ZERO PRESSURE, MULTIPLE ZONE – PRIMARY SECONDARY PUMPING:
Another pump/piping strategy that can allow for a better control, smaller pumps and fewer design calculations is a “primary secondary pumping system” (refer to the drawing on the following page). This drawing details a single GARN® unit providing heat to two separate buildings, a home and a shop. Note the following: Pumps P1 and P3 circulate water from the GARN® WHS unit to a pair of closely spaced tees within each building and then back to the GARN® WHS unit. The two pumps are sized based upon the head loss of the underground piping and the manifolds at the GARN® WHS unit. The head loss for the piping within either building is NOT taken into account. This makes for simpler piping head loss calculations when interfacing with an existing system.
The underground piping and the GARN® manifold are considered the “primary piping loop.”
Pumps P2 and P4 simply circulate warm water (a mixture of cool system return water and hot supply water) to the heat delivery system in the building. The two pumps are sized based upon the piping and equipment head losses within the building without taking into account the head loss of the underground piping or the manifold at the GARN® WHS unit. This allows a good match between pumps P2 and P4 and the heat delivery equipment (air coil, hot water baseboard, radiant floor, or any combination thereof). In fact multiple small pumps may be used to split the building into independently controlled heating zones. Again, this makes for simpler piping head loss calculations when interfacing with an existing system because the existing pump generally does not have to be replaced as it experiences no net change in its resistance to flow.
The piping in the building is considered the “secondary piping loop.”
One could further increase the energy efficiency of this system by using variable speed pumps for P1 and P3. The speed of the pumps would be controlled by an optional temperature sensor or even an indoor-outdoor reset temperature controller. In this case, with the GARN® WHS unit hot (say 195°F) P1 and P2 would run slowly as only a small volume of hot GARN® WHS water would be required to warm the water within the secondary piping loop. When the GARN® WHS unit was cool (say 125°F) the pumps would provide a greater flow to warm the water within the secondary piping loop.
Some specifics about the closely spaced tees:
The tees should be no more than 6 pipe diameters apart . The tees should be located on the return side of any existing hot water heating system. Flow between the tees may reverse direction when the secondary system pumps (P2 and P4) are activated. The piping reducers are beyond the 12” of pipe and the two tees. Activation of P1 and P3 may be interlocked with P2 and P4 except when there is a possibility of the underground piping freezing.
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Hydronic System Design Manual ©DECTRA CORPORATION - March 2013
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