This almost got built

The Glitch:

A recent inquiry involved a heating system for a school in which two electric boilers were being added to supplement the output of a gas-fired cast-iron boiler, and take advantage of low “off-peak” electrical rates. The load was four zones of low water temperature floor heating. The designer planned to operate the gas-fired boiler at temperatures high enough to avoid sustained flue gas condensation and to install a “bypass circulator” as another means of boosting boiler inlet temperature. A 3-way motorized mixing valve would be used to reduce the supply water temperature in each of the four zones. The proposed schematic for the modified system is represented (very closely) in Figure 1. Can you spot several details that would be better handled through an alternate design?

Are you an ace troubleshooter?

Within the pages of this magazine, PM’s Hydronics Editor John Siegenthaler, P.E., will pose a question to you, our readers, to review a system’s schematic layout and discover its faults, flaws and defects. Discover archived “The Glitch & The Fix” exercises at its radiant-focused website, www.radiantandhydronics.com. Good luck!