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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.

Heating time for a 1100 litre hot tub using Luxwel Wood Heater

The chart below shows how the heating up of our Classic hot tub works while using a Luxwel Wood heater. To warm up the water from 6°C to 37°C takes around 3 hours. The chart also demonstrates that water, which is rising towards the surface is warmer than the water on the bottom of the hot tub.

This process is what we call “the thermosiphonsprinciple”. Warm water rises towards the surface whereas cold water descends towards the bottom. In using this you obtain an effective self-circulation provided that the pipes, which go in and out of the hot tub are placed in the right position.

The Luxwel Wood Burner has got a very high efficiency compared to other heaters which are placed on the inside of the hot tub, where the heat evaporates directly via the chimney pipe. Another problem with those types of heaters is that the temperature is concentrated around the heater itself. The Luxwel hot tubs that including the Luxwel Wood Burning heaters, which are placed on the outside of the hot tub have been specially designed to heat up water quickly and equally.

The metered measurements have been registered by a 4-channel HOBO logger and then compiled graphically in Excel. Each of the 4 lines represents its own measuring point. The red line shows the temperature of one of the spots, where warm water flows into the hot tub. Here it is also possible to see the burn-up effect (difference between the top and the bottom of the curve) and the fact that it is really important to fill up the heater with wood constantly. This and the quality of wood show the result of the total time for the heating up.

Luxwel Burner Chart

The green line shows the temperature of the second spot where warm water flows out.

The blue line demonstrates the temperature of 20 cm beneath the water surface.

The temperature on the bottom of the hot tub is indicated by the yellow line.

Graph Points

Hot Tub Water Hygiene

Remember that it is your responsibility to ensure that the water used in your Luxwel hot tub is clean and safe. Water that looks clean and fresh can contain bacteria as the filtration is only supposed to remove particles; it has no influence on preventing the growth of bacteria or removing them. Therefore, always use cleansing products in the water where you are going to bathe. This is regardless of the material the hot tub is made of.

Please be aware that a person who has not washed before entering a hot tub will pollute the water 200 times more than somebody who has taken a shower beforehand. Sun lotion, skincare products, soap should not get into the water and that’s why we suggest you always take a quick shower before enjoying a bath. If the climate permits you can even have a shower area outside next to your hot tub.

Bathing in a traditional hot tub is very relaxing, even if it requires some work with planning and heating. Unfortunately, many people forget that bacteria also can be found in “clean” water, bathing temperatures are ideal for bacteria to multiply. An effective and easy way to provide protection from bacteria is to use treatments, for example chlorine. Different brands can be obtained, but the important thing is to use products for the correct water temperature. Cleansing products used in swimming pools are not suitable for warmer water and vice versa. If you don’t have filtration, we recommend that the water is used only once. However, it is not a health risk to use the same water many times if a suitable amount of chlorine is added.

You must always ensure you use the correct amount of chlorine. If the chlorine levels are too high then the PH-value will be increased and has a strong smell. In the worst case it can even peel off the sealing compound (if your tub has such). The chlorine levels should not exceed 1.5 ppm and pH 7.2-7.6. For bromine the level should not exceed 2.0 ppm and the pH should be 7.2-7.8. If automatic dispensers (floating, etc.) are used, careful adjustment should be made to avoid incorrect dosage.

Cleaning the hot tub

After using the water in a hot tub ideally it should always be drained out. Luxwel  hot tubs have a drainage system in the lower pipe, where also the water from the heater is drained. Once draining is complete the hot tub should be rinsed with clean fresh water. If the water has been in the hot tub for a long time, the hot tub must be scrubbed with pine soap for example. Remember also to clean under the benches. If you have a wood-burning heater from Luxwel, the burner must always be drained.

In you don’t wish to use chemicals in conjunction with filtration then the water should be renewed often. It is always best to empty the tub after each use and to rinse it thoroughly. Should you leave the water in the tub in order to keep it leak-proof then it can lead to an accumulation of bacteria.

