Disdvantages of 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, 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 2x4 walls). In such a house, putting in an expensive heating system doesn't make good economic sense. 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 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.

Time lag of heat movement through radiant floor heating

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 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.

Heat loss into the ground

With slab-on-grade radiant floor heating systems, there is potential for significant heat loss into the ground. This reduces the overall efficiency of the radiant-slab system. 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. (

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. 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.

Predicted vs. actual savings