Which Wood-Fired Heater? 27kW Vs 35kW
Our leading European manufacturer of wood-fired hot tubs and heaters, conducted a practical side-by-side test of two popular models on a 1400-liter Quattro hot tub. The results show a clear performance difference.
The Two Heaters Compared
1. Aluminium Heater 27kW
Compact, more affordable, and simpler design. It lacks adjustable air supply control. Air enters through basic holes in the door. Suitable for smaller setups or budget-conscious buyers, but heats slower.
2. Aluminium Heater 35kW
Higher output and more efficient. Features adjustable air supply via a handle on the door — open for fast heating, close for maintaining temperature. Includes a double-wall partition for better heat transfer to the water. Ideal for larger tubs and precise control.
Test Conditions
- Hot tub: Quattro (1400 liters, fiberglass-lined, square model for up to 8 people)
- Starting temperature: 14°C
- Target temperature: 40°C
- Ambient temperature: +3°C to +5°C outdoors
- Fuel: Dry wood (14–18% humidity), preferably birch logs 5–10 cm thick, 30–50 cm long
- Other: Water mixed before each reading (to avoid layering — up to 5°C difference top to bottom). Insulated lid (50–70 mm thick) used during heating.
Heating Time Results
- 27kW heater: 2 hours 45 minutes (165 minutes) to reach 40°C
- 35kW heater: 1 hour 30 minutes (90 minutes) for the same +26°C rise
Key takeaway: The 35kW heater was approximately 80% faster — a huge difference, especially on cold days