Heating, Cooling and HVAC

So What’s Wrong with Radiant-Floor Heating?
In the right application, radiant-floor heating is a superb heat-delivery system—in fact, perhaps the very best. You usually pay more for it, but the enhanced comfort, potential energy savings, and other benefits can easily justify the extra cost. That said, however, super-energy-efficient green buildings may not be as well-suited to radiant-floor heating. Here’s why:

Economics
It can be reasonably argued that a green home in a moderate-to-cold climate should have very high levels of insulation (at least R-25 walls and R-40 ceiling/roof), extremely low infiltration rates, high-performance glazings (unit U-factors below 0.3), and at least some passive solar gain or suntempering.

We’re not talking about conventional houses, mind you, but high-performance green homes. Such a house will use very little heating energy—probably less than 2.0 Btu/ft2 · degree-day (41 kJ/m2 · °C), which would translate into very low heating costs. To achieve that level of energy performance requires a significant investment in the building envelope (for example, double 2×4 walls). In such a house, putting in an expensive heating system doesn’t make good economic sense. As Rosenbaum notes, “It just doesn’t make sense to put in a $10,000 heating system to provide $100 worth of heat per year.”

Investing so much money in the building envelope and still putting in an expensive radiant-floor heating system eliminates the potential for offsetting much of the extra cost in building envelope improvements through savings in the mechanical equipment—one of the key principles of integrated, whole-systems building design. In most highly energy-efficient houses, the same high level of comfort provided by a radiant-floor heating should be achievable simply by installing one or two small, quiet, high-efficiency through-the-wall gas heaters (such as those produced by Rinnai) or a few short sections of electric baseboard heat. At $1,000 to $2,000 apiece for Rinnai heaters (installed) or a few hundred dollars for electric baseboard vs. $10,000 for a typical radiant-floor heating system, savings of $6,000 to over $9,000 would be possible—and that savings could pay for most of the envelope improvements required to bring the heating load so far down that space heating (instead of distributed heat) becomes a viable option.

Even Larry Drake, a strong proponent of radiant-floor heating systems as executive director of the Radiant Panel Association in Loveland, Colorado, admits that radiant heat is more difficult to justify in high-performance buildings. “The tighter the envelope, the less the amount of savings of a radiant system,” he told EBN.

Heating performance with micro-loads
Along with the economic questions about the wisdom of radiant-floor heating systems for high-performance green homes, there are building science reasons why this may not be a great fit. Heat is transferred from an exposed slab to the space at a rate of about 2 Btu/ft2 · hr · °F (11 w/m2 · °C), according to Rosenbaum. In a well-insulated house, this rate of heatflow means that even when it is very cold outside, the slab can only be a few degrees warmer than the rest of the room or the room will keep heating up. For a concrete slab to feel warm, however, it needs to be about 80°F (27°C). Thus, for most of the heating season, the greatest feature of radiant-floor heat—a warm floor—won’t occur. With moderate solar gain, heat delivery from a floor slab will be even less. Because the floor is insulated underneath, it will be more comfortable to walk on than most slab floors, but the benefit will be from the insulation, not the radiant heat.

The time lag of heat movement through concrete can also be a problem. In a very well-insulated house, that lag time can result in overheating, particularly if there are other sources of heat being delivered to the space, such as passive solar. If a concrete slab is “charged” with heat during the early morning hours and the surface is warmed to the point where it cannot readily absorb solar radiation striking it, that solar heat will more directly heat the air, increasing the risk of overheating. The same thing happens to a much greater extent in high-performance passive solar homes with masonry heaters because the surface of an operating masonry heater is at a higher temperature. In such houses, occupants usually need to check weather forecasts—if they load up the masonry heater firebox in the morning and it turns out to be a bright, sunny day, the space will very likely overheat. A radiant floor maintains a much lower surface temperature than a masonry heater, so the floor will effectively “turn off” as the room warms up with solar gain. “If the floor temperature is 76°F,” says Rosenbaum, “then the radiant system can’t heat the place to hotter than that.” Therefore, this isn’t a huge problem with radiant-floor heating systems, but it may mean that homeowners will have to open windows periodically in the winter and their overall energy savings from solar energy will not be as great. Shapiro counsels against the use of radiant slabs in areas of houses with passive solar heat. “It’s a waste of energy,” he says, though just how much waste occurs is unclear.

The risk of overheating with concrete-slab radiant-floor heating systems in very energy-efficient buildings leads some designers to incorporate sophisticated control systems. Rather than a simple room thermostat, many radiant-floor designers install control systems that also adjust the circulating water temperature based on outside air temperature and the temperature of the slab. It can also be important to have different zones in a concrete-slab radiant-floor heating system—so that less heat can be delivered, for example, to portions of the slab that are warmed by solar gain. However, according to Rosenbaum, a radiant-floor slab is somewhat self-regulating when it comes to solar gain. If the floor slab begins absorbing solar heat and warms up, it will extract less heat from the circulating water; that heat will return to the boiler and can be circulated to nonsolar zones.

