We are testing 2 popular models of heaters for Hot Tub:

1. Aluminum heater 27kW

It is a more compact, cheaper and less efficient model.

It also lacks the possibility of regulating the air flow supply into the furnace combustion chamber. There is a cast iron ashpit grate on a steel frame in the furnace, and its upper part has a retractable steel partition providing a more effective fuel combustion. The furnace door is made of aluminum and has several holes in its bottom part to supply air into the furnace.

2. Aluminum heater 35 kW.

It is a more efficient and functional model. It is possible to control the air supply into the furnace, which makes it easier to control the water temperature in the barrel. The air supply is regulated by the handle at the bottom part of the door, this handle can be used to open or close holes, thus we can cut off or pump air into the furnace during the combustion. For example, it is easy to keep the constant water temperature in the barrel for a long time if to put some firewood and cut off the air supply into the furnace. The firewood will be slowly burning, but it will be enough for the barrel water to remain constant.

The upper part of the furnace has a 2-wall partition where water circulates, which significantly increases the furnace surface area, through which the heat from the burnt gases is transferred to the water.

Testing of the heaters for Hot Tub is performed outdoors at the same external temperature under the same meteorological and weather conditions. The external temperature is +3…+5 °C. As a test tank was used  wood-fired Hot Tub Quattro (1400 liter) , the manufacturer is Baltresto. It was important to use the same fuel, namely, dry wood of 14-18% humidity.  The temperature readings were taken at intervals of 15-30 minutes after mixing the water. The water must be mixed, as the water temperature difference at the bottom and the top of the bath can be up to 5 degrees. During the heating process baths were covered with an insulated lid of 50 – 70 mm thickness.

A few tips on how to heat a furnace properly and efficiently:

• use dry firewood from non-pitchy deciduous trees. The most suitable fuel is birch logs with the cross section of 5-10cm and the length of 30-50cm
• it is necessary to put logs so that air can flow freely from below and between logs, so they should be put at a slight angle to each other but not parallelly
• the furnace must be filled with wood up to 2/3 of its volume. And this volume should be kept while heating the water
• if you want to heat the water faster, you need to open air supply holes in the door.

It took 2 hours and 45 minutes to heat 1400 liters of water from 14 to 40 °C with the aluminum heater for Hot Tub 27 kW. The heater for Hot Tub  35 kW heated the same amount of water in 1 hour 30 minutes.

This means that heater for Hot  Tub 27 kW heated the water 80% slower than furnace 35kW.