylinder's heating, it doesn't replace it.
For the wider picture on cylinder types, see our hot water cylinders page.
Cylinders that suit a wetback
Wetback-compatible cylinders come in both low and mains pressure versions, Rheem says. Rinnai's low pressure copper cylinders, made in New Zealand for working pressures up to 76 kPa, can be fed by a wetback on many wood, pellet and burner systems. Its coiled mains pressure cylinders work with pumped wetbacks and other closed-loop heat sources.
So a wetback doesn't lock you into low pressure. It does change the consent position when the cylinder is replaced, as below.
Replacing a cylinder that has a wetback
This is where the rules differ from an ordinary swap. Under Schedule 1 clause 37 of the Building Act, replacing a storage water heater connected to a solid-fuel heater, or another supplementary heat exchanger, is exempt from consent only if the replacement is a comparable open-vented water storage heater, fixed in the same position and using the same pipework.
Change any of those, for example going to mains pressure, moving the cylinder or running new pipes, and the clause no longer describes your job. Then the plumber or the council needs to look at consent. As with every exemption in that part of Schedule 1, an authorised person must do the work.
Will a wetback work in my house?
Rheem's answer is that a local plumber has to judge it. The position of the fire relative to the cylinder is what makes or breaks a wetback, and no two installations are the same. A plumber will look at both, and at the route the pipes would take between them.
Signs the wetback side needs a look
- Banging, rumbling or gurgling from the cylinder or pipes while the fire is going.
- Steam or water coming from a vent or relief pipe.
- No extra hot water even with the fire well alight.
- Weeping joints near the fire or the cylinder.
If the cylinder bangs or steams with the fire lit
Stop adding fuel and let the fire burn down. Keep well clear of any pipe discharging steam or hot water, and ring a plumber. Clause G12 of the Building Code requires features that relieve excessive pressure and limit temperature, so never block or cap a vent or valve.
Your part, the plumber's part
You can
- Let the fire die down if the cylinder is noisy or steaming.
- Keep the cylinder's vent and relief pipes clear.
- Tell the plumber what kind of fire you have and roughly how old it is.
- Keep the fire's paperwork with the cylinder's.
Leave to an authorised tradie
- All pipework between the fire and the cylinder.
- Replacing or changing the cylinder.
- Valves, vents and their drains.
- Deciding whether a new wetback install is possible.
Questions about a wetback job
- Is our cylinder open vented, and will the replacement be too?
- Will the new cylinder go in the same place on the same pipework?
- If not, does the job need a building consent?
- Is our fire suited to a wetback, and where would the pipes run?
- Who handles the fire side and who handles the cylinder?
What we would do
If your cylinder already has a wetback, say so before anyone quotes a replacement: it decides which consent clause applies and which cylinders will work. If you're thinking of adding one, ask a plumber who fits wetbacks to look at the fire and cylinder together before you buy anything.