The key to low temperature operation is large emitter surface area. The greater the heated surface area, the lower the required water temperature for a given rate of heat output. By embedding tubing in floors, walls, and ceilings, it’s possible to create very large heated surfaces within a room, called hydronic radiant panels. Radiant floor heating is the best-known form, and can be installed in several ways that allow it to operate at relatively low water temperatures.
For a heated slab-on-grade floor, the tubing should be placed about mid-depth within the slab, and the underside and edge of the slab should be well-insulated. These details are crucial for low-temperature performance. It’s also important to use low-resistance (or even no-resistance) floor coverings. A painted, stained, or stamped concrete slab surface is ideal. If that doesn’t suit your tastes, consider ceramic tile or vinyl flooring. If you must have carpet, it’s best to use only 1/4-inch-thick commercial-grade loop carpet, and glue it directly to the slab. Covering a radiant slab with a pad and carpet will insulate your living space from the slab’s heat.
The “Radiant Heat Output” graph can help guide tube spacing and floor covering selections. Assuming you’re satisfied with a 70°F indoor air temperature, this graph gives you the required average water temperature in the floor tubing based on tube spacing, the R-value of the floor covering (if any), and the rate at which heat must leave the floor under maximum heating conditions (in Btu/hr./ft.2).
For example, assume the room’s maximum heating load (which is calculated manually or by using one of several available software packages) divided by the heated floor area is 15 Btu/hr./ft.2. You’ve decided to space the tubing at 6 inches, and finish the concrete slab with a sealed stain (RFF = 0). The required average water temperature in the embedded tubing is only 83°F. It’s common to design floor circuits with overall temperature drops of 16°F to 20°F. Thus, the supply water temperature is half this overall temperature drop (8°F to 10°F) above the average water temperature. The temperature drop along the circuit is due to the heat being released from the water flowing through the circuit. Under these assumptions, the slab needs to be supplied with 93°F water to cover the maximum heating load. The supply water temperature can be lower under partial load conditions.
Such temperatures give excellent performance from a renewable energy heat source. Although the floor surface temperature will likely only be in the low- to mid-70s, that’s as hot as the floor needs to be to release all the heat the room needs. I always stress this point so that occupants understand that a “toasty warm” floor implied by some advertisements simply doesn’t occur in low heating load conditions, and in systems optimized for low water temperature operation.
The graph only reflects a maximum finish flooring resistance of 1.5 and a maximum average water temperature of 110°F (with the assumption of a maximum supply water temperature of 120°F). If you’re serious about building a good performing system, don’t exceed these limits. Here are some more suggestions for a heated slab floor with a renewable energy heat source: