Active Closed Loop Solar Water Heating for
Swimming Pools and Spas by Twin Coil Tank

A Closed Loop system can be used to link domestic hot water (DHW), pool and spa heat to the same collector. It is normal to include domestic hot water in any multiple application as it is a year round requirement.
A special twin coil DHW tank can be used. This tank has a coil at the bottom of the tank connected to the Collector which heats the domestic hot water in the tank. The coil at the top of the tank is connected to the other application but only allowed to function when the tank temperature is above a set temperature, say 130F.
Linking with twin coil domestic hot water tank is an excellent way of storing solar heat for the hot tub to draw from, while still providing the domestic hot water.

1. Spa Pump

3. Globe Valve (Bypass) 5. Motorized Valve
2. Filter 4. Check Valve 6. Differential Controller

Operation

  • Setup: The globe valve (3) is slightly closed so that a small quantity of water will pass through the top coil when the water is directed that way. Do not close too much.
  • The Differential Controller will turn the motorized valve (5) to direct the flow to the top coil heat exchanger and bypass when the following conditions are met:
    • The temperature at the top coil sensor C is above the set temperature (say 120F) and
    • The temperature at the top coil sensor C is higher than the return temperature R by at least the pre-selected amount.
  • Some spa water will collect heat from the heat exchanger, (but most go through the bypass).
  • When either of the conditions in 2. are not met, the motorized valve will revert.
  • This will also happen if the return water has reached a preset temperature at T.

    Note:
    When plumbing a swimming pool or spa heat exchanger a bypass should be included. This is to avoid putting too much back-pressure on the pool/spa pump.

Linking Applications

There are two main methods of linking applications with a closed loop system:
  • Each application has a dedicated heat exchanger and the heat is directed using zone valves.
  • A special twin coil DHW tank can be used. This tank has a coil at the bottom of the tank connected to the  Collector which heats the domestic hot water in the tank. The coil at the top of the tank is connected to the other application but only allowed to function when the tank temperature is above a set temperature, say 130F.

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Thermo Technologies


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