Finding Better Performance

The Glitch

Here is a multitemp/multiload system with several types of heat emitters. The modulating/condensing boiler supplies space-heating loads through a buffer tank. It also supplies an indirect DHW tank as a priority load.

Exercise: As it exists, this system works relatively well, but it could be further refined. See if you can spot some design details that would provide equal or better performance while using less hardware.

The Fix

Let’s start on the boiler side of the buffer tank. Because the buffer tank has very low flow resistance, there’s no need for a separate circuit to hydraulically separate it from the boiler. Provided that the piping to the buffer tank is relatively short and generously sized the vast majority of head loss in the boiler circuit is the boiler itself. A circulator can be sized to handle this requirement and the buffer tank circulator can be eliminated.

Moving to the right side of the buffer tank we find a “primary loop” with its own circulator. However, the zone circulators are not connected as secondary circuits (e.g., they are not connected using closely spaced tees). This arrangement can cause heat migration in the parallel-connected zone circuits due to head loss around the piping loop whenever the primary loop is operating.

The only reason for this “primary circulator” is the set of closely spaced tees leading to the injection mixing assembly. One option would be to move the “primary circulator” adjacent to the closely spaced tees, equip it with a check valve and flow restrictor valve, and operate it only when the injection mixing assembly is active. However, there’s an even better solution shown in the “fix” schematic.

Because the buffer tank has very little head loss it acts like a pair of closely spaced tees. This allows the injection pump to be connected in parallel with the other zone circulators, and eliminates the need for the “primary circulator.” Keep the header piping short and generously sized to minimize head loss along its length. Also be sure the injection pump has an internal check valve since it’s now in parallel with the other circulators.

Another improvement is converting the diverter tee circuit supplying the panel radiators into a homerun distribution system. This allows all panels to receive the same water temperature and provides for flow balancing of each homerun circuit. It’s also far easier to purge than the diverter tee circuit.

Purging valves have also been added to the return side of the low temperature manifold subassembly to ensure good air removal.

Summary: Two circulators eliminated, reduced piping lengths, no heat migration in non-operational circuits, and better panel radiator performance. A little “tweaking” at the schematic stage pays off.