From a winter refill to your first soak, here is what to actually expect from a UK hot tub heat-up, plus three honest ways to make it quicker.
Short answer: most domestic hot tubs heat at roughly 1 to 2 degrees Celsius per hour, so going from a 10C refill to a 38C soak takes anywhere from 14 to 28 hours. The honest answer depends on your heater rating, your tub size, your cover, the outside temperature, and whether the jets are running while it heats.
Three things make the biggest difference, in this order.
The heater. A 3kW UK plug-in heater is the slowest. A 6kW hardwired heater is roughly twice as quick. Some swim spas have two heaters and are quicker again.
The water volume. A small two-person tub holds around 700 litres. A large six-seater can hold 1,500 litres or more. Twice the water means roughly twice the heat-up time, all else being equal.
The cover and the weather. A well-fitting, dry, insulated cover roughly halves the heat loss compared with a thin or waterlogged one. Cold wind across an uncovered surface is the single biggest leak we see in winter.
Refilling on a mild April day from a 10C mains supply with a 3kW heater and a good cover, expect around 18 to 22 hours to reach 38C.
In January with a wind-chill on the cover, the same refill can take 30 hours or more. That is normal. Plan your refill for a Friday morning so you are ready for Saturday evening.
If the readout has not moved in two hours, that is a warning sign rather than slow heating. Check the heater is calling for power and that the high-limit reset has not tripped.
Top up with warm water from inside the house. A few buckets from the kitchen tap take a couple of degrees off the gap straight away. Stop short of the recommended fill line and let the heater do the rest.
Leave the jets on low while it heats. Circulating water heats more evenly and the readout will catch up faster than with still water.
Keep the cover on the whole time. Lifting it to peek genuinely costs you ten or fifteen minutes of progress in cold weather.
Yes. Add your initial dose of pH adjuster and sanitiser with the jets running once the water is above about 20C. Below that, granular products dissolve poorly and you risk staining.
Most of the loss happens at the surface. Once the water nears the air temperature for the upper 20s, you are fighting the cover's insulation rating for the rest of the climb.