crushed stone with 4 in. of porous concrete as the working surface. Voids between the crushed stones are filled with water to provide the heat transfer medium. Warm water is circulated through the system to maintain the sump at the desired level. Figure 5–5 shows such a system. Pipe loops can be installed in the rock fill as a heat exchanger. Bench heating systems are similar to floor systems, except pipe loops require closer spacing. Three systems are available in small diameter tubes (approximately 0.2 in. I.D.) of EPDM elastomer that are used for both under floor and bench heating
systems. One is a single tube, another is a two- or four-tube design with tubes 3/4 in. on center. A third system is a multi-tube design with tubes 3/8 in. on center. Water temperature for bottom heat is generally in the 90–110°F range, depending on plant
requirements. Flow rate is set to produce an average
water velocity of 2–3 ft./sec. in the pipes to obtain
acceptable heat transfer without excessive friction
loss.
Electric resistance heaters are also available as
mats or grids. They can be used for bottom heat
with pots or flats set directly on them. Follow the
manufacturer’s directions for installation and use.