Swimming pool heat exchangers, models more technically referred to as shell-and-tube types, are today produced by many different companies and generally have a similar construction, i.e., they consist of a bundle of tubes covered by a steel shell. How to connect them correctly - below you will find a step-by-step instruction.
Most of the advice contained in this post, which we can rightly call an instruction manual for connecting pool heat exchangers, is universal, meaning it can be applied to practically all brands of similar heat exchangers. However, we specify that the frame of reference will be Nordic Tec brand pool heat exchangers.
There are a number of contraindications for using plate heat exchangers in pools, which you can find extensive information about online. Therefore, we will not elaborate broadly on this topic, but it is worth remembering:
Therefore, we strongly recommend using dedicated pool heat exchangers, designed specifically for this application. Other types of heat exchangers should be used in the fields for which they were created, which will guarantee trouble-free operation. If you are interested in this topic, we invite you to read the following posts where we expanded on it:
This is also worth knowing before connecting the heat exchanger. If we plan to use salt for water treatment in any form - whether it's physiological salt, salt for antibacterial protection, salt for a water softener, or for a chlorine generator - we must abandon it if we plan to install a steel pool heat exchanger, or find other water treatment methods, e.g., using ready-made chlorine that does not require salt.
If we plan for saltwater or even an occasional addition of salt - we must use a titanium heat exchanger, which unfortunately is much more expensive than a steel one, due to the cost of the material itself. Titanium is one of the more expensive and less available materials.
This is the first question that should come to mind before installing pool heating. There are hydraulic devices that can only be mounted in specific positions. If it were a plate heat exchanger - it would need to be mounted vertically to avoid airlocking, which is related to its specific construction. It is different with the installation of pool heat exchangers.
✅ Pool heat exchangers can be mounted horizontally or vertically - it doesn't have much technical significance, airlocking will not occur in this case, so you can basically be guided by installation convenience. For ease, in the diagrams added to this post, we will use the vertical position, but you don't need to be guided by it.
When connecting pool heat exchangers, we have more convenience than with some devices - because the flow direction of the pool water or heating fluid does not matter much. In both directions, the channels of the pool heat exchanger will be equally permeable; there is no need to worry that they might create greater hydraulic resistance in one direction, etc. For this reason, Nordic Tec shell-and-tube heat exchangers - and probably most often those of other brands as well - do not have specific markings. The markings you will find on the diagrams below are merely conventional; you can apply them to the device in a more or less permanent way to make your work easier.
However, once we choose the connection direction for one channel, there is no longer freedom of choice for the second channel. According to the counter-flow principle, the fluid in the second channel must flow in the opposite direction to the first channel, not the same one - as described below.
Pool heat exchangers work better when the counter-flow principle is maintained, which is why the manufacturer does not recommend parallel-flow connection. The counter-flow principle refers to what we briefly described above - i.e., the media in both channels should flow in opposite directions relative to each other. If they flow in the same direction, the temperature gradient will be different, resulting in a much weaker effect. Therefore, you should refer to the diagram below.
K1-K4 - i.e., the channel flowing along the pool heat exchanger, through the thin tubes - has lower throughput than the remaining channel, K3-K2. Therefore, it should be chosen for the heating medium. Obviously, the specifics of pool heating suggest higher flow rates on the pool water side - so the choice is obvious, the K1-K4 channel should be used for the heating medium.
K3-K2 - this is the channel intended for pool water. Because we must maintain the counter-flow connection principle, the entry of cool pool water must occur at connection K3 - this way the pool water and the heating medium will flow in opposite directions relative to each other.
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Of course, there are different assumptions, different heat sources, different temperatures and flows on the supply side - so it's impossible to answer this question definitively. However, it should be noted that heating a pool is not the same as domestic hot water or house heating. The specifics of the flows are also different - and the flow of pool water in a properly executed installation is sometimes even several times greater than on the supply side. Therefore, the differences between the pool water inlet and outlet temperatures are not meant to be huge and impressive as in the case of central heating via radiators.
Pool water is meant to circulate through the heat exchanger for a longer period (i.e., circulate in the system), pool heating doesn't last an hour - and it really shouldn't worry us that in some cases the water only gains a few degrees Celsius more in the heat exchanger than at the inlet. We can reduce the pool water flow - in that case, a smaller water flow will achieve higher temperatures at the outlet. However, this is not at all advisable or necessary. Regardless of whether we gradually heat larger masses of water or heat more intensely - the total pool heating time and the result will be similar.
We are, of course, talking about a situation where the heat exchanger is correctly sized, but also where the heat source is correctly sized. We must remember that the heat source should also have adequate power. If it is, for example, an undersized heat pump, then heating the pool will be an ordeal and may not end successfully. It is also important not to use unrealistic assumptions, which involve expectations that the pool can be heated quickly by supplying relatively low temperatures on the "hot" side, such as 30 degrees Celsius. It is surprising that some people expect that if it's enough for heating underfloor systems (i.e., for example, 150 liters of water) then it should also be enough to heat 15 or 30,000 liters of water. This is a common mistake made by heat pump owners.
BLOG:
Undersizing of a heat exchanger - what does it mean in practice?
Oversizing the heat exchanger - what is it for?
What is the production process of brazed plate heat exchangers?
How to choose a heat exchanger for a gas boiler?
How to properly connect a Nordic Tec plate heat exchanger? Can I reverse the flow directions?
History of plate heat exchangers
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