The safe maximum
The US Consumer Product Safety Commission warns that water above 40 °C (104 °F) can raise body temperature to dangerous levels, and US spa controls are built not to exceed it. Keep sessions short at the top of the range, keep children's water cooler, avoid alcohol in the tub, and ask a doctor first if you are pregnant or have a heart condition.
Typical settings
- 37–38 °C for long, relaxed soaks and warm summer evenings.
- 39–40 °C for shorter sessions in cold weather.
- Lower when the tub is not in use for several days, if your spa heats back up fast enough.
What each degree costs
An insulated spa loses heat all day through its shell and cover, and the loss grows with the difference between the water and the air. The table is our heat model for a typical spa with a thermal cover:
| Water | Per month at 32 °F outside | Per month at 50 °F outside |
|---|---|---|
| 98 °F | 165 kWh · $30.16 | 140 kWh · $25.63 |
| 100 °F | 168 kWh · $30.78 | 143 kWh · $26.26 |
| 102 °F | 172 kWh · $31.40 | 147 kWh · $26.88 |
| 104 °F | 175 kWh · $32.03 | 150 kWh · $27.50 |
Heat model for an insulated acrylic spa with a thermal cover, three 30-minute uses a week, circulation pump included; electricity $0.18/kWh (United States average). Older spas and worn covers use more.
A worn or waterlogged cover can easily matter more than a degree or two. Work out your own tub with the hot tub running cost calculator.
Heat-up time
After a fresh fill, the heater's output sets the pace: a 3 kW heater warms 1,500 litres by about 1.7 °C per hour before losses, so heating from 10 to 38 °C takes most of a day. Wood-fired tubs with 20–30 kW stoves heat in a few hours (wood-fired hot tubs).
Check the water
Thermostats can drift. A floating thermometer is a cheap way to check that the water is really where you set it.