Active Closed Loop Solar Water Heating for
Radiant Underfloor Space Heating using Twin Coil Tank


A Closed Loop system can be used to link domestic hot water (DHW) and space heating to the same collector. It is normal to include domestic hot water in any multiple application as it is a year round requirement.


NOTE: The information presented in this page is for guidance only - no part of this may be used for any agreement, whether express or implied, or to form any contract. THERMO TECHNOLOGIES reserves the right to change specifications and prices without prior notice.

In multiple applications such as this, a special twin coil DHW tank is often used. This tank has a coil at the bottom of the tank connected to the VM 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 130ºF.
In this configuration the domestic hot water tank becomes a double heat exchanger, providing solar heat for the Radiant Underfloor System, as well as solar Domestic Hot Water.


  • The Solar Loop operates normally with a Differential Controller to heat the Twin Coil Tank through the bottom heat exchange coil.
  • In the Radiant Underfloor Loop the pump activates through the zone valves when either Room Thermostat calls for heat.
  • This loop flows through the top coil in the tank. If the upper tank is below, say, 130ºF boiler controller redirects the flow through the boiler, activating the boiler at the same time. This preserves hot water in the tank for domestic use.
  • The bypass, incorporating a Mixing Valve and check valve, ensures that the floor system cannot overheat by mechanically recirculating it when a set temperature is reached.
  • Normally the set temperature of the immersion heater (just below the top coil in the tank) is set far enough below the boiler set temperature to ensure that the radiant system only takes the solar heat from the tank. In emergencies the immersion heater could be used to supplement the floor heat by raising the immersion heater set point.


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