In last month’s Heating with Renewable Energy column, we discussed piping options for multiple air-to-water heat pumps. The simplest approach — a 2-pipe header system — allows for staging and modulation of each heat pump, but it doesn’t allow some heat pumps to supply heating while others supply cooling. When simultaneous heating and cooling is required a 4-pipe header system with motorized valves allows any heat pump in the “plant” to be called into heating or cooling mode operation. The system in Figure 1 shows such a system.