Filtration

An easy way to always have clean and warm water whenever you wish, is to use our filtration system with connections which have been developed according to the Clean water concept. The filter is used to filter away particles and deposits and in order to obtain an effective filtration, the water is sucked both from the base and surface through the filter and is pumped back at the base on the opposite side from where the surface water is sucked in. This way all the water will pass through the filter which is very important. Ensure that the filtration system you use really does what it promises. You can be confident that the system offered by Luxwel is superior and guarantees clean water.

The water is sucked out through the base suction and the skimmer before it passes through the filter. The dirt will stick there and clean water is sucked in through the pump and pressed back to the hot tub where it is mixed and cleaned again as long as the circulation pump is working. All water passes through the filter many times per hour and is cleaned effectively.

Five easy tips for maintaining good water hygiene if you have got a filtration system.
1. Take a shower before you get into the hot tub.
2. Add some bactericidal solution before and after taking a bath.
3. Control the ph-value and balance it if necessary.
4. Oxygenate your water on a weekly basis.
5. Clean the filter cartridge frequently and let the filtration system run sufficiently.

Ph-Value- it affects water quality. It should be around 7.2-7.6 and can be measured with a test stick. If necessary please balance the water by increasing or reducing Ph value.

Good to know! Is the water cloudy, it is always a result of bad filtration. Either the filtration periods have been too short and therefore could not filter the impurities.

Filter cartridge: The task of the filter cartridge is the removal of soil, sand, minerals, phosphate and dirt from bathing persons from the bath water.

Cleaning of the cartridge: The filter should be cleaned and rinsed with high water pressure, in addition the cartridge also needs to be cleaned thoroughly in depth. Even if it does not look dirty, it should still be cleaned on a regular basis. Minerals and other impurities can get stuck deep in the filter lamella and that can lead to problems with the flow. It is always recommended to have an additional filter. A filter needs to pass a cleaning- and drying process while the other one gets installed. The lifespan and efficiency of the filter can be doubled through the use of thorough and regular cleaning.

AquaClarus Hot Tub Filtration system

AquaClarus

A question we often is receive is ‘How often should I change the water in my hot tub?’. Our response is that the same water should not be used multiple times unless it is correctly treated. It is possible to get rid of bacteria with chemicals such as chlorine, but that does not make the water clean and you can often find that the water looks quite uninviting, being  cloudy and greyish in appearance.

To remove such pollution, filtering is required. Traditional filtering requires connections in a hot tub for incoming and outgoing water. At freezing temperature, there is the risk of damage if the filtration equipment is not protected correctly with insulation.

Now there is an easier way to keep the water in your Luxwel hot tub clean. AquaClarus is a filtration system that hangs over the edge of the hot tub, and it fits all hot tubs on the market. Depending on the quality of the water it should run between ½ -2 hours. Impurities trapped in the filter can be easily removed and cleaned and there is also a changeable filter. The AquaClarus filter system is suitable for hot tubs and small pools up to 10m³.

We also recommend that chemicals are used in the hot tubs to maintain  the pH value between 7.2-7.6.

There are currently two versions of the system available, the AquaClarus 2000 or AquaClarus 4000.

The filter is adjustable vertically and the water is suctioned from the surface, at a rate of up to 4000 l/hour, and is then passed through a “pre-filter” that catches larger particles. The water then passes through a removable filter that can be cleaned after use. At the bottom of the filter the purified water is pushed out and creates a circulation in the water so that impurities from the bottom are also lifted up to the surface and passed through the filter.

This filtration system also works well as a “water stirrer” and is ideal for hot tubs with heaters that lack a circulation system. Power is supplied from a standard plug socket.

Note! The pump in AquaClarus 2000 gives 50% lower flow (2000l/hour) in comparison with AquaClarus 4000 where the amount of water that passes through the filter is 4000l/hour. AquaClarus 4000 filters significantly more water.

AquaClarus can also be combined with a 3 kW thermostatically-controlled electrical heater, connection and hole for water circulation. It is a simple but well-working solution that can be used for filtration and combined with electrical heating during all the seasons. The heater also has an insulated module that is fastened on the side of the Hot Tub.

The images below demonstrate this.

AquaClarus and 3kW heater

Below is a short video of the AquaClarus in action