In this configuration the storage tank employs a double heat exchanger, providing solar heat for the Radiant 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.
The space heating pump runs when the 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.
This is only an example. Actual layouts vary from job to job.