Short answer: you don’t need a pump at all — you need a thermosiphon coil sitting in the fire. Hot water rises, cold water sinks, and that difference alone pushes water from the tub, through the coil, and back again. The fire is the engine. Gravity does the pumping.
That single principle is what makes an off-grid soaking barrel possible. Miss it, and you end up with a tub of cold water and a very hot fire doing nothing useful — which is exactly where most first-time builds go wrong.
Why “no pump” is the hard part
Most people who build a backyard barrel tub start the same way: they get a beautiful wooden barrel, set a firebox beside it, fill it with water, and wait. And wait.
The problem is not the fire. The problem is that heat doesn’t move itself. Water sitting in a tub is a terrible conductor of heat. A firebox warming the water directly around it creates a thin layer of warm water that just… stays there. You’ve built an expensive campfire with a bathtub next to it.
The usual fix is a pump and a heat exchanger — but now you need electricity, wiring, a waterproof connection, and a power source at the far end of the yard, or off the grid entirely. That defeats the purpose of the simple, self-sufficient setup most people wanted in the first place. If that’s the trade you’re trying to avoid, the answer isn’t a smaller pump. It’s removing the pump from the design altogether.
How a thermosiphon actually moves water
Thermosiphon circulation (also spelled thermosyphon) is one of those elegant bits of physics that feels like cheating once you see it.
- Heat the coil. The coil sits inside your fire pit or stove, surrounded by flame.
- Water inside the coil expands. Hot water is less dense than cold water, so it becomes lighter.
- Lighter water rises. It climbs through the coil’s spiral and out the upper outlet, back into the tub.
- Colder water falls into the coil. As warm water leaves, cooler water from the bottom of the tub is drawn into the lower inlet to replace it.
- The loop repeats continuously. No motor, no impeller, no electricity — just a temperature difference that keeps feeding itself as long as the fire burns.
The key insight: the coil is not a heater that warms the tub directly. It’s a pump that runs on heat. The tub water circulates through the fire rather than sitting next to it. Once you think of it that way, every installation decision below makes obvious sense.
What the coil itself has to do
Not every coil works for this. For circulation to start and keep going, the coil needs three things:
Enough surface area in the flame. More contact with fire means a bigger temperature difference between the water going in and the water coming out — and a bigger difference is what drives faster flow. An 8-circle spiral design packs a lot of tubing into the firebox without needing a wider footprint.
Material that survives fire and water at the same time. This is where cheap coils fail. A coil sitting in a fire while carrying water is being attacked from both sides at once — heat oxidation outside, corrosion inside. Food-grade 304 stainless steel with seamless tubing is the practical choice here, because it resists both rust and the thermal cycling that cracks lesser metals.
The right size for a real firebox. Too small and it can’t absorb enough heat to move water. Too large and it won’t fit the fire pit you already built. The common barrel-tub size runs roughly 35cm across and 30cm tall, with the spiral doing the work inside that envelope.
This is exactly what the Premium Stainless Steel Thermosiphon Wood Fired Hot Tub Coil (Model A1) is built to do: passive circulation, no electric pump, no wiring, and a sealed 8-circle spiral that drops straight into a wooden barrel, metal stock tank, or foldable soaking tub. If you’d rather start from a complete kit with hoses and fittings included, Smartclima also offers the wood fired hot tub heater coil kit in 24 model configurations.
Installing it: the one rule that decides success
The coil itself is the simple part. The install is where thermosiphons succeed or fail, and it all comes down to one rule:
Hot water must be able to rise the entire way, with nothing in its path to trap air.
Everything below is just that rule applied:
- Set the coil low. The coil must sit below the tub’s water level. Gravity-fed circulation needs the cold inlet lower than the hot outlet — ideally the inlet of a coil is the lowest point in the whole loop.
- Run the return line uphill, continuously. From the coil’s top outlet back into the tub: no dips, no sagging hose. Every low spot becomes an air pocket, and an air pocket stops circulation dead.
- Keep bend-light. Every sharp elbow adds resistance that a pump would normally overcome. A thermosiphon has no such muscle.
- Draw the cold inlet from the bottom of the tub. That’s where the coldest, densest water is — feeding it to the coil keeps the temperature difference (and therefore the flow) as strong as possible.
- Fill completely before firing. The loop must be full of water with no trapped air. Fill, then check for bubbles at the return outlet before you light anything. Important, must keep the outlet must be immerged under water.
- Light the fire and give it a few minutes. Circulation starts once the coil is hot enough to create a real density difference. Once it begins, you’ll often see and hear the flow at the return.
For a barrel tub this is genuinely a beginner-level DIY job — basic hand tools, no plumbing certification, because there’s no pressure and no electrical work involved. If you’re still assembling the tub itself, the step-by-step guide to building a wood fired hot tub with a heater coil covers drilling and fitting the bulkheads.
Mistakes that kill circulation
- Outlet of Coil level with or above the tub water line. The single most common failure. No height difference, no gravity feed.
- A sagging return hose. Looks fine, silently traps air, tub never heats.
- Air left in the loop at first fill. Bleed it before lighting the fire.
- A coil too small for the firebox. If flames lick past it without real contact, you’re heating the yard, not the water.
- Expecting instant heat. Thermal mass is real. More water and a colder start mean longer to reach soaking temperature — this is a slow, steady system, not a switch.
What this setup gets you
- No electricity. Nothing to plug in, no wiring at the tub — it works at a cabin or a property with no power run.
- No pump to fail. The only moving part is the water itself.
- Wood as the only fuel. Cheap, local, and often already on hand.
- A tub that still works in a power cut. Which, for a lot of owners, is the entire point.
The honest trade-off: you’re swapping convenience for independence. You’ll be splitting wood and tending a fire instead of pressing a button. For most people building a barrel tub in the first place, that’s not a downside.
If you’d like a coil sized to a specific tub volume, Smartclima builds custom stainless steel tubing coils to order.
Frequently asked questions
Do I need any electricity at all?
No. The coil uses passive thermosiphon circulation — no electric pump, no wiring, nothing to plug in. The only input is firewood.
Can you heat a hot tub without a pump?
Yes, provided the outlet of coil sits below the tub’s water level and the return line rises continuously back to the tub. Gravity and the density difference between hot and cold water do the work a pump would otherwise do.
Will it work with my tub?
It’s designed for standard outdoor soaking setups: wooden barrels, metal stock tanks, and foldable plastic soaking tubs. The main variable is matching the coil to your tub’s volume and your firebox size.
How long until the water is hot?
It depends on your water volume, how cold it started, your firewood, and your airflow. Plan on tending the fire for a while rather than expecting a quick result — and expect the second and third firings to feel faster as you learn your setup.
What if the water isn’t circulating?
Check height of outlet pipe first (outlet of coil below water level), then look along the return line for any low spot holding air. Those two causes cover the large majority of cases.
Is it hard to install?
It’s rated beginner-friendly full DIY with basic hand tools — but respect the two rules that matter: coil low, return line always rising.
Bottom line
Heating a wood fired hot tub without electricity isn’t a workaround — it’s a genuinely better design for off-grid soaking, as long as your coil can move water on heat alone. Start with the right coil, install it lower than you think you need to, and let physics do the rest.
👉 Get the Premium Stainless Steel Thermosiphon Wood Fired Hot Tub Coil (Model A1) — food-grade 304 stainless steel, 8-circle spiral, no pump and no wiring required.
Questions about sizing a coil for your tub? Contact the Smartclima team and we’ll help you match one to your setup.