Wood-fired hot tub: heat-up time, firewood and sizes

Heat-up time and firewood

Heating water takes a fixed amount of energy: 1,500 litres warmed by 28 °C needs about 49 kWh of heat. A wood stove delivers part of the wood's energy into the water, so the stove's output, the tub's insulation and the weather decide the time.

Wood-fired hot tub: heat-up time and firewood to 100 °F (25 kW stove)

Water	Summer: air 68 °F, tap water 59 °F	Spring/autumn: air 46 °F, tap water 50 °F	Winter: air 23 °F, tap water 41 °F
264 gal	1.1 h · 23 lb	1.3 h · 28 lb	1.6 h · 33 lb
396 gal	1.6 h · 34 lb	2 h · 41 lb	2.3 h · 49 lb
528 gal	2.2 h · 45 lb	2.6 h · 55 lb	3.1 h · 65 lb

Heat model for a fibreglass-lined tub with wood cladding, kept under its wooden lid while heating, from a fresh fill; the stove delivers 25 kW into the water, firewood at about 4 kWh per kg with 65% of its heat reaching the water. With the lid on, the colder tap water matters more than the air. Stoves and wood differ: treat these as a guide.

A lid during heat-up makes a big difference: open water loses heat many times faster than a covered tub.

Internal or external stove

Internal stove: sits inside the tub behind a guard, so all its heat goes straight into the water. It takes up seating space and needs a guard to keep bathers off the hot metal.
External stove: stands next to the tub; water circulates through it by itself (hot water rises) or with a pump. More room in the tub and easier to tend, with a little heat lost through the pipes.

Materials

All-wood tubs (spruce, larch, cedar or thermowood) look traditional but are harder to clean and the wood must stay wet to stay tight.
Fibreglass or acrylic liners with wood cladding are the most common today: smooth, easier to clean, and some have insulation between liner and cladding.
Insulated covers cut heat loss between uses and keep leaves out.

Water care

Without filtration and chemicals the water should be changed often: many owners refill every few days to a week. Filters, UV units and chemicals let the water last longer. Plan for a hose, a drain and somewhere for the water to go.

Running costs and alternatives

Firewood is the main cost; some tubs add an electric heater for keeping the water warm. Compare wood and electric in wood-fired vs electric hot tub, or work out your own case with the hot tub running cost calculator.

Questions and answers

How long does a wood-fired hot tub take to heat up?

Usually a few hours, depending on the size, the stove, the lid and the weather. The table on this page shows our heat model for common sizes at several outside temperatures; keep the lid on while heating.

How much firewood does a wood-fired hot tub use?

Heating 1,500 litres by 28 °C takes about 49 kWh of heat, which our model turns into firewood with the stove's efficiency. The table gives the kilograms for common sizes; dry hardwood burns hotter and cleaner.

Is an internal or external stove better?

Internal stoves heat a little faster because all their heat goes into the water, but they take seating space. External stoves leave the tub free and are easier to tend. Both work; choose by space and how you like to use the tub.

How often do you change the water in a wood-fired hot tub?

Without filtration or chemicals, every few days to a week is common. With a filter, UV or chemicals the water lasts longer; follow the tub maker's advice.