The Off-Grid Wood Fired Coil Kit

The Wood Fired Coil Kit turns any drillable rigid tub into a wood-fired hot tub.

The coil sits external to your tub on a non-combustible base. You light a fire under or beside it. As the water inside the coil heats up, it rises through the upper port back into your tub; cooler water from the bottom of the tub flows down into the coil to be reheated. This thermosyphon cycle is silent, self-sustaining, and runs on nothing but firewood — no power, no pump, no plumbing.

The kit ships with everything except your tub, the firewood, the fire enclosure, and the silicone for the port seal.

The kit suits most rigid drillable tubs:

- Galvanised steel stock tanks (1.8m oval, 2.4m round, similar) — ideal. The coil was designed around stock-tank wall thickness.
- Stainless steel hot tubs — yes.
- Rigid steel oval and round tubs — yes, provided the wall is thick enough to hold a 48mm port.
- Cedar or hardwood barrel tubs — yes, with port lining (a thin metal sleeve fitted through the drilled hole to support the seal).

Not compatible:
- Plastic IBC totes (1000L) — wall too thin to seal safely.
- Inflatable or vinyl tubs — material can't seal a port.
- Standard bathtubs or clawfoot baths — walls too thin.

If you're not sure, email hello@terratanks.com.au with photos and dimensions before ordering. A 10-minute reply saves a return shipment.

Most customers have the kit fitted in 30–60 minutes. The high-level steps:

1. Drill two 48mm holes in your tub wall using a standard hole saw.
2. Fit the through-tank ports with silicone and gaskets, hand-tighten the clamping rings from inside.
3. Connect the hoses to the coil and to the ports with hose clamps and quick-connect couplers.
4. Fill the tub, light a small fire under the coil, soak.

The full step-by-step is in the Setup section above, and there's an install video at the bottom of this page. A printed guide ships in every kit. Phone support is included if you get stuck — 0421 824 725.

After fitting the ports, allow silicone to cure before filling the tub — most are water-ready in 4–6 hours, but a 24-hour cure gives the best long-term seal.

No plumber required for most installs. If you've used a power drill and a spanner before, you've got the skills.

On a 500L tub with the cover on, expect 2–4 hours from cool tap water to bathing temperature (around 38–40°C).

Approximate cover-on heat-up times in mild Australian conditions:

- 500L tub: 2–4 hours
- 800L tub: 3–4 hours
- 1,000L tub: 4 hours (insulated tub: ~3 hours)
- 1,500L tub: 5 hours (insulated: 4 hours)
- 2,000L tub: 6 hours (insulated: 5 hours)

Cover-off, allow roughly double. Fire size, wood quality, starting water temperature, and wind exposure all move these numbers — a small fire with damp wood and the cover off can more than triple them.

To shorten the heat-up: keep the cover on, burn dry seasoned hardwood, start with a slightly larger starter fire, and stir the water every 20–30 minutes with any clean stirrer to even out hot and cold zones. Once you're in, a small ongoing fire holds the temperature.

The coil produces an open fire while in use, so it's an outdoor product only — not for indoor use, garage use, or covered patios that aren't fully open to the sky.

Outdoors, the tub and coil need to sit on a non-combustible surface — compacted cracker dust, gravel, concrete, stone pavers, natural earth, or slate. Not on a timber deck, decking composite, artificial turf, or any combustible surface.

The kit doesn't include a fire enclosure. Most customers build a simple firebrick ring under the coil, use a steel fire pit insert, or burn in an existing backyard fire pit. Treat it the same way you'd treat any backyard fire:

- At least 3m clear from main structures (house walls, pergolas, sheds, fences).
- At least 3m clearance from eaves, vegetation, washing lines, hanging branches.
- Always burn dry, seasoned hardwood — green or wet wood smokes heavily and tars the coil internally.
- Some councils restrict outdoor wood smoke. Check local fire restrictions before lighting if you're in a built-up area.

Across most of Australia, freezing isn't a real concern. Perth, Sydney, Brisbane, most of VIC and SA — water sits well above freezing year-round, and a tub used regularly never gets close to freezing.

For alpine or sub-zero areas, drain the tub between sessions as a default. If you're leaving water in overnight in freezing conditions, drop the level so any expansion has somewhere to go, or float a partially-inflated inner tube or a couple of sealed empty plastic bottles on the surface to absorb the expansion if a surface ice layer forms.

The coil itself must be drained between uses if there's any chance of freezing — water trapped inside a frozen coil can crack the metal. Disconnect the quick-connects after each soak in winter and let the coil drain by gravity.

A few dollars of dry hardwood per soak — or free if you've got a wood pile.

There's no electricity cost (the kit doesn't use any). No filter cartridges. No ongoing chemicals beyond what your tub needs anyway — most customers drain after each session and skip sanitiser entirely.

Compared to a powered spa (which can run $300–600 a month just to keep hot in cold weather), the running cost difference is dramatic. Compared to an electric tub heater (which typically adds $80–150 a month to your power bill at regular use), you're saving every soak.

Free delivery to most major city metro areas and many rural areas. Flat-rate depot shipping to the rest of Australia. Estimated 2 to 4 weeks.

We offer a 90-Day Money-Back Guarantee.

If you don't love the kit — or it doesn't fit your tub the way you expected — return it within 90 days of delivery, even if you've installed and used it, for a full refund of the purchase price. The only condition: you cover the cost of returning the kit to our nearest metro warehouse.

If your order arrives damaged, faulty, or not as described, that's separate — we'll repair, replace, or refund under your Australian Consumer Law rights regardless of timing. Email hello@terratanks.com.au.

You can absolutely DIY this from raw parts. A handful of customers ask before they order.

Three reasons most still buy the kit:

1. The coil is the hard part. Bending stainless steel into an efficient thermosyphon isn't a Bunnings purchase — it's a manufactured part. Sourcing one separately runs $350–$600 plus weeks of lead time.
2. The fittings are matched. Through-tank ports, gaskets, quick-connect couplers, and hoses are sized to work together. Mixing parts from different suppliers usually means re-buying when something doesn't fit.
3. Time. A weekend of sourcing, returns, and re-ordering vs one kit that arrives ready to fit. Most DIY-ers value their weekend.

If you're an experienced fabricator and want to build everything from scratch, you don't need this kit. If you'd rather skip the parts-hunting and install something that's been tested to work, this is what we sell.