How the heating works
The stove either sits inside the tub behind a guard, or stands outside it and connects through two pipes. Either way the principle is the same. Water next to the hot metal warms up, rises, and colder water moves in to take its place.
That has one consequence people miss. The top of the tub gets hot long before the bottom does. Many tubs come with a paddle for this reason. Stir the water before you trust what the thermometer says.
How long it takes to heat
Hot tub retailer Layzee Living puts it at between one and three hours for a standard tub. The gap is wide because five things change it. How cold the water is to begin with, how big the tub is, whether the stove is inside or outside, how dry the wood is, and the weather.
In practice, a small tub in July is at the quick end. A big one filled from a cold tap in January is at the slow end. If you’re arriving at six and want to be in by eight, light it the minute you’ve unloaded the car.
How hot is hot enough
Layzee Living says most people aim for 36°C to 40°C. Argyll and Bute Council, in advice written for businesses that offer these tubs, is firmer. The water should never go above 40°C, and 36°C to 38°C is considered safe for a healthy adult.
You can’t turn a fire down the way you turn a dial. So stop feeding the stove a little before you reach the temperature you want, because the water keeps warming for a while. If it overshoots, add cold water and stir.
Using one on holiday
The council’s advice says operators should supply the right fuel and give guests instructions for lighting the stove safely. Read them. Stoves differ, and the site knows its own.
The usual routine goes like this. Check the tub is full before you light anything. Keep the lid on while it heats, which makes a big difference. Use small, dry logs and feed the fire little and often. Stir every so often. Check the temperature after stirring, then get in.
How long it stays hot
With the lid on, a tub holds its heat for a good while after the fire dies down. With the lid off on a cold, windy night it cools quickly. Keep a few logs back if you fancy a second dip after dinner.
Why the water is fresh each time
A wood fired tub has no pump and no filter. The water isn’t cleaned as it sits. For that reason, Argyll and Bute Council’s advice says chemical disinfection isn’t appropriate for this kind of tub. Instead, the water should be drained after each bathing session, and it shouldn’t stay in the tub for more than five hours.
That’s one council writing for the businesses in its area, and other sites may work differently. It does show the thinking, which is clean water every time.
Electric tubs are a different animal
An electric hot tub is kept hot around the clock, filtered and treated with chemicals. You lift the lid and get in. The Health and Safety Executive’s guidance on spa pools, known as HSG282, covers these when they’re part of a business, and it names holiday park rental units as an example.
Which is better? Wood fired gives you the fire, the smoke and the satisfaction. Electric gives you the whole evening. Our pages on stays for two and last minute trips say when we’d choose each.
Sources
- Layzee Living, How long do wood fired hot tubs take to heat up?
- Argyll and Bute Council, Scandinavian spas, advice for commercial operators (January 2024)
- HSE, HSG282, Control of legionella and other infectious agents in spa-pool systems (2017)
Published 3 October 2026. Spotted a mistake? Tell us.