| Session | Energy required (BTU) | Wood required (lbs) | Heating time (hours) |
|---|
Why wood-fired hot tubs need careful planning
Wood-fired hot tubs tap into a primal ritual—hauling firewood, stoking the stove, and watching wisps of steam rise as the water warms. Because there is no plug-and-play thermostat, owners must choreograph fuel, time, and safety. A tub full of cold well water might take hours to reach a comfortable soak. Overshooting the temperature wastes wood and forces you to dilute with cold water. Underestimating fuel means cutting the soak short. This planner crunches the numbers so each session feels restorative rather than frantic.
The physics hinge on specific heat. Water requires one BTU per pound to raise its temperature by one degree Fahrenheit. Multiply the volume by 8.34 pounds per gallon to determine the mass, then multiply by the temperature rise. That gives the theoretical energy demand. Real stoves lose heat to the air, the chimney, and the tub shell, so we divide by efficiency to capture the wood energy required. Finally, we divide by firewood BTU per pound to convert energy into fuel weight and by the burn rate to find the hours of active firing.
Expressed in MathML, the energy balance looks like this:
, where is gallons, is final temperature, is initial temperature, and is stove efficiency as a decimal. Divide by firewood BTU per pound to get the pounds of wood required.
Worked example: cedar barrel in a mountain cabin
A family installs a 450-gallon cedar barrel tub beside their cabin. Spring water flows from a nearby cistern at 55°F. They aim for a 103°F soak. The tub uses a stainless submersible stove rated at 45% efficiency when fired with seasoned hardwood. Their woodpile includes locally split oak averaging 7,200 BTU per pound. The stove comfortably burns about 7.5 pounds of wood per hour. Firewood costs $325 per cord delivered, each cord weighing 3,800 pounds. They host six soaking sessions per month during shoulder seasons.
The calculator reports that each session requires roughly 180,000 BTU, translating to about 34.7 pounds of wood. At the chosen burn rate, the warm-up takes 4.6 hours. Monthly consumption totals 208 pounds of wood, or 0.055 cords, costing about $17.69. The CSV export lists energy demand, wood weight, and time per session, giving the family a quick reference when scheduling guests. Seeing that warm-up takes nearly five hours encourages them to light the stove by mid-afternoon if they want an evening soak.
Comparison table: wood species
Different firewood species produce different heat. Use the table as a starting point when purchasing cords or harvesting your own.
| Species | BTU/lb | Notes |
|---|---|---|
| White oak | 7,300 | Dense, long burn; ideal for overnight soaks. |
| Maple | 6,900 | Stable coals, easy splitting; good shoulder-season fuel. |
| Douglas fir | 6,300 | Common in the Northwest; moderate heat, pleasant aroma. |
| Alder | 5,900 | Lighter wood; burns fast, useful for quick temperature boosts. |
Strategic tips for efficient soaks
Season firewood to below 20% moisture to avoid wasting energy boiling water out of logs. Stack cords under cover with airflow. Preheat make-up water in black hoses or solar barrels to shrink the required temperature rise. Use an insulated cover immediately after filling the tub to trap heat during firing. Stir water periodically with a paddle to even out stratification—otherwise the surface may feel ready while deeper water remains cool.
Wood-fired tubs thrive on rhythm. Keep a logbook of start times, ambient temperature, wood species, and the moment you hit target temperature. After a few sessions you will develop intuition: perhaps 35 pounds of oak is perfect on a calm evening, but windy nights demand 40 pounds. Pair the planner with a moisture meter to ensure your firewood is ready. If the tub is used infrequently, schedule a deep clean and ash removal after each firing session to protect the stove body.
Budgeting the ritual
Most owners think of wood-fired tubs as “free heat” because the stove runs on forest fuel. In reality, cords cost money, even if you cut your own (chainsaw fuel, time, permits). The calculator converts pounds into cord fractions so you can assign a realistic dollar value. The monthly cost might be modest compared to electricity or propane, but it still deserves a line in the household budget. For off-grid retreats, accurate consumption forecasts prevent mid-winter shortages. The CSV export doubles as a procurement checklist—note when you hit half a cord and reorder before peak season.
Limitations and assumptions
The model assumes the tub is filled with fresh water for each session. Many owners reuse water for several soaks, losing only a few degrees between sessions. In that case, reduce the temperature rise accordingly. Heat loss from wind and ambient air is not explicitly modeled; expect longer warm-ups on frigid nights. Stove efficiency is highly variable—clean chimneys, full combustion, and submerged stove walls all boost performance. If you use a snorkel stove sitting outside the tub, efficiency may drop, requiring more wood.
Firewood BTU values assume seasoned wood at 20% moisture. Green wood can cut output by 30% or more. Likewise, altitude affects boiling point and combustion; at 8,000 feet the stove may draft differently. The burn rate input should reflect an average across the session. During startup you may load more wood; once the tub nears target temperature, you can taper the fire. Despite these simplifications, the planner captures the essential energy balance so you can prepare properly, entertain guests confidently, and respect fire safety.