How Long Does It Take a Hot Tub to Heat Up?
It depends on what is doing the heating. A wood fired hot tub, the wood burning kind, heats from cold in around two hours for a Small, three for a Standard and five for a Large. An electric cedar hot tub takes up to fourteen hours from a cold fill, at roughly three degrees Fahrenheit an hour, which is why it is held at temperature or scheduled rather than heated on demand. A conventional acrylic spa is similar, typically eight to twenty-four hours from cold depending on its heater and the outside temperature.
The number most people actually live with is a different one: how long it takes to get back to temperature after the last soak. On an insulated tub with the lid on, that is a small top-up rather than a heat-up, and it is the figure worth asking any manufacturer for.
By system
| From a cold fill | Between soaks | What you do | |
|---|---|---|---|
| Wood fired, AlumiTub | About 2 hours Small, 3 Standard, 5 Large | Loses 2 to 3°F overnight with the lid on; a handful of wood brings it back | Light the fire two or three hours before you want to soak |
| Electric, AlumiTub | Up to 14 hours at about 3°F per hour | None. It holds the set temperature, 50 to 104°F | Set it once, or schedule it from the app |
| Hybrid, AlumiTub | Wood times above, faster with both running | Either | Fire for the heat-up, electric to hold it |
| Acrylic spa, typical | 8 to 24 hours depending on heater and weather | Holds temperature continuously | Leave it on |
The wood fired figures assume a cold fill at typical outdoor temperatures with dry seasoned hardwood. The full method, and the five things that make it faster, is in how long a wood fired hot tub takes to heat.
What moves the number
- Starting water temperature. A January fill from a lake or a cold well starts thirty degrees behind a summer hose. If you can fill with warm water, do.
- Water volume. 315 US gallons on a Small, 420 on a Standard, 650 on a Large. More water takes longer to heat, though not longer to hold.
- Heater output. A submerged wood firebox transfers heat directly into the water and outperforms an external stove. An electric heater has a fixed output, which is why its time from cold is long and predictable.
- Insulation and the lid. Most heat leaves through the water surface. A floating insulation layer under a well-fitted lid, kept on while heating, makes the largest single difference on any system.
- Air temperature and wind. Wind strips heat from the surface faster than cold air alone. Shelter matters.
- Wood quality, on a wood fired tub. Dry hardwood turns its energy into heat. Wet wood spends it boiling off moisture.
Why the second soak is different
Time from cold is the number in the brochure. Time between soaks is the number you live with, and it depends almost entirely on insulation. An AlumiTub carries 3.5 inches of rigid board beneath the base, a full wrap behind the cedar, and a floating layer on the water surface, so a wood fired tub loses only two to three degrees overnight with the lid on. Two evenings in a row is one heat-up and one small maintenance burn.
Sunday morning 8am. Hadn't added a log for 11 hours and it was still 103.
Marc Young, Bowyer Island BC
On an Electric there is no second-soak question at all. The system holds whatever you set it to, and the same insulation means the heater cycles rarely to do it. The trade is the slow start from a cold fill, which is why it is designed for holding rather than heating on demand, and why the fill is done well ahead.
How to shorten it
- Fill with warm water where you can
- Keep the floating insulation and lid on while heating
- Put rigid insulation beneath the tub, which is required on concrete, gravel and treated wood anyway
- Shelter the tub from wind
- On a wood fired tub, burn dry seasoned hardwood and stop adding wood around 95°F, since the momentum carries the water to a 101 to 104°F soak on its own
- On a Hybrid, run the fire and the electric side together
Every AlumiTub arrives pre-assembled and sets up in about an hour, so on a prepared site the first soak is the same evening on a wood fired tub and the next day on an electric one. The three systems are compared in full in the heating options guide.
Common questions
How long does a hot tub take to heat up from cold?
Two to five hours on a wood fired AlumiTub depending on size, up to fourteen hours on an electric cedar tub at about three degrees an hour, and typically eight to twenty-four hours on a conventional acrylic spa. Starting water temperature, water volume, insulation and wind move all three.
Why does an electric hot tub take so long to heat?
An electric heater has a fixed output, so bringing several hundred gallons up from cold takes many hours. Electric tubs are designed to hold a set temperature continuously, or on a schedule, rather than be heated on demand, and with good insulation the heater cycles rarely once the water is there.
How long does a hot tub take to reheat between soaks?
On an insulated wood fired tub with the lid on, the water loses only two to three degrees overnight, so reheating is a handful of wood and a short wait rather than a full heat-up. An electric tub holds its set temperature, so there is no reheat.
What is the fastest way to heat a hot tub?
On a Hybrid, run the fire and the electric heater together. On any tub, fill with warm water, keep the floating insulation and lid on while heating, insulate beneath the tub and shelter it from wind. On a wood fired tub, dry seasoned hardwood matters more than anything else.
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