How long does a hot tub take to heat up?
Quick answer
It depends on two things: how much water and how big the heater. A wired 5.5 kW tub warms a typical 350 to 400 gallon spa about 5 to 6°F per hour, so a 40-degree heat-up takes most of a working day. A plug-in 1.5 kW tub manages closer to 1.5°F per hour and can take a full day from cold.
Heating water is fixed physics, so the answer is predictable once you know your gallons and heater. No guessing required.
What sets the time
- Gallons. Twice the water takes twice as long to heat with the same heater.
- Heater output. A wired 4 to 6 kW heater is several times faster than a 1 to 1.5 kW plug-in.
- Starting temperature. A bigger rise takes proportionally longer; filling with warmer water shortens it.
- Heat loss. On a cold, windy day the tub loses heat while it warms, adding time, so always heat with the cover on.
Typical figures
| Heater | Rate (350 to 400 gal) | 60 to 100°F takes |
|---|---|---|
| 1.5 kW plug-in | ~1.5°F/hr | ~26 hours |
| 4 kW wired | ~4°F/hr | ~10 hours |
| 5.5 kW wired | ~5.6°F/hr | ~7 hours |
Ideal times with the cover on, computed at 8.34 lb per gallon and 3,412 BTU per kWh. A cold day, wind, or a poor cover adds to them. Enter your own numbers on the calculator for an exact figure.
Common questions
Can I speed up the heat-up?
Keep the cover on the whole time, fill with the warmest water you have available, and make sure the filter is clean so the pump moves water freely. Beyond that the heater output sets the pace.
Why is my plug-in tub so slow?
A 110-volt plug-in tub has a small heater, often around 1.5 kW, which only adds heat slowly to a few hundred gallons. That is the tradeoff for not needing an electrician; from cold it can take a day to come up.
Does heating overnight cost less?
The energy is the same whenever you heat, but if your utility charges less off-peak, heating overnight is cheaper. Either way, leave the cover on so you are not paying to heat the air.