A wood-fired hot tub is a water vessel connected to a stove, not a bonfire beneath a bath. The fire heats a metal water jacket; hot water returns to the tub while cooler water moves back to the heater. The quality of that circulation, together with water volume, cover, fuel and weather, determines how the tub behaves.
The heat path, one stage at a time
- The tub is filled above the required heater connections. The stove must never be fired without the water condition specified by its manufacturer.
- A controlled fire heats the stove’s water jacket. Energy passes through the metal wall into the water rather than directly into the tub.
- Water circulates. In a correctly designed passive system, warmer water rises back to the tub and cooler water returns. Other systems use a pump.
- The whole tub is mixed and measured. Water near the return can be hotter than the average, so temperature must be checked away from the outlet.
- The fire is reduced before the target. Heat remains in the fuel and metal after the flames fall, so late firing can overshoot.
Internal, external and integrated heaters change the experience
An internal or snorkel stove sits in the bathing vessel behind an approved guard. The short heat path can be compact, but the heater occupies water and seating space and needs careful loading around bathers. An external stove preserves the interior layout and is easier to approach from outside, although its connecting pipes, distance and height relationship must be correct.
An integrated stove is built into the tub’s outer form. It can give a tidy footprint and convenient loading, but it is not the same as an internal heater: construction, circulation and service access depend on the model. Compare the final water volume and usable seating, not just the outside diameter.
Natural circulation is simple, but not casual
Thermosiphon flow relies on the density difference between warmer and cooler water. It works only when the heater and pipework are designed for it: restrictions, air locks, unsuitable hose, incorrect slope or an over-long route can reduce flow. A pump should not be added as an improvised cure unless the heater system is approved for pumped circulation and has the required controls.
Powered filtration is a separate question. A log stove can provide the heat while a pump circulates water through a filter, and massage or lighting can add further electrical loads. This is why some wood-fired tubs work without mains electricity and others do not. The final options schedule is decisive.
What controls the heating time?
The water volume and temperature rise set the basic energy requirement. A larger tub filled with cold water needs more energy than a small tub starting mild. The useful heat reaching the water then depends on the stove, fire, circulation and losses to the air. Wind, an uncovered surface and wet fuel can turn a headline estimate into a much longer session.
- Configured water volume: bench shape and heater position can change it substantially.
- Starting and target temperature: compare claims using the same two figures.
- Dry fuel and fire management: split, seasoned logs burn more predictably than damp, oversized pieces.
- Cover: use the approved cover during heat-up when the manufacturer allows it, while maintaining the required flue clearance.
- Mixing: confirm the whole volume is warm rather than measuring a hot layer beside the return.
From cold fill to bathing: a practical routine
Inspect the stove, flue, guard and connections before filling. Fill to the prescribed level, vent trapped air where required and prove circulation. Light a small fire first and check for leaks, smoke spillage and an improving return temperature. Build the fire gradually, keep the bathing area supervised and stir or circulate the water before each measurement.
Stop adding fuel early enough for residual heat. Confirm a comfortable, even temperature before anyone enters, then keep children and guests away from hot metal and the flue. After bathing, follow the manufacturer route for burning down, water treatment or drainage. Never drain while the stove or embers can still heat an empty jacket.
Water care is independent of the fuel
Wood heat does not disinfect the complete water volume. The care plan depends on bather load, retention time, filtration and the approved treatment method for the tub materials. A simple fill-and-drain routine may suit occasional private use; retained warm water needs deliberate circulation, testing and treatment. Business-use tubs require the risk assessment and monitoring appropriate to the operation.
Choose the model by the routine you want
Choose an internal heater when a compact layout and close fire management suit you; an external heater when interior space and outside access matter; or an integrated design when the combined footprint and finish fit the site. Then compare real water volume, normal occupancy, fuel loading position, chimney route, delivery access and maintenance. The best model is not the one with the largest quoted output, but the one whose heat path and operating routine fit your garden.
Continue with the hot-tub heating-time calculation and the wood-fired versus electric comparison. DEFRA’s Burn Better guidance covers suitable fuel and cleaner domestic wood burning.