The operating temperature range of 70ºF to 90ºF is close enough to typical conditioned space temperatures that the water loop is usually not insulated and not subject to condensation.
Each heat pump branch contains a zone valve with a high Cv rating, or a motorized ball valve, that opens when the heat pump is operating. Each branch also contains a FlowCal™ balancing valve that automatically maintains a set flow rate through the heat pump when it operates. Most heat pump manufacturers specify flow rates of 2 to 3 gallons per minute per ton of heat pump heating capacity. Reinforced hoses are used to reduce vibration transfer between the heat pump and rigid piping. Each heat pump branch is also equipped with isolation and purging valves. These allow any heat pump to be isolated from the distribution loop and removed for servicing if necessary, without affecting operation of the other heat pumps.
Flow through the building loop is typically maintained 24/7. It’s provided by a duplex set of variable-speed pressure-regulated circulators that automatically adjust flow rate based on proportional differential pressure control. As more heat pumps turn on, flow in the loop is automatically increased, and vice versa. Only one of the two circulators operates at a time. The other serves as a backup. A controller automatically determines which circulator is operating and attempts to creates approximately equal elapsed run time for each circulator.
A DiscalDirtMag® separator provides high-efficiency air, dirt and magnetic particle separation for the system.
Some water loop heat pump systems use closed-loop cooling towers. The same fluid that passes through the water loop also passes through closed piping paths within the cooling tower. There is no contact between this water and the sump water used to enhance the tower’s evaporative heat dissipation. This is the type of cooling tower shown in Figure 8-3.
If the building loop system operates with all water, the cooling tower must be protected from freezing in winter. In some systems, the tower is isolated from the balance of the system and drained during winter. In other systems, a small flow of water from the loop is maintained through the tower to keep it from freezing. The modulating 3-way motorized valve shown in Figure 8-3 can provide this flow. The latter approach allows the tower to come online quickly during an abnormally warm day in winter.
Other systems use open cooling towers. In these systems, the tower is typically separated from the closed/pressurized building water loop by a plate & frame heat exchanger. That heat exchanger should be sized for a small approach temperature difference in the range of 2 to 4ºF. This helps keep the loop temperature down during cooling mode operation on hot summer days, and thus minimizes the drop in heat pump cooling performance.
In most systems, the cooling tower is located outside the building. However, it is possible to use a cooling tower mounted inside the building. Outside air enters the tower through a large louvered panel in an exterior wall. After passing through the tower, this air is discharged back outside through another larger louvered panel. When this type of tower is used, the discharge air should be directed away from nearby windows, doors, parked vehicles or areas of outdoor gatherings. This prevents water droplets in the discharge air, or residual water treatment chemicals in the sump water, from being discharged where they could cause adverse effects.