The Kiwitub burner took five years to develop. We went through seven prototypes and over a hundred time trials to arrive at the final production design. We're scared to change it any more as we think we've got it right. We think it'll last for 200 years - this isn't hype.
Immense thanks to Barry Whitfield of Valley Industries in Dunedin who loaned me old plumbing textbooks ("This is OLD technology - it's not modern.") and refused to take any money for the prototypes: "This is the most fun I've had in years, and if it flies you'll get it made by us anyway." I don't think this happens in Auckland.
Production has now shifted to Barry's brother Allan, at Allan's Sheetmetal & Engineering Services, also in Dunedin, with Chris taking over from Neal as the copper boilermaker.
The main Kiwitub burner is hand-made out of copper with a cast brass door, feet and fittings. We use copper because it'll transfer heat through itself a staggering 25 times better than stainless steel. So if you seriously want the heat from the flames to transfer through the surrounding metal into the tub water, then copper is what you use. That's why good stainless steel saucepans have copper bottoms.
All the brass is cast at Giltech Precison Castings - my friendly neighbourhood brass foundry, just around the corner from where I live. It's a foundry in the middle of a residential area - it was there first over 100 years ago, and the houses just grew up around it (including mine).
There are no motors or pumps to make noise or wear out - water heated in the burner circulates into the tub purely by convection, just like a wet-back loop on a domestic hot water cylinder. It's rated at 36kW output - which is huge. A big electric heater in your house would be about 3kW, and most spa pools are 1kW. This is how we can heat the tub water as fast as we do.
The burner runs on various fuels. If you want to run it on gas, just connect the stainless steel gas tube burner to a 9kg LPG barbecue-type gas bottle, slide it inside the central chamber, and light the gas. If you want to run it on firewood, just slide in the stainless steel grate instead, and build a wood fire on that.
The PVC, neoprene and closed-cell foam insulation jacket comes as standard with the burner. It cuts the heat loss from the top of the burner in cold, wet, windy or snowy conditions. It seems a pity to cover up the copper and brass but the burner heats the tub water faster in cold, windy, rainy or snowy conditions if it’s wearing its insulation jacket. We’ve trialled lots, and the strap-on half-jacket of closed-cell foam with the red weather cover is the best balance between efficacy and simple construction. It is provided as part of the Kiwitub package.