How long does it take to heat a hot tub?

Short answer: a wood-fired Q-tub reaches bathing temperature in around 2.5 to 3.5 hours. Electric heating takes longer and depends on the output: around 16 hours with a 3 kW element, around 5 hours with 6 kW, and around 5 hours with a 7.5 kW heat pump. How fast it goes in your situation is determined by four factors: the water volume, the temperature difference between start and target, the output of the heating, and the heat loss along the way. Below we work through the numbers and show step by step how to keep your heating time as short as possible.

Heating times in practice

The times below apply to heating cold mains water (around 10 to 15 °C) to bathing temperature (37 to 38 °C), with the cover on.

For reference: a Q-tub XXL holds 1300 litres (internal heater) to 1400 litres (external heater). Smaller models hold less water and heat up correspondingly faster.

Calculating it yourself: the rule of thumb

Heating water costs a predictable amount of energy. The rule of thumb:

required kWh = litres x temperature rise in °C / 860

Worked example with a 1000-litre tub that needs to go from 12 to 38 °C (a rise of 26 degrees): 1000 x 26 / 860 = around 30 kWh. With a 6 kW element, that's a minimum of 30 / 6 = 5 hours, which matches practice. The same sum explains why a 3 kW element takes almost a full day to heat, and why the wood-fired heater is so much faster: it delivers many times more output and drives the heat straight into the water.

The formula gives the lower bound. In reality, additional time is added due to heat loss through the water surface, the walls and the wind. This is exactly why the cover, insulation and a sheltered spot make such a difference.

Wood-fired: step by step for minimum time

1. Fill the tub first. Always fill the shell well above the top heater connection, to the level given in the assembly manual, before you so much as light a match. A heater that burns without water suffers irreparable damage.
2. Put the cover on. From the first minute. The water surface is by far the biggest source of heat loss; an insulated cover noticeably saves time and wood.
3. Light with small, dry wood. Use firelighters or wood wool with thin kindling. Never petrol, spirits or other flammable liquids, and preferably no paper or cardboard either: the light ash blows straight into the bath water.
4. Open the air vent fully. Maximum draught in the first phase means a hot, clean fire. Once the water is nearly at temperature, close the air vent down to temper the fire.
5. Fire with small, split, dry wood. Wood with less than 20 percent moisture (dried for at least two years) burns hottest. Split the logs small: more contact area with the fire means faster heat transfer. Light wood such as birch ignites quickly; harder types such as beech or ash hold the fire longer afterwards. Never burn painted, treated or wet wood.
6. Keep feeding it actively for the first hour and a half. Keep the heater well fed and add wood every 20 to 30 minutes. A half-full heater slowly dying out is the most common reason heating takes longer than it should.
7. Stir the water and measure in the right spot. Warm water rises: the top layer can be more than 5 degrees warmer than the bottom. Stir the water halfway through and before every measurement (with a paddle or by switching on the jets), and measure with a floating thermometer in the middle of the tub.
8. Stop firing in time. Such a large mass of water reacts slowly: wood you add now won't be felt for another quarter of an hour. Ease off around 35 to 36 °C and let the fire finish the last bit. Keep a garden hose or bucket of cold water on hand to correct things if it does get too warm.

Electric and heat pump: always ready instead of repeatedly waiting

An electric element heats steadily, roughly 1 to 3 °C per hour depending on power and volume. So the smart strategy is not "heat cold water whenever I want to bathe", but "keep the water constantly at temperature":

• Regular users: leave the thermostat at bathing temperature. The spa is then always ready, and thanks to insulation and the cover, the element only has to make up the small loss.
• Weekend users: lower the setpoint to around 30 °C on weekdays and raise it a few hours before use. This is faster and more economical than letting it cool completely.
• Heat pump: the most economical option. A heat pump draws heat from the outside air and, as a result, delivers several times more heat into the water per kilowatt-hour of electricity. The system automatically scales back when little output is needed.
• Hybrid: fire with wood to heat up quickly and let the electric element or heat pump hold the temperature. This combines the speed of wood with the convenience of electric heating.

Where the heat is lost (and how to limit it)

• The water surface: by far the biggest loss. Cover on while heating, between sessions and immediately after bathing.
• Wind: a tub in full wind cools noticeably faster. A sheltered spot or windscreen shortens every subsequent firing.
• The shell: the double insulation layer of the Q-tub limits loss through the walls; an insulated cover does the rest.
• Using residual heat: once heated, water under a good cover cools only slowly. Topping up the heat the next day from, say, 30 °C costs a fraction of a full firing from cold.

Heating in winter

In winter you start with colder mains water (5 to 8 °C instead of 12 to 15 °C) and the tub loses more heat to the air and wind. Allow for 30 to 60 minutes of extra firing time and plan your first winter session with plenty of margin. If you won't be using the hot tub for a while during a frost period, read winterising a hot tub first: frozen pipework and pumps are the most expensive winter mistake.

Common mistakes

• Firing without the cover on: the heat disappears as fast as the heater delivers it; heating takes hours longer.
• Wet or treated wood: wet wood first boils off its own moisture and produces barely any heat; painted or treated wood should never go in the heater.
• Measuring without stirring: measuring only the warm top layer means getting into a tub that's still cold underneath.
• Lighting the heater with too little water: always fill above the heater connections first.
• Continuing to fire up to the target temperature: the lag from a full heater will overshoot the target; ease the fire back from 35 °C.

If the water barely heats up despite a good fire, work through the causes in hot tub water not heating. If you use an electric element or heat pump, read electric heating and heat pump.