An external wood-fired hot-tub stove is a water circuit wrapped around a fire. Cooler water must reach the heater and warmer water must return to the tub without a closed valve, undersized fitting, sagging hose or trapped air pocket. The safe geometry comes from the drawing for the exact tub and stove, not from a photograph of somebody else’s installation.
Understand the loop before selecting fittings
Most external-stove systems use a lower connection to carry cooler water towards the heater and an upper connection to return warmed water. Natural circulation—often called thermosiphon flow—depends on density difference and an unobstructed height relationship. Pumped systems add a circulation pump and controls, but they do not make pipe layout irrelevant; the heater may still retain heat if power or flow stops.
Mark the ports on the approved drawing and on the installed equipment. Reversing them, reducing diameter or creating a high point can leave part of the water jacket without reliable circulation. Do not fit a valve merely because it makes servicing convenient unless the design shows how accidental isolation is prevented.
| Connection detail | Why it matters | Evidence at handover |
|---|---|---|
| Correct lower and upper ports | Supports the approved direction of circulation | Labelled drawing and photographs |
| Full-bore pipe and fittings | Restrictions increase resistance and local overheating risk | Component schedule with internal diameters |
| Continuous supported route | Prevents sagging, air traps and movement at hot unions | Visible route before decking closes |
| Drain and service access | Allows frost protection, inspection and replacement | Marked drain points and removable panels |
Distance and slope do not have one safe universal number
A short direct route is generally easier to circulate than a long route with bends, but acceptable distance depends on port height, pipe diameter, heater resistance and whether the system is passive or pumped. Moving the stove to the far side of a terrace or dropping it below the approved level changes the hydraulic problem. Ask the supplier to approve the finished arrangement in writing.
Likewise, “slope the hoses” is incomplete advice. Each run must follow the model drawing and remain supported when warm and full. Flexible hose that looks straight while empty may sag into an air- or water-trapping loop. Keep hot pipework away from sharp edges and use only material rated for the temperature and water treatment.
Build the safe site around both tub and firebox
The heater needs a stable non-combustible base, loading access and the clearances specified in its manual. The flue must be the approved system, supported and positioned so combustion gases discharge safely. OFTEC notes that solid-fuel appliances require a compatible chimney or flue and suitable combustion air; dry fuel is important for efficient burning and reduced deposits.
Do not hide the stove inside a timber enclosure for appearance. Keep people away from hot surfaces and the flue, protect the route used to add fuel and prevent log storage, covers or furniture from entering the clearance zone.
A commissioning sequence that finds faults while they are still cold
- Complete and level the permanent bases; put the tub and heater in their final positions.
- Match every supplied port, union and hose to the approved drawing.
- Support the route and leave unions, valves and drain points accessible.
- Fill the tub slowly to the required operating level with the entire circulation path open.
- Vent trapped air using the documented method; prove pump flow where a pump is part of the design.
- Inspect all joints with cold water and record the result.
- Use a small controlled first fire and observe return temperature, circulation, joints and flue behaviour throughout the heat-up.
Common layout errors
- A hidden valve can close the water path while the stove still contains fire.
- A convenient reducer or decorative narrow pipe defeats the approved full-bore route.
- An unplanned high point traps air in the upper run.
- Permanent decking prevents a leaking union or drain from being reached.
- The heater cannot drain completely before freezing weather.
- The flue is altered to clear a roof, fence or cover without manufacturer approval.
Operating and seasonal checks
Before every firing, confirm water level, open flow path and absence of visible leaks. Keep the return area unobstructed and monitor temperature throughout. Burn the approved dry fuel and reduce input before the target because a solid-fuel fire cannot switch off instantly.
For winter shutdown, drain the heater and every low point exactly as shown in the instructions. For continued use, the freeze plan must protect both exposed lines and the heater during power loss or an unattended cold period. Keep the final drawing, manual, labelled valve positions and first-heat observations with the tub records.
To choose between internal, integrated and external positions before the base is built, use the hot-tub heater selection guide.