Kiwitub burner running on firewood
If you don't want to run the dual-fuel Kiwitub burner on gas, just slide in the stainless steel grate and build a wood fire in it instead.
The grate has two stainless steel tubes mounted under it, open at both ends, and with laser-cut holes along their lengths. These act as big secondary air intakes, sucking in fresh fully-oxygenated outside air and spilling it underneath the fire grate. This turbocharges the performance - we can get the entire flue red-hot, which is 1,040˚C for stainless steel. That why there's the perforated aluminium flue safety guard.
Both the gas and firewood options come as standard with the burner. It takes only a few seconds to switch from one form of heating to the other.
Firewood usually takes about 50% longer than gas to heat the tub water. Although you can get a hotter fire on firewood, to do so you have to pull open the flue damper more to allow a stronger air flow, and this means more of the heat from the fire goes up the chimney. Note that there is almost no smoke coming out of the flue cowl - it's a very clean burning design. Also of course firewood is carbon neutral. And free.