An experiment of heating water with Baltresto 35kW stove

An experiment of heating water with Baltresto 35kW stove

The speed of heating water with Baltresto 35kW stove

Wood burning stove 35kW of Baltresto Company designed for heating water in small tanks up to 3000 liters. The wood burning heater is made of salt-water-resistant aluminum Almg3 brand. Aluminum is the most suitable metal for wood-burning stoves intended for heating water, because the thermal conductivity of aluminum in 10 times greater than the thermal conductivity of stainless steel, this means that the wood-burning stoves made of aluminum sheet with a thickness of 3mm in 5 times more effectively transfers heat from the furnace to the water than stainless steel sheet with a thickness of 1.5 mm.

We conducted an experiment to test the efficiency and speed of water heating by an aluminum Wood burning stove 35kW.

For the experiment we used small dry pieces of spruce as fuel. It was prepared 2 bags of 40 liters (about 25 kg) of firewood. In the process of burning, firewood was constantly added to the optimal amount of fuel in the furnace to maintain the maximum temperature in the combustion zone. If you use the conventional firewood with higher humidity and do not constantly monitor the burning them in the furnace, the heating time will be slightly longer than in our experiment.


The volume of heated water was 1400 liters. We used Wood-fired Hot Tub “Quattro” with Fiberglass liner by Baltresto Company.

During the experiment, continuous shooting was conducted with a constant video recording of the water temperature in the wood-fired hot tub. The temperature was recorded using a float transmitter and a digital receiver with a clock.

To avoid large heat loss, the wood-fired hot tub was covered with a thermal insulation cover thickness of 10cm.

The water in the wood-fired hot tub is heated unevenly; it is warmer at the top by a few degrees than at the bottom. Therefore, to obtain the correct temperature, the water must be mixed by using a paddle or, with the help of air massage in our case.

During the experiment, the air temperature dropped from 13°C to 8°C in 2 hours.

Nothing was cut out of the video. The video was only accelerated in 50 times, which allowed to reduce the video from 2 hours to 8,5 minutes.


The first 5 minutes the water is not heated, because the first occurs heating the Wood-fired Stove and heating the water in it. Starting from 5th minute after the ignition of firewood, the water in the wood-fired hot tub begins to slowly warm up. By the 15th minute, the water warmed up from 7 to 10 degrees.

In the process of heating water, we observed a periodic decrease the water temperature by        1-2°C.  This happened after mixing the water using the aero massage, when warmer water on the surface mixed with cold water at the bottom of the wood-fired hot tub.

By the 30th minute the water was already 14°C

During the 1st hour of burning, 40 liters of firewood were consumed and the water warmed up to 24°C

By 1:30 the water has warmed up to 30°C

The most effective heating has been fixed from 1:30 to 1:40. For 10 minutes, the water is heated to 4°C. If you calculate the power of the Wood-fired stove based on these parameters, you’ll get:

*Q = (m*с*dT)/t = (1400*4,2*4)/600 = 39 kW

Q = Required cooling/heating power in kW

m = Weight of substance in kg

c = Specific heat (of water = 4,2)

dT = Specified temperature difference in °C

t = Heating time in *seconds

In 1:40 we put the last 80 liters of firewood in the furnace

By the end of the experiment, in 1 hour 49 minutes after the start of burning, we recorded a temperature of 36.3°C, which is optimal for taking a warm bath.

Water heating graph over time.

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