You can build a wood-fired hot tub yourself, and it costs less than you think. You need a watertight tub, a wood-burning stove, a solid base and a safe connection between the stove and the tub. The stove heats the water through natural circulation (thermosiphon): warm water rises, cold water sinks, so you need no pump and no electricity. Expect a heat-up time of roughly 1.5 to 3 hours and 60 to 120 litres of dry wood per session (source: hot tub stove manufacturers including Welltub and Spabuiten).
This guide walks you through the whole thing: stove choice, base, heat-up time, water care, safety, planning rules and realistic costs. I’m Fred, and I like projects you can finish with your own hands. This is one of them.
What exactly is a wood-fired hot tub?
A hot tub is a large bath of warm water that several people can sit in together. You may also see them called wood-fired tubs or bathing barrels. The wood-fired version heats the water purely with fire, without any electricity.
That is the big difference from an electric spa or jacuzzi. A spa runs on electricity for the heater, the filtration and the jets. A wood-fired hot tub needs no power in its basic form: no filter, no jets, no connection. Just water, wood and fire.
That makes it perfect for a spot at the bottom of the garden with no socket nearby. Or simply if you like the quiet and the crackle of a real fire.
Internal or external stove: the decision that matters most
There are two ways to position the stove, and this is really the choice everything hinges on.
Internal stove: the stove sits inside the tub, in the water. Upside: it heats a little faster because the hot metal is in direct contact with the water. Downside: it takes up seating space, and you can burn yourself on it. The metal gets red hot. With children around, that is a genuine risk.
External stove: the stove stands next to the tub. Two hoses or pipes connect it to the tub. Cold water enters the stove at the bottom, heats up, rises and flows back into the tub at the top. That is the thermosiphon principle: the water circulates on its own from the temperature difference, with no pump (source: Spabuiten).
For a self-build, and certainly with children around, I’d go for the external stove. You keep the fire away from the bathers, you get more seating room, and the burn risk inside the tub disappears.
The tub: which material do you choose?
The tub determines the look, the lifespan and a good chunk of your budget. These are your options.
| Material | Pros | Cons |
|---|---|---|
| Wood (larch, red cedar, thermowood) | Natural look, good insulation, classic feel | Can start leaking if it dries out, more maintenance, must stay damp |
| Plastic / fibreglass (polyester) | Watertight, smooth, easy to clean, lighter | Less natural look, scratches easily |
| Masonry / concrete | Very durable, fully made to measure | Heavy, labour-intensive, a permanent structure (planning permission may apply) |
For wood: larch and red cedar are popular because they naturally cope well with moisture. A wooden tub does need to stay damp, though. Leave it standing empty and dry for a long stretch and the wood shrinks and starts leaking. That is the most common mistake with wooden tubs.
A widely used middle road is a plastic inner tub with a wooden surround. You get the watertightness of plastic and the warm look of wood. Plenty of self-build kits are put together exactly this way.
Size and volume: how many litres do you need?
Volume drives everything: how many people fit in, how long heating takes and how much wood you burn.
As a rule of thumb, count on roughly 150 to 250 litres of water per person, depending on how much elbow room you want. A tub for 4 to 6 people often sits around 1,000 to 1,500 litres.
For context: a popular self-build kit with a diameter of 175 cm suits 5 to 6 people (source: supplier Welltub). Larger models run up to 2,000 or 2,500 litres.
Worth remembering: the more litres, the longer the heat-up time and the more wood you burn. So don’t go bigger than you actually need.
Step by step: how to build your hot tub
Step 1: choose and level the base
Don’t underestimate the weight. Water weighs 1 kilo per litre. A 1,000 litre tub is therefore 1,000 kilos of water, plus the weight of the tub itself and of four to six adults. You are quickly over 1,500 kilos in one spot.
Your base has to carry that load. Never just set a hot tub on grass or loose-laid slabs. Good options:
- A concrete slab or concrete floor
- Firmly compacted, stabilised paving on a level sand bed
- A load-bearing timber deck, provided it is designed for this weight
Make sure the base is completely level. A tilted tub gives you uneven water distribution and puts stress on the structure.
Step 2: position the tub
Set the tub up level. Check with a long spirit level in several directions. For a wooden tub: fit the steel tension bands evenly so the staves (the vertical boards) close tightly against each other.
Step 3: position and connect the stove
For an external stove: place it a short distance from the tub, on a non-combustible, heat-resistant surface such as concrete or slabs. Keep a generous distance from fences, planting and other combustible material.
Connect the two fittings: the lower one (cold water in) and the upper one (hot water out). The lower one sits below the upper one so that natural circulation gets going. Check that every through-fitting is sealed fully watertight.