Heat loss into the ground
With slab-on-grade radiant-floor heating systems, there is potential for significant heat loss into the ground. According to Paul Torcellini, Ph.D., P.E., of the National Renewable Energy Laboratory, even with insulation under the slab, 20% of the heat entering the slab can be lost into the ground. This reduces the overall efficiency of the radiant-slab system, offsetting the potential savings described above. Typical manufacturer recommendations for 1” (25 mm) of XPS insulation beneath a radiant slab are clearly inadequate; even 2” (50 mm) may not be enough. Shapiro recommends up to 4” (100 mm) in cold climates. In place of ozone-depleting XPS, one can use high-density expanded polystyrene (minimum 1.5 pcf, 24 kg/m3 foam recommended).

It is ironic that most people want radiant floor heat because they don’t like a cold floor, yet there has long been resistance to insulating beneath concrete floor slabs—which would dramatically reduce the cold-floor problem. They solve the problem with an expensive radiant-floor heating system (including rigid insulation under the slab) when the rigid insulation alone would solve most of the problem. (To be fair to radiant-floor heating proponents, the only way to make a slab floor actually warm to the touch is to provide radiant-floor heating—because the high conductivity of concrete makes a slab feel cool even when it is at or slightly above room temperature.)

Challenges with cooling
Most radiant-floor heating systems cannot provide cooling, and most homes and small commercial buildings are being built today to provide cooling—even in relatively cool climates. This is why forced-air systems are far more popular than hydronic heating systems nationwide—the ducts used for forced-air heating can also be used to deliver chilled air (see further discussion under “Radiant-Floor Heating vs. Forced-Air Heating” below). One of the problems in turning a floor into a heat sink is the risk of condensation on the cool surface. (Condensation occurs when a surface temperature drops below the dew point—which can be quite high in more humid parts of the country.)

Radiant cooling (generally with ceiling panels) is used quite commonly in Europe, where humidity levels are generally not as high as in eastern North America and where the comfort envelope of building occupants (the temperature range at which they are comfortable) is wider than here. That said, there is some interesting research underway in the U.S. on radiant cooling. This concept is being tried out, for example, at an architecture school studio at Penn State University. Chilled water is circulated through ceiling panels to provide radiant cooling, with 100% fresh air used for ventilation. The key is that the ventilation air is dehumidified before delivery to the conditioned space, thus eliminating the potential for condensation on the radiant ceiling panels. This system is saving energy in two ways: because pumping water requires less energy than moving air, and because the chilled water has to remove only the sensible heat loads—not the latent loads. With the 100% outside-air supply, the total amount of circulated air is reduced by about 80%, compared with conventional recirculating systems.

Predicted vs. actual savings
The final concern with radiant-floor heating systems is that much of the assumed energy savings may not be occurring. There is very little hard data to back up the common claim that radiant-floor heating systems save a lot of energy because people with this form of heat are comfortable at lower temperatures and thus keep their thermostats lower. In fact, the only study we could find shows this not to be the case.

Last winter, the Canada Mortgage and Housing Corporation (CMHC) carried out a study of 75 houses in Nova Scotia: 50 with radiant-floor heating and 25 with other heat distribution systems—research that was first reported in the December 2001 issue of the Journal of Light Construction. These houses were visited during daylight hours on weekends, and thermostat settings were recorded. Thermostat settings in the houses with radiant-floor heating averaged 68.7°F (20.4°C), while settings in the control houses averaged only 67.6°F (19.8°C). Although the sample size was small, this study shows no evidence that homeowners with radiant-floor heating keep their thermostat settings lower; in fact, it shows the opposite. Don Fugler of CMHC, who managed the research project, told EBN that they launched the study after a radiant-floor heating product manufacturer contacted CMHC asking for more detail on standard information the agency had been giving out about the energy savings from radiant-floor heat. He cautions that this was a very superficial study, but that it points out the need for additional research into the common claim about energy savings.

Larry Drake of the Radiant Panel Association says that the CMHC study was very interesting and the conclusions being drawn from it are misleading. “To assume that people don’t feel comfortable at lower temperature is conjecture,” he said. He argues that the relationship between comfort and mean radiant temperature has been well established by ASHRAE for decades. He speculates that if homeowners with radiant heat have opted to keep their thermostats about where they keep them without radiant heat, they have opted to increase their level of comfort rather than going for the energy savings. He also suggests that homeowners may tend to set their thermostats numerically, irrespective of comfort—so that if they used to keep their thermostats at 70°F and then put in radiant-floor heating, they may well still keep their thermostats at 70° (and end up being more comfortable).

Andy Shapiro prefers not to make claims about energy savings with radiant-floor heat. “Radiant heat can be a wonderful amenity in a house,” he says, “but to sell it as an energy saver stretches the point.”

Source:http://www.radiant-heat-floor-methods.com