Radiant panels don’t have to cover an entire floor, wall or ceiling.

Figure 1

Radiant panel heating has matured from the upstart of the hydronics industry in the 1990s into a respected technology that can provide excellent comfort in a wide range of applications.

Most of those reading this column have probably designed and/or installed several radiant panel systems. In many cases, your plans involve covering an entire floor area with some type of radiant panel construction detail: slab-on-grade, thin slab, tube and plate, etc.

In recent columns, we’ve discussed the fact that, as the design heating load per unit of floor area decreases, so does the average floor surface temperature. In very well-insulated houses, the average floor surface temperature may only be a few degrees above the room air temperature. The reason is the floordoesn’t needto get any warmer in order to satisfy the heating load as determined by the setting of the room’s thermostat. Forcing the floor to operate at higher temperatures would quickly overheat the space, and likely lead to energy waste in the form of open windows.

From the standpoint of thermal efficiency of the heat source, lower surface temperatures are not a problem. Heat sources such as condensing boilers, hydronic heat pumps and solar thermal subsystems all operate at high efficiency in combination with low water temperatures. The lower the water temperature, the higher their efficiency.

The “problem” is the owner’s reasonable expectation of warm floors will not be realized. And as many of you know, unfulfilled customer expectationsarea problem, even when the heating system is working at peak efficiency.

Formula 1

When Less Is More

There are several alternatives that provide a reasonable balance between heat source efficiency and the owner’s desire for warm surfaces. One is to make the surface area of the radiant panel smaller by not covering the entire floor area with tubing.

Imagine a room with a design heating load of 3,000 Btu/hr., and a corresponding indoor temperature of 70 degrees F. The room measures 20 feet by 30 feet. If the entire floor area was covered with radiant panel, the upward heat flux requirement at design load would be:

Formula 2

The average floor surface temperature can be estimated using Formula 2.

Tsurface = average floor surface temperature (degrees F)
q = upward heat flux (Btu/hr./ft2)
Tair = room air temperature (degrees F)

Thus, for the stated example:

This temperature is a few degreeslowerthan normal skin temperature for hands and feet. The infrared thermograph of a normal (thermally comfortable) hand in Figure 1 shows fingertip temperatures in the low- to mid-80s.

A floor surface at 75 degrees F will feel slightly cool to the touch of this hand, even though that floor is releasing sufficient heat to maintain the room at 70 degrees F.

Figure 2

Figure 2 shows an infrared thermograph of a radiantly heated floor. You can clearly see evidence of heat absorption from the hand in Figure 1 after it was pressed against the floor for a few seconds.

Keep in mind the 75-degree F average surface temperature exists on adesign day, when outside temperatures are at or close to their lowest values. This average surface temperature will be even lower under partial loading conditions.

The issue now becomes one of customer expectation. If the customer was informed that the floors would not feel warm, even though interior temperatures would still be maintained at a comfortable setpoint, and if they understood and agreed to this operating condition, there should not be any unfulfilled expectations. However, if the customer can’t get his brain past all those cozy barefoot advertisements for radiant floor heating and still expects warm floors regardless of load, the result is likely to be serious disappointment.

Figure 3a

The retort, “But I paid for warm floors…” will surely be heard and your prospects for a good customer relationship are headed south. The fact that the mod/con boiler you just installed is operating at 97 percent rather than 92 percent thermal efficiency is probably not going to smooth things over.

If the size of the radiant panel in this example was cut in half, the necessary upward heat flux would double from 10 to 20 Btu/hr./ft2. This would bring the average floor surface temperature on a design day from 75 degrees F up to 80 degrees F. Although still a tad low, such a temperature may appease those looking for barefoot-friendly floors. Reducing the panel area to one-third of the room’s floor area would boost the average floor surface temperature to about 85 degrees F, a recommended maximum for floors in which there is prolonged foot contact.

Figure 3b

Interestingly, the idea of not covering the entire floor area with tubing was common in the days when copper tubing was used for radiant floor heating installations. Each panel was sized to the room load assuming a specific upward heat flux. A room with half the load of another would get half as many square feet of panel area. Assuming floor coverings of comparable R-value, this approach allows the system to work with a single supply water temperature, and thus eliminates multiple mixing devices.

I used this approach when designing the floor heating system in my own house in 1979. Figure 3 shows a couple of images of the floor heating panel in our dining area. The panel was constructed using 3/8-inch copper tubing. We installed it under the eventual location of the table, right where feet rest on the floor. It feels great on a cold winter morning.

Formula 3

Other Options

Another approach is radiant wall or ceiling heating. Since occupants don’t rest their feet on these surfaces, neither is constrained to a maximum average surface temperature of 85 degrees F. Instead, a practical surface temperature limitation for a gypsum surface is about 120 degrees F. This is based on not causing long-term degradation of the joint compound.

The heat output of a radiant wall can be estimated using Formula 3.

Formula 4

The heat output of a radiant ceiling can be estimated using Formula 4.

The variables in both these formulas are as follows:

q = outward heat flux (Btu/hr./ft2)
Ts = average surface temperature (degrees F)
Tr = room air temperature (degrees F)

Formula 2 implies that a radiant wall with an average surface temperature of 100 degrees F, releasing heat into a 70-degree F room, would yield an output of 1.8 x (100-70) = 54 Btu/hr/ft2. A radiant ceiling operating under the same conditions would yield 1.6 x (100-70) = 48 Btu/hr.

These are both significantly higher heat fluxes than what is possible with floor heating when the latter is constrained to an average surface temperature of no more than 85 degrees F. At that surface temperature, the floor releases about 30 Btu/hr/ft2.

The numbers for radiant wall and ceiling panels imply that smaller panel areas are possible. Smaller panels mean less materials and lower installation costs, a definite plus.

Figure 4

Watch The Water Temperatures

If you’re designing radiant panels for well-insulated buildings, there’s a good chance that a high-efficiency hydronic heat source will also be used. The thermal efficiency of condensing boilers, solar thermal collectors and hydronic heat pumps are all dependent on the water temperature at which the heat distribution system operates. The lower, the better.

My suggestion is to select and size the heat emitter so that the supply water temperature doesn’t exceed 120 degrees F on a design day. This temperature is attainable by all the aforementioned heat sources, and allows a reasonable compromise between surface area, surface temperature and heat source efficiency.

Formula 5

The radiant wall construction shown in Figure 4 has proven to be a good performer when it comes to heat output vs. water temperature. Its output can be estimated using Formula 5.

Figure 5

Q = heat output of wall (Btu/hr./ft2)
Twater = average water temperature in wall panel (degrees F)
Troom = room air temperature (degrees F)

Figure 5 shows this type of radiant wall during operation. The upper image is what your eyes see. The lower image is what a FLIR thermographic camera sees. Notice how effectively the aluminum heat transfer plates are laterally dispersing heat away from the tubing.

Formula 6

If one uses the same construction shown in Figure 4 on a ceiling, the panel’s heat output can be estimated using Formula 6.

Q = heat output of wall (Btu/hr./ft2)
Twater = average water temperature in ceiling panel (degrees F)
Troom = room air temperature (degrees F)

It’s hard to overestimate the importance of warm surfaces in conveying physical as well as psychological comfort. This basic human need can be balanced with the desire for low energy use and high equipment efficiency by using smaller radiant panels. As you plan future systems for energy-efficient homes, be sure to have a conversation on these tradeoffs so customer expectations will be met and your company’s reputation will remain “radiant!”