We all want the most energy efficient, low cost heating system available for our homes. And we know that radiant heat is more efficient than hot air heat. But do we know why?

First we need to understand the differences between hot air and radiant heating. Hot air heating occurs when hot water or gasses transfer heat to a large surface area (furnace/coil or finned element) and cool dense air passes over the area, therefore absorbing some of the heat. Radiant heating heats objects, such as your furniture, walls or ceiling — any solid object that the radiant surface “sees.” So what. So what?! Radiant heat controls the rate at which you lose your own heat. Convection simply replaces a building’s heat loss.

Q: So how does a radiant heating system control our own heat loss?

A: Mean Radiant Temperature (MRT) is where it’s at. MRT is the average temperature of all objects within a space. The closer/higher the temperature is of all objects in a space, the higher the MRT. If the MRT is high, then the comfort level goes up and the air temperature becomes less important. Period. Why? A high MRT, in simple terms, is a room’s way of telling you to “keep your radiant heat to yourself” and let the other solid objects in the room do the heating for you.

Q: What the heck does that mean?

A: All objects emit radiant heat. How much depends on surface temperature. (The sun emits a lot more radiant heat than an ice cube!) For example, the human body, at a surface temperature of about 82-85 degrees F, emits a tremendous amount of radiant heat. On a 0 degree F day, the surface of a single pane, sliding glass door could approach 35 degrees F. Pretty cold, eh? When you stand in front of the glass, 85 degrees minus 35 degrees equals a 50 degree F temperature difference. You’ll feel as if a cold draft is coming off the glass. But it’s not a draft. The window is pulling radiant heat off the surface of your body at such a tremendous rate, that your own heating system can’t keep up. You shiver in an attempt to generate internal heat and offset your heat loss to the cold glass. This principle occurs whenever you are near an object that is cooler than you. Any exterior wall, cold ceiling, any single, double, or triple pane window, furniture, fixture or door.

Since radiant heat waves are emitted from all objects, the warmer the object, the higher the intensity. Turn your floor into a radiator that heats all the objects in a room and watch what happens. The average temperature of the objects in the room rises. The MRT of the room is increasing. The radiant floor is saying to you, “I’ll heat the cooler objects in the room so you don’t have to.” What actually happens is you will turn your thermostat down because the air temperature becomes too warm for the MRT. Huh? In a high MRT environment air temperature becomes secondary. Your body feels comfortable at a lower air temperature since the room wraps you with warmed surfaces. Don’t believe me? Read this from ASHRE HVAC Applications chapter 48.5, 1991. I didn’t believe it until I installed a thin slab tile floor in my sunroom addition.

“In general, the human body has a great ability to sum sensations spatially caused by radiant heat from many hot and cold sources on the skin’s surface. For example, Australian aborigines sleep unclothed next to open fires in the desert at night, where ta [the temperature of outside air] is 43 degrees F. The tr [the average temperature of the fires] caused directly by the three fires alone is 171 degrees F, but the cold sky tr is 30 degrees F; the resulting ta during sleep is 82 degrees F, a value acceptable for human comfort when the subject is unclothed and acclimatized to cold.”

Pretty neat heating system!!

Q: So why is radiant heat more efficient than hot air heat?

A: For every degree cooler a space is maintained, the energy saving is between 3 and 5 percent (DOE Studies). This is because heat loss is based on the temperature difference between outdoor and indoor temperatures. The greater the difference, the greater the heat loss and vice versa. If the thermostat in a hot air system is set at 72 degrees F, you will feel just as comfortable at 65 to 66 degrees F in a radiant heating system. Remember that radiant heat has a high MRT. Hot air has a lower MRT. Conservatively speaking, the temperature differential between radiant and hot air heat would translate into a 15 percent energy savings.

OK, skeptics. Let’s throw out comfort and look at shear energy savings based on indoor/outdoor temperature differences. Let’s use a setback thermostat with 12 degrees F setback and a room temperature of 72 degrees F for 12 hours. The average temperature is 66 degrees F. If we kept the radiant system at 65 degrees F, it is still more efficient than hot air. Radