By The Zagosa Team. Published .
Reviewed or updated .
Plan for a wait. As a rule of thumb, a home hot tub climbs at about 1 to 2 degrees Celsius each hour, so lifting a 10C refill up to a 38C soak lands somewhere between 14 and 28 hours. Where you fall in that range depends on the heater fitted, the size of your tub, the state of your cover, the outdoor temperature, and whether the jets are circulating while it warms.
The things that decide your heat-up speed
Three factors dominate, and it is worth knowing them in order of impact.
Start with the heater. A 3kW plug-in unit, the common UK setup, is the slowest of the lot. Step up to a 6kW hardwired heater and you roughly halve the time. A handful of swim spas run twin heaters and are faster again.
Next comes water volume. A compact two-seater holds around 700 litres, whereas a big six-seater can be 1,500 litres or more. Double the water and, broadly, you double the wait.
Finally, the cover and the weather. A snug, dry, well-insulated cover roughly halves your heat loss against a thin or waterlogged one. In winter the biggest leak we come across is cold wind blowing straight across an uncovered surface.
A timeline that matches real UK weather
Fill up on a mild April day from a 10C mains supply, with a 3kW heater and a decent cover, and you are looking at roughly 18 to 22 hours to hit 38C.
Do the same fill in January with wind-chill on the cover and it can stretch to 30 hours or beyond. That is not a fault, it is physics. The trick is to time your refill for a Friday morning so the tub is ready by Saturday night.
One caveat worth flagging: if the display has not budged in a couple of hours, that points to a problem rather than slow progress. Check the heater is actually calling for power and that the high-limit reset has not tripped.
Three genuine ways to speed things up
Add some warm water from indoors. A few buckets from the kitchen tap knock a couple of degrees off the gap immediately. Stop a little short of the fill line and let the heater finish the job.
Run the jets on low as it heats. Moving water warms more evenly, and the readout climbs quicker than it would with still water sitting in the shell.
Keep the cover shut throughout. Peeking under it in cold weather genuinely wipes out ten or fifteen minutes of progress every time.
Common questions
Can I dose chemicals during the heat-up?
You can. Once the water passes roughly 20C, add your first dose of pH adjuster and sanitiser with the jets running. Below that mark granular products struggle to dissolve and you risk staining.
Why does the reading race up then crawl?
The surface is where most heat escapes. As the water approaches air temperature in the upper 20s, the rest of the climb comes down to how well your cover insulates.