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Open. Open collectors are called collectors that work without additional pressure in the water circulation system. Usually, such a device is equipped with a tank in the upper part, into which a supply of water is poured; after that, the water by gravity flows to the tap. At first glance, open systems are not very convenient: they need to be placed higher (and the pressure will depend on the height difference between the collector and the water tap), while it is necessary to think over the way to fill the tank (bring a hose with a pump to it), and the purpose of such devices is limited domestic hot water supply and heating pools. On the other hand, such collectors are extremely simple, inexpensive, do not require a connection to the mains and can work even in the absence of electricity (as long as there is water in the tank).
Another design option is devices without a tank powered by a circulation pump. However, they are less common, mainly among models for heating pools (see "Suitable for").
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Closed. Closed collectors require high-pressure operation (about 5–6 bar) and are designed for direct integration into the DHW supply system. In this case, the indirect principle of operation is usually used for heating — the transfer of heat from the water in the collector to the water in the DHW system through a special heat exchanger.
Such devices are noticeably more complicated
...and expensive than open ones. At the same time, they are more versatile and efficient; they can be used for domestic hot water and heating. In addition, you can install the heater at any height. It does not affect the pressure in the system, unlike open design.
