Wetback Hot Water Systems: NZ Wood Burner Integration Guide
Understanding Wetback System Requirements
If you heat your home with a wood burner through winter, adding a wetback is one of the smartest ways to cut your power bill. A wetback system uses the fire you’re already burning to heat your household hot water—giving you practically free hot water all winter long.
Because wood fires are an uncontrolled heat source (you can’t just flick a switch to turn off a log fire), wetbacks require precise plumbing, proper expansion controls, and building consent to run safely.
Here’s a practical look at how wetback systems work, open vs. closed options, and what a safe installation involves.
How a Wetback System Works
A wetback is essentially a pipe loop or heat exchanger fitted inside your wood burner’s firebox.
It relies on a natural process called thermosiphon circulation—no electric pumps required:
Cold water flows from the bottom of your hot water cylinder into the wetback coil inside the fire.
The fire heats the water, causing it to rise naturally up into the top of your cylinder.
Cooler water sinks back down to take its place, keeping the cycle going continuously while the fire is roaring.
Because it relies on gravity and natural flow, the pipework between your wood burner and cylinder must be installed at a steady upward gradient.
Wetback System Key Figures
Open Vented vs. Closed Mains Pressure: Which Do You Need?
When pairing a wetback with your hot water cylinder, you generally have two setups to choose from:
1. Open-Vented (Low-Pressure) Systems
This is the traditional and most bulletproof way to run a wetback. It uses a small header tank in your roof cavity to feed water down to the cylinder. If the fire gets extremely hot and the water expands or boils, the excess pressure simply vents out safely through an open pipe on your roof.
Best for: Older homes with existing low-pressure plumbing, or anyone wanting a simple, low-maintenance setup.
2. Closed Mains-Pressure Systems
Modern homes often rely on high-pressure showers fed directly from the mains supply. Connecting a wetback to a closed mains-pressure cylinder is doable, but it requires specialized safety gear—including correctly sized expansion vessels, valve packs, and temperature relief valves to handle pressure spikes safely. Mains pressure are a lot less common than the low pressure systems.
Best for: Newer builds or renovations where strong shower pressure is non-negotiable. Can You Add a Wetback to an Existing Wood Burner?
Can You Add a Wetback to an Existing Wood Burner?
Not every wood burner can take a wetback. Contemporary burners are usually engineered with specific wetback mounting ports and firebox ratings.
Retrofitting a coil into an older or non-approved firebox can void your heater’s clean-air certification and create a serious safety hazard. A qualified plumber will check:
Firebox size: If the coil is too large, it strips too much heat out of the fire, causing smoky burns and creosote buildup in your flue.
Cylinder capacity: If your cylinder is too small for your fire’s output, the water will boil rapidly; if it’s too big, you’ll struggle to get a hot shower.
Essential Safety Gear & Summer Backups
Because a wetback fire can heat water well beyond standard cylinder temperatures, proper safety features are mandatory under NZ Building Code (G12):
Tempering Valves: Solid-fuel fires can easily push stored water above 80°C. A thermostatic tempering valve blends boiling cylinder water with cold tap water before it reaches your showers and sinks, keeping your household safe from scalding.
Pressure & Temperature Relief Valves: Essential safety valves vent excess hot water and steam safely to the outside drain if the system starts over-pressurising.
Summer Heating Backup: When the weather warms up and the fire stays off, you’ll still need hot water. Wetback cylinders include an electric element (or can integrate with gas or heat pumps) so you have hot water year-round.
Council Consent & Getting It Done Right
In New Zealand, installing a new wetback or altering an existing solid-fuel heating setup requires a building consent from your local council. This ensures all plumbing, flue clearances, and seismic restraints meet current building regulations.
Things to ask your installer before starting:
Is my current wood burner approved for a wetback retrofit?
Will this setup work best as an open low-pressure or valve-controlled mains-pressure system?
Have you factored in the local council consent process and sign-off documentation?
Ready for Free Hot Water This Winter?
When installed correctly, a wetback is a reliable, eco-friendly upgrade that slashes your winter energy bills for decades.
If you’re looking to install a new wetback cylinder, upgrade your fire, or check if your current plumbing is wetback-ready, give the team at The Cylinder Guy a call on 0800 234 800. We’ll assess your setup and make sure you get a safe, compliant system built for your home.