Different heaters





"How fast can you heat up the water?" This is one of the most common questions we are asked concerning hot tubs. Two other common questions deal with the number of people who can sit in our hot tubs and how long the same water can be used.

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. Accordingly, the effect needed is then 12 kWh.

The effect for a wood-burning heater, for example, is known as the nominal effect, which is the theoretical maximum effect. However, the real effect can be much longer in practice (as a comparison we can mention the effect of a wind turbine, which is also given as the nominal effect. This could be 1 MW, for example, although the average effect can be 0.2 MW and even 0 MW when there is not enough wind).

One important thing to consider when making use of the effect 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 very much. A large nominal effect but bad energy use is usually known as a bad output degree. Then there are small heaters, which give the same energy with 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.



Heater, type 1 (SpaDealers Vilpra heaters)


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


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 Chofu Propane was CE-marked. The same system is also used for Vilpra Wood. Also look at the heating curve, which clearly shows how the heating works. Ask if our competitors have anything similar!



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.
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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.
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Disadvantages:


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.