How long does a hot tub take to heat up?
How long does a hot tub take to heat up? About 8 hours are needed for a 5.5 kW heater to warm a 350 gallon (1,330 litre) tub from 50 up to 102°F (10 to 39°C). With a 4 kW heater, expect about 11 hours, while a 120 V plug-in model may need more than a day.
Choose your heater and tub, plus how warm the water is now, and you'll see the hours.
- Heating 350 gal from 50 to 102°F using 5.5 kW
- Roughly 8 hours
- The same tub paired with a 4 kW heater
- Roughly 11 hours
- Inflatable model holding 290 gal uses 2.05 kW
- Roughly 18 hours
- 1 kW 120 V plug-in for 250 gal
- Roughly 32 hours
How long does a hot tub take to heat up, by heater?
These numbers rely on heater output and the total weight of the water. The EIA states that 1 kWh equals 3,412 Btu and one Btu raises the temperature of 1 pound of water by 1°F. Per the USGS, water weighs 8.34 pounds per gallon.
| Heater | Tub size | Heat rate | 50 to 102°F | 50 to 104°F |
|---|---|---|---|---|
| Inflatable pump heater, 2.05 kW | 180 gal (670 L) | 4.7°F (2.6°C) an hour | Roughly 11 hours | About 12 hours |
| Inflatable pump heater, 2.05 kW | 290 gal (1,100 L) | 2.9°F (1.6°C) an hour | Roughly 18 hours | About 19 hours |
| 120 V plug-and-play, 1 kW | 250 gal (930 L) | 1.6°F (0.9°C) an hour | Roughly 32 hours | About 33 hours |
| 240 V hard tub, 4 kW | 350 gal (1,330 L) | 4.7°F (2.6°C) an hour | Roughly 11 hours | About 12 hours |
| 240 V hard tub, 5.5 kW | 350 gal (1,330 L) | 6.4°F (3.6°C) an hour | Roughly 8 hours | Roughly 8 hours |
| 240 V hard tub, 4 kW | 500 gal (1,900 L) | 3.3°F (1.8°C) an hour | About 16 hours | About 16 hours |
| 240 V hard tub, 5.5 kW | 500 gal (1,900 L) | 4.5°F (2.5°C) an hour | About 12 hours | About 12 hours |
The 2,050 W figure for the inflatable heater comes from Lay-Z-Spa's Ibiza spec. On 120 V, Jacuzzi gives a range of 1 to 1.5 kW, while 240 V models use 4 to 5.5 kW. A water temperature of 102°F (39°C) is the goal in California's spa energy test.
Reasons real tubs heat slower | Why actual tubs take longer | Factors slowing real tub heating
These estimates assume all power goes directly into heating the water. Brand times run slower because real tubs lose heat to the air while warming.
The heat rate Intex lists for its 1,300 W PureSpa sits between 2 and 3°F (between 1 and 1.5°C) per hour, on 210 gallons (795 litres). At 290 gallons, the rise falls to between 1 and 2°F (between 0.5 and 1°C). MSpa lists its Meteor at between 2.0 and 3.0°F (between 1.1 and 1.7°C) each hour.
Lay-Z-Spa quotes a pace of about 1 to 1.5°C (between 2 and 3°F) each hour, which means a cold fill needs 24 to 48 hours. Hard tubs warm more quickly. Hydropool gives the pace as between 5 and 10°F (between 3 and 6°C) each hour, so a 60°F (16°C) fill needs 3 to 8 hours.
Cold weather and keeping the cover on
Throughout the entire heat-up, keep the cover on. Hydropool notes that over 90 percent of heat leaves the surface if the tub is uncovered. Its times for a hard tub:
| Starting point | Typical time |
|---|---|
| Top-off after a soak | 30 to 60 minutes |
| Full refill, mild weather, good cover | 3 to 5 hours |
| Full refill, cold weather or a worn cover | 6 to 8 hours or more |
| Swim spa refill | 12 to 24 hours |
Cold air also adds hours to inflatable models. Colder weather slows heating down, according to Lay-Z-Spa. Intex recommends avoiding use if the air temperature falls below 39°F (4°C).
Inflatable hot tub heat up time vs hard shell
A pump heater for an inflatable uses about 2 kW from a standard household socket. The same water warms about 2 to 2.7 times faster in a 240 V hard tub running 4 to 5.5 kW.
Lay-Z-Spa says its Ibiza warms to 40°C (104°F) in 6 to 12 hours on a typical UK summer day. The slowest type is the plug-in hard tub. Also, Hot Spring's 115 V models stop heating whenever the jet pump runs.
Ways to heat it faster | Steps for faster heating | How to speed up heating
- Turn on the heater while filling, keeping the cover mostly shut. Hydropool cites skipping this step as a common cause for long first fills.
- Warm tap water is okay only when the hose is rated for hot water and the tub allows it. Your water heater must also be able to keep up.
- Keep the tub running between your soaks. Hydropool notes that turning it off completely costs more to reheat than it saves.
- Never set it above 104°F (40°C), the safe hot tub temperature.
- Work through hot tub not heating if the water barely warms.
Common questions
How much time is required to heat a hot tub from cold?
About 8 hours are needed for a 5.5 kW heater to warm a 350 gallon (1,330 litre) tub from 50 up to 102°F (10 to 39°C). With a 4 kW heater, you need about 11 hours, and cold wind adds time.
What is the heat-up time for an inflatable hot tub?
Count on 24 to 48 hours from a cold start, says Lay-Z-Spa, with the water gaining about 1 to 1.5°C (between 2 and 3°F) each hour. Depending on size, the PureSpa from Intex gets a rating of 1 to 3°F per hour.
What is the reheating time once you have used the tub?
You will typically wait less than sixty minutes. Because the heater only has a few degrees to add, Hydropool says a top-off after a soak runs 30 to 60 minutes.
Is it better to keep the hot tub running constantly?
Yes, for most owners. Keeping the temperature steady uses less energy than starting from cold, according to both Hydropool and Lay-Z-Spa. Lay-Z-Spa notes this is true when you soak 3 or more times each week.
Will turning on the jets make the hot tub heat up quicker?
Models running on 120 V power do not see faster heating. Since Hot Spring's 115 V models shut off the heater during jet pump operation, you should heat with jets off and the cover closed.