Mixed results

The Glitch:

An installer is asked to connect a single speed geothermal water-to-water heat pump to a highly-zoned low temperature hydronic distribution system. The installer realizes that when a single speed heat pump is connected to such a distribution system, it is essential to use a buffer tank, so one is installed, as shown in Figure 1. 

The system uses a temperature setpoint controller to operate the heat pump so the buffer tank is maintained between 110° F and 130° F. It also uses a 4-way motorized mixing valve to control the supply water temperature to the distribution system based on outdoor reset control. At design load, the distribution system requires 125°. The heat pump is enabled to operate whenever any of the zone thermostats call for heat. The system has a 2 gallon expansion tank.

Can you identify several details that are wrong, and propose a better 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!