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Figure 6-2 60 40 20 6-inch tube spacing 12-inch tube spacing Rff=0 Rff=0.5 Rff=1.0 Rff = finish flooring resistance 0 0 10 20 30 40 50 60(oF/hr/ft2/Btu) "Driving ∆T" (Tw-Tr) (oF) Average water temp. - room air temp. Figure 6-3 finish flooring subfloor 6" wide alum. plates 1/2" tubing, 8" spacing underside insulation Thus, in a room maintained at 70oF, the average water temperature in the circuit needs to be 87.5oF. The supply water temperature to the circuit would likely be in the range of 95–98oF. This is a relatively low supply water temperature that would allow an air-to-water heat pump to operate at high COPs. For comparison, consider supplying the same 20 Btu/hr/ft2 load using a heated floor slab with 12-inch tube spacing and a finish floor resistance of 1.0oF•hr•ft2/Btu. The driving ∆T must now be 42.5oF. The average circuit water temperature required to maintain a room temperature of 70oF would be 70 + 42.5 = 112.5oF and the supply temperature likely in the range of 120–123oF. This higher temperature would significantly reduce the heat pump’s COP. The following guidelines are suggested in applications where a heated floor slab will be used to deliver heat derived from a hydronic heat pump: • The tube spacing within the slab should not exceed 12 inches. • The slab should have a minimum of R-10 underside and edge insulation. • The tubing should be placed at approximately 1/2 the slab depth below the surface, as shown in Figure 6-1. This placement decreases the required water temperature required for a given rate of heat output relative to tubing placed at the bottom of the slab. Lower water temperatures improve heat pump performance. • Bare, painted or stained slab surfaces are ideal because the finish floor resistance is essentially zero. • Any finish flooring layers installed on the slab should have a combined total R-value of 1.0 or less. UNDERFLOOR TUBE & PLATE RADIANT PANELS Another possible radiant panel construction is called an underfloor tube & plate system. Figure 6-3 shows a typical cross section for this panel. This panel provides a way to heat wood- framed floors. It relies on aluminum heat transfer plates to diffuse heat away from the tubing and spread it across the floor area. These plates are critically important for good performance. To achieve compatibility with low water temperatures, tube spacing should not exceed 8 inches. The maximum allowed R-value of the subfloor and finish floor coverings should also be kept as low as possible. Figure 6-4 plots a coefficient called “k,” versus the R-value of finish floor covering installed over a 3/4-inch thick plywood subflooring and assumes 8-inch tube spacing. The heat output of this floor panel can be estimated using Formula 6-1. 47 upward heat output (Btu/hr/ft2)PDF Image | Heat Pump Systems 2020
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