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The Hot Tub Heating Effect

“How fast can you heat up the water?” This is one of the most common questions we are asked concerning hot tubs.

Raising the temperature with a certain number of degrees during a certain time requires energy (kWh). The formula for Energy = Effect (kWh) x Time (in hours). The theoretical energy needed to heat 1000 liters of water by 10 degrees in one hour is 12 kWh.

One important thing to consider when heating a hot tub is the construction of the heater. For example; there are large heaters on the market that require a lot of wood, and a large part of the energy produced goes up the chimney without heating the water. Then there are small heaters, which give the same energy output using a much smaller amount of wood. The output degree is much higher with a good construction, and instead of letting most of the heat escape via the chimney, the energy is used for heating the water.

In practice, wood-burning heaters come in 3 different main categories.

  1. Free-standing outer heater, with simple effective self-circulation and even heating.
  2. Outer heater with self-circulation attached to the wall of the hot tub.
  3. Traditional heater (old technology) placed inside the hot tub.

Below is an example of the 3 types of wood heaters on the market.

Heater, type 1 (Luxwel Wood Burning heaters)

This type also has self-circulation and the construction of the incoming and outgoing water system forces the water to heat up evenly. The differences in temperatures in different places of the hot tub are very small, and no stirring is needed.

Advantages:

  • Free-standing heater outside the hot tub with self-circulation
  • No risk of scalding yourself on hot water
  • The entire space in the hot tub can also be used when the heater is used
  • High energy output (high output degree) which means that little wood is used
  • The heating can start when there is 15 cm of water in the hot tub
  • Reaches working temperature rapidly, which prevents covering on the inside walls of the heater
  • Easy to move and connect, weighs only 26 kg

Disadvantages:

  • More expensive than inside heaters
  • If there is a risk of freezing, the water must be emptied from the heater

Blue colour represents colder water and red colour represents warmer water. The arrows show the theoretical circulation in and out from the heater and in the water.

The water is taken into the heater from the bottom and the warm water goes out under the bench through a special pipe. Simultaneously as the warmer water rises towards the surface, the colder water is pulled from the bottom. The effective circulation makes the temperature differences between warmer and colder water max. 6° C, so there is no risk of scolding. This circulation system was developed when the Luxwel Propane was CE-marked. The same system is also used for Luxwel Wood. Also look at the heating curve, which clearly shows how the heating works.


Heater, type 2 (competitor)

Self-circulation is based on the fact that warm water rises to the surface and creates a flow in the heater. When the incoming water and outgoing water are next to each other, the result is that the surface water and the water closest to the heater will be very warm. This provides an uneven temperature in different places in the hot tub. It is necessary to stir the water to get a uniform temperature.

Advantages:

  • The heater itself doesn’t take up space inside the hot tub

Disadvantages:

  • Relatively expensive construction
  • Since the heating takes place where the water comes in from the heater and goes out to the heater, it is practically impossible to sit there when the heater is being used
  • It is not possible to start heating before the water covers the heat-generating surfaces of the heater
  • If there is a risk of freezing, the water must be emptied from the heater

Blue colour represents colder water and red colour represents warmer water. The arrows show the theoretical circulation in and out from the heater and in the water.

Even though this heater has self-circulation, the construction and the bad-water circulation cause the hot water to rise rapidly towards the surface. It is practically impossible to sit near the place where the pipes for the incoming and outgoing water are positioned when the heater is being used. Temperature differences between warmer and colder layers can be over 50° C, and only stirring can change it.


Heater, type 3 (competitor)

The heat for the water comes from the walls and heater pipes. The warm water rises to the surface and the heat is limited to a relatively small surface. The water must be stirred in order to have a uniform temperature. At first when the water is cold it is also difficult to get the wood to burn properly.

Advantages:

  • Simple and cheap construction
  • No risk for freezing, since there is no water in the heater

Disadvantages:

  • Bad output degree
  • Coating on the inside of the heater, which is difficult to remove and weakens the output degree even more
  • Takes up a lot of space inside the hot tub
  • Difficult to heat and keep clean
  • Litter often falls into the water when filling up wood and cleaning
  • It is not possible to start heating before the water covers the heat generating surfaces of the heater

Blue colour represents colder water and red colour represents warmer water. The arrows show the theoretical circulation in and out from the heater and in the water.

The heat is concentrated around the heater and the warm water rises rapidly towards the surface. The temperature difference between the coldest and the warmest water is at least 40° C, so it is necessary to stir the water often.

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