The hydronic system shown above was created to connect three gas-fired, cast-iron boilers to a low-temperature distribution system that supplies 12,000 ft. of 3/4 in. PEX tubing embedded in the floor slab. A boiler staging controller is used to fire each boiler as needed. The circulator on each boiler is controlled by a standard temperature limit controller. Water temperature to the radiant floor circuits is regulated by a variable-speed injection pump.