The Glitch:

An installer needs to connect a thermal storage tank that’s heated by a renewable energy heat source, along with a mod/con auxiliary boiler. Both will supply a common load.

When a heating load is present, and the load circulator turns on, the tank is to supply the load if the water at the aquastat is at or above 140°. In this case circulator (P1) needs to run and circulator (P2) and the auxiliary boiler should be off. If the temperature at the aquastat is less than or equal to 130°, the auxiliary boiler should be on, circulator (P1) should be off, and circulator (P2) should be on.

During a follow up site visit the installer notices that the temperature at the top of the tank is 145° F, (P1) is off, and the auxiliary boiler and circulator (P2) are supplying heat to the load. It’s not supposed to work like that.

Circulators (P1) and (P2) both have internal spring‐loaded check valves. Assuming that the aquastat is operating correctly, and that essential details such as an expansion tank, PRV, etc. are all present and correctly installed (they’re just not shown), why is the system behaving this way? Do you see any other details that should be changed?

Check out the full article here.