How Long Does It Take to Heat a Wood Fired Hot Tub?

A wood fired hot tub takes between 2 and 5 hours to heat, depending on size, starting water temperature and conditions. On an AlumiTub, expect around two hours for the Small, three for the Standard and five for the Large. Fill the tub first, build the fire at the rear of the firebox, stop adding wood around 95°F, and let the residual heat carry the water to a 101 to 104°F soak.

That is the method. What follows is the numbers by size, what moves them, how to light and run the fire well, which wood to burn, and how to heat it faster when you want to.

Heat-up time by size

Model	Water volume	Seats	Approximate heat-up from cold
Small, 5 ft	315 US gallons	2 to 3, or 3 to 4 on Electric	~2 hours
Standard, 6 ft	420 US gallons	4 to 5, or 5 to 7 on Electric	~3 hours
Large, 7 ft	650 US gallons	6 to 8, or 9 to 10 on Electric	~5 hours

These assume a cold water start at typical outdoor temperatures using dry seasoned hardwood. Ideal soaking temperature is 101 to 104°F. Once there, an AlumiTub holds heat with as little as an armful of wood a day, and a second evening in a row is a small maintenance burn rather than a full heat-up.

Sunday morning 8am. Hadn't added a log for 11 hours and it was still 103.

Marc Young, Bowyer Island BC

Four things move the number:

• Outdoor temperature and wind
• Starting water temperature
• Firewood quality and dryness
• Insulation and how consistently the lid is used

Why an AlumiTub heats faster than a traditional wooden tub

Traditional all-wood cedar tubs lose heat through the walls and base once the wood becomes saturated. Wet wood lets heat pass through, which means longer heat times, more firewood, and faster loss between soaks.

An AlumiTub is built differently:

• A marine-grade aluminum structure that does not absorb water
• Insulation where it counts: 360 degrees around the tub and a floating layer on the water surface, under the lid, with 3.5 inches of rigid board laid beneath the base during site prep
• An internally submerged firebox, the largest of its kind, heating the water directly and taking regular-length logs

Unlike an external firebox, an internal one uses natural convection to heat the water directly. That cuts heat time, uses less firewood, and produces minimal smoke when the wood is dry. How wood fired hot tubs work covers the mechanics.

How to heat it, step by step

Fill the tub before lighting the fire. Fill to the line marked on the tub, or at least 4 inches above the chimney collar, and keep the water there the entire time the firebox is hot. Firing below that line warps the firebox, risks fire, structural damage or forceful ejection of the chimney assembly, and voids firebox coverage.

Two more things before the first light of the season: keep at least ten feet of vertical clearance from the top of the chimney cap to anything overhanging, and never operate during a fire ban. Check with your local fire authority if you are unsure what applies where you are.

1. Build the fire at the rear of the firebox, below the chimney, starting with paper and dry kindling loosely arranged for airflow
2. Allow airflow. Keep the firebox lid open about 4 inches while the fire is establishing
3. Build gradually. Add small pieces of seasoned hardwood as it catches, then larger logs. Do not pack the firebox tightly, and do not load wood above the internal pipes. An overfilled firebox produces more smoke, less heat and a slower warm-up
4. Stir three or four times during heating with a paddle or a length of wood about four feet long, circulating water from the base to distribute heat and prevent hot spots near the firebox
5. Stop adding wood at 95°F. The fire keeps transferring heat as it burns down, and the momentum carries further than you expect
6. Let residual heat finish the job. The water rises to 101 to 104°F without overshooting

Adding wood past 95°F is the usual cause of an overheated tub. AlumiTubs hold heat well, so the challenge is rarely getting hot enough. It is stopping short of too hot. If it goes past 104°F, let it cool or add cold water away from the stove. Never drain below the fill line until the firebox has fully cooled.

We didn't really think that two little pieces of wood would heat it back up to temp after sitting all night and day, but holy, did it. We had to add some cold water.

