Is in-floor radiant heat efficient?

The answer to whether radiant heat is efficient is yes and no. Radiant floor-heating is nothing but a form of radiator.

Instead of using finned-baseboard, cast-iron floor radiators or flat-panel wall radiators, you're using tubing on or below the floor as the source of heat-radiation. And like any of the other radiator options, it can be designed and installed efficiently or it can be made to run like a hog.

Several factors can make or break the efficiency of your radiant heating system.

1) Insulate under the floor

Unlike a forced-air heating system, which causes warm air to rise, radiant heating radiates in any and every direction. Moreover, due to the laws of thermodynamics, the heat emitted from the tubing will want to go towards the coldest path. If that path happens to be down (toward a cold basement, perhaps), that's where the heat will go.

The better you insulate below the floor, the more efficient the system will be at delivering heat into the room in which it was intended. Most manufacturers recommend a minimum of R-13 (about 4" of fiberglass). Don't rely on thin Astro-Foil (the stuff with bubbles) to do the job, as many people make the mistake of doing.

One can reasonably argue that radiant floor-heating is less efficient than radiators installed within the room. If the radiators are over-sized, like many old cast-iron radiators are, this is true, since there are no downward heat losses associated with radiators.

2) Minimize water temperatures

A well-designed radiant floor heating system should never exceed water-temperatures of (approx.) 135 degrees (F). The lower your water temperature, the more efficient the system will be. In many cases, it's possible to operate a system with maximum water-temperatures of 110-115 degrees (F). You can accomplish this by installing tubing at closer spacing (6" OC versus 9" or 12").

3) Utilize a modulating control system

When we talk about a system's design-temperature, we're talking about the water-temperature needed to heat the home on the coldest day of the year. The rest of the time we can run the system at a lower supply-water temperature.

This can be accomplished automatically with a modulating control system that utilizes a boiler-reset control (w/ either an indoor and/or outdoor temperature sensor), and a modulating mixing-valve or variable-speed injection pump. I prefer mixing-valves over injection pumps, as mixing valves require just one pump versus three (primary loop pump + injection pump + secondary loop pump) for an injection system. That said, both methods modulate water temperatures based on outdoor and/or indoor temperatures. Lower water temperatures mean higher efficiency.

4) Use the right boiler

When shooting for efficiency, the standard boiler has become wall-hung condensing models. Condensing boilers often reach efficiencies of up to 95-96 percent. Sophisticated models often have built-in outdoor-reset controls and fully modulating burners to minimize water-temperatures and maximize combustion-efficiency. Condensing boilers run the gamut in terms of price and quality. Like anything else, you really get what you pay for -- and it pays to pay more. Look for models with stainless-steel heat-exchangers (avoid aluminum or copper) and fully modulating burners (versus one or two stage).