The Glitch:

An installer pipes up the system shown in Figure 1. Its purpose is to provide two zones of space heating.  The heat emitters are fan-coils with a relatively high pressure drop characteristic. The system also provided domestic hot water using an indirect tank. The boiler loop is intended to keep the boiler operating above the dewpoint of the flue gases and protect it from thermal shock. Can you identify some problems that are likely with this set up, and propose an alternative design to eliminate any such problems?

Figure 1:

The Fix:  

To create reasonable flow rates through the fan-coils, it’s necessary to generate a differential pressure between the supply and return pipes. The only differential pressure that will be created in this system is that due to head loss around the “primary loop” from where the supply pipes for each zone connect to where the return pipes for each zone connect. Assuming that the loop is relatively short, and the piping is sized to limit flow velocity to 4 feet per second, it’s very unlikely that sufficient differential pressure will be created around the right side of the “primary loop.”  This will cause weak flow in the heating zones, and probably lead to complaints of inadequate heating.