Christina Crain

Which wood to burn

The wood you burn directly affects how quickly the tub heats, how much fuel you use, and how pleasant the process is. Dry, dense hardwood burns best.

• Oak. Dense, high heat output, long burn. Benefits from eighteen months to two years of seasoning
• Maple. Burns hot and clean, widely available across Canada and the northern US
• Ash. Easy to split, seasons relatively quickly, reliable
• Birch. Lights readily and burns well, good for starting and shorter sessions

All share the key quality: moisture below twenty percent. Dry wood turns its energy into heat. Wet wood spends it evaporating moisture first, which cuts output, produces heavy smoke and leaves residue in the firebox and chimney.

Avoid pine and spruce, which burn fast and deposit resin; cedar, which is beautiful on the outside of the tub and poor as fuel; wet or green wood, the most common mistake; and anything treated, painted or laminated, wood with nails, pellets, accelerants or fire starter logs, which release harmful compounds and can damage the firebox. Never use construction offcuts or timber of unknown origin.

Buying rather than sourcing your own? Kiln-dried hardwood is ready immediately and performs consistently. Bulk cord wood from a local supplier is more economical for regular use, provided it has been properly seasoned. More in the best firewood to use.

Five ways to heat it faster

1. Insulate the base

Two layers of rigid insulation beneath the tub, 1.5 inch and 2 inch for about 3.5 inches total. This is required on concrete, gravel and pressure-treated wood as a protective barrier, and owners report noticeably faster heat times. The bottom band accessory hides the edge.

2. Use the lid and the floating insulation

Place the insulation sheet on the water and the cedar lid on top, while heating and between uses. Most heat loss happens through the water surface. Overnight loss then comes down to two to three degrees Fahrenheit.

I can't believe the tub stays so hot. It heats so easily and the next morning when we went to use it, it had only dropped 3 degrees. It's well insulated and well made.

Jess Krutz

3. Keep the wall insulation intact, and out of the wind

The insulation between the aluminum structure and the cedar cladding is what holds heat through a long soak. Siting the tub out of direct wind helps in the same way.

4. Burn only dry firewood

Dry, seasoned hardwood burns hotter and cleaner. Wet wood spends its energy evaporating moisture, which means smoke, poor combustion and slower heating.

5. Fill with warm water where you can

Starting higher shortens heat-up and cuts fuel. Any hot source will do it.

Using both heat sources

If speed matters, the Hybrid carries the wood fired system and the electric heating and filtration system in the same tub, managed from the touchpad or an app.

Running both together shortens heat-up considerably, and it suits guest properties, residential installations, and anyone who wants the tub ready without tending a fire. A common approach is to hold a low baseline on electric, say 80°F, and bring it the rest of the way by fire.

One thing to know: keep the wood stove from taking the water past 106°F on a Hybrid, or the touchpad throws an error and the electrical system needs resetting.

On an Electric, heating from a cold fill takes up to fourteen hours at roughly three degrees Fahrenheit an hour, so it is designed to hold a temperature or run on a schedule rather than be heated on demand. There is no overnight drop to plan around, since the system holds whatever you set it to. More in the cedar tub that stays hot.

Why speed is not always the goal

It is entirely possible to heat a wood fired tub quickly. Speed is not the heart of it though.

Heating a wood fired tub does not happen in the background. Kindling catches, smoke rises briefly and clears as the fire finds its rhythm, the cedar carries its scent, and the water begins to move as heat transfers upward from the submerged firebox. The process is unhurried and completely visible. For people who choose wood over electric, that is usually the reason rather than the compromise.

Two to three hours is not long, but it is long enough to do something with. In the morning it is coffee and the kind of quiet that is rare otherwise. In the evening it is the slow approach to the end of the day, and by the time the tub is ready you are already in a different state of mind than the one you arrived with. It is the difference between instant coffee and a French press on a Sunday morning.

Explore the Wood Fired hot tub, or read the Ultimate Guide to Wood Fired Hot Tubs.