Go for a stainless steel stove. Stainless lasts a long time and you can use chlorine with it. With aluminium stoves you cannot, because chlorine attacks the aluminium (source: Aqua Kristal / Serena Spa).
Step 4: fill and test
Fill the tub above the upper stove fitting. Important: never light the stove when the water level is below that upper fitting. Circulation won’t start and the stove can overheat (source: Spabuiten). Check for leaks before you light anything.
Step 5: fire it up
Load dry, split wood into the stove and light it. Top the stove up every 10 to 15 minutes while it heats (source: Spabuiten). Put the lid on the tub as soon as you start burning. It makes a huge difference to the heat-up time.
Heat-up time and wood use: what’s realistic?
This is where a lot of people are too optimistic. Here are the real numbers.
Heat-up time: a stove of around 35 kW heats about 1,000 litres in roughly 1.5 hours to 38-40 degrees. For most small and medium hot tubs you land at 1.5 to 2.5 hours (source: Wellnesstuinier, Welltub). With a large tub of around 1,450 litres and a cold start (0 degrees outside), that stretches to 2 or 3 hours (source: Spabuiten).
Hot tub stoves usually deliver between 20 and 55 kW (source: manufacturers). More kW means faster heating, but also a fiercer fire.
Wood use: count on 60 to 120 litres of dry wood per heat-up, depending on the season (source: Spabuiten, Haardhout.com). In winter you’ll be at the top end: colder starting water and more heat loss. In summer, at the bottom end.
Heat-up time and wood use depend on:
- The water volume (bigger = longer)
- The outside temperature and the wind
- The starting temperature of the water
- The wood quality (dry wood gives far more usable heat; wet wood can double the heat-up time, source: Spabuiten)
- Whether you use a lid and insulation
Insulation and a lid: the biggest time saver
A well-insulated lid is the best investment you can make. It holds the heat in during the heat-up and afterwards, and it stops evaporation.
Without a lid you are heating a good part of the garden rather than the water. So put it on as early as you can, the moment you start burning.
Where you can, insulate the tub wall and the connecting pipes too. Site the tub out of the wind, for example against a fence or wall (keeping a fire-safe distance to the stove). Wind cools the water faster than you’d expect.
Safe water temperature: not too hot
Hotter is not better. A comfortable and safe bathing temperature sits around 37 to 40 degrees. Always use a thermometer to keep an eye on it.
Take particular care with these groups:
- Pregnant women: keep the water at a maximum of 37.5 to 38 degrees. A body temperature above 39 degrees can affect the development of an unborn child, especially in the first trimester (source: Wellis, 24Baby). If in doubt, talk to your midwife or doctor.
- People with heart or circulatory conditions: don’t stay in longer than about 10 minutes, and check with a doctor first if needed (source: Loviaspa, Wellis).
- Everyone: if you feel dizzy or overheated, get out. Drink water, and don’t get in after a lot of alcohol.
Water care: how often do you change it?
A wood-fired hot tub has no filter in its basic form. That means the water gets dirty faster.
Without a filter or water treatment, change the water roughly every 4 to 5 days, or after heavy use (source: Serena Spa, Aqua Kristal). Otherwise you end up with cloudy water, algae and bacterial growth.
With a simple filter and disinfection system you need to change it far less often: with light use (1-2 times a week), every 3 to 4 months (source: Aqua Kristal). Then you keep the water values in range:
- pH between 7.0 and 7.6
- Total alkalinity (TA) between 80 and 120 ppm
- Test the water once or twice a week (source: Hottub Houten)
If you’d rather work without chlorine, you can disinfect with active oxygen. Note: chlorine is only suitable for a stainless steel stove, not an aluminium one (source: Aqua Kristal).
Practical tips for keeping the water clean longer: rinse your feet before you get in, always use the lid when the tub isn’t in use, and scoop out leaves and dirt straight away.
Safety: what to watch out for
A hot tub with a live fire asks for common sense. The key points:
- Fire safety: keep the stove well away from fences, planting and canopies. Put it on a non-combustible surface. Don’t let sparks blow towards anything flammable.
- Hot surfaces: the external stove and the flue get red hot. With an internal stove that goes double. Screen it off or keep children away from it.
- Children: never leave children unsupervised near the tub or the stove.
- Slipping: a wet wooden rim and wet slabs are lethally slippery. Provide a non-slip step in and out, plus a handrail or small ladder.
- Smoke: you are burning in the open air, so carbon monoxide poisoning isn’t the risk it is indoors. Do think about smoke nuisance for yourself and the neighbours, and position the stove so the smoke drifts away.
Planning permission: do I need it?
In most cases a freestanding, movable hot tub in your back garden doesn’t need planning permission. An above-ground hot tub or jacuzzi up to around 1 metre high in the rear garden generally falls within what you can do without an application.
There are exceptions where you should look more closely, and where permission may be needed:
- If you sink the tub into the ground (partly or fully) or build it in as a permanent masonry structure
- If you build a canopy or roof over it
- If you are in a conservation area or your property is listed
- If outbuildings and structures together take up a large share of the garden, since there are limits on how much of the plot you can cover
Rules differ from one local authority to the next. Not sure? Check with your local planning department before you start. If you rent, or live in a leasehold or managed development, clear it with your landlord or management company too. It’s a quick call and it saves you trouble later.
Maintenance and winterising
Routine maintenance: clear the ash out of the stove before you light it again, check the water level before every burn, and keep the tub and lid clean. With a wooden tub: don’t leave it standing empty and dry for long, or the wood will shrink.
Winterising: if you’re not using it over winter, drain all the water before the first frost. Freezing water expands and can permanently damage your stove, pipes and tub (source: spa maintenance sources). Drain the tub completely, let the stove and pipes empty out, and blow out any residual water if you can. A fully drained system overwinters without any trouble.
Using it through the winter instead? Then the heat-up takes longer and you burn more wood, and you need to make sure no water freezes in the pipes between sessions.
Common mistakes
- Underestimating the base. A filled tub quickly weighs over 1,500 kilos. Without a solid, level foundation you get sinking or tilting.
- Burning without enough water. If the water level is below the upper stove fitting, circulation stops and the stove can overheat.
- Using wet wood. It can double your heat-up time and produces far more smoke. Burn only dry, split wood.
- Skipping the lid. Without a lid you heat up much more slowly and lose heat incredibly fast.
- Letting a wooden tub dry out. Leaving it empty and dry causes shrinkage and leaks.
- Leaving the water too long. Without a filter that means changing it every 4 to 5 days, no longer.
- Putting the stove too close to combustible material. Keep your distance from fences, planting and canopies.
Costs: what does a wood-fired hot tub cost?
Costs vary widely, depending on whether you make everything yourself or buy a kit.
| Approach | Indication | Notes |
|---|---|---|
| Full self-build | Widest range, heavily dependent on materials | Sourcing tub, stove and fittings yourself; cheapest if you buy smart |
| Self-build kit | Around 1,900 to 2,000 euros for a complete 5-6 person kit | For example a 175 cm kit with a 32 kW stove (source: Welltub) |
| Ready-made model | Higher | Delivered fully assembled |
Don’t forget the running costs: count on 60 to 120 litres of dry wood per heat-up. Used once or twice a week, a wood-fired hot tub is usually cheaper on fuel than an electric spa, provided you insulate well and burn dry wood (source: Hottub Houten).
I give ranges on purpose: the exact price depends on your material choice, the stove, and whether you can find parts secondhand. Always ask the supplier for current prices.
Frequently asked questions
How long does it take to heat a wood-fired hot tub?
Usually 1.5 to 2.5 hours for a small to medium tub. A 35 kW stove heats about 1,000 litres in roughly 1.5 hours to 38-40 degrees. With a large tub and a cold start it can take 2 to 3 hours (source: Wellnesstuinier, Spabuiten).
How much wood do I need per session?
Count on 60 to 120 litres of dry wood per heat-up, depending on the season and the outside temperature (source: Spabuiten, Haardhout.com). More in winter than in summer.
Is an internal or external wood stove better?
For most self-builders the external stove is the safer choice. The fire and the hot metal sit outside the tub, so less burn risk and more seating room. An internal stove heats a little faster, but you can burn yourself on it (source: Spabuiten).
Do I need planning permission for a hot tub?
Usually not for a freestanding, movable hot tub up to around 1 metre high in the rear garden. Do take care if you sink it into the ground, build a canopy over it, or live in a conservation area or listed property. Check with your local planning department if you’re unsure.
How often should I change the water?
Without a filter or water treatment, roughly every 4 to 5 days. With a filter and disinfection system and light use, every 3 to 4 months (source: Serena Spa, Aqua Kristal).
Which water temperature is safe?
Around 37 to 40 degrees. For pregnant women a maximum of 37.5 to 38 degrees, and people with heart or circulatory problems are better off not staying in longer than 10 minutes. Always use a thermometer (source: Wellis, Loviaspa).
Get started
A wood-fired hot tub is one of the finest garden projects you can take on yourself. With a solid base, a stainless steel stove at a safe distance, a good lid and dry wood, you’ll get years out of your own outdoor bath.
Want to be sure your foundation, dimensions and structure are right before you start? At Fred’s DIY Plans you’ll find clear build drawings and plans for your garden project, so you build it right first time: https://www.fredsdiyplans.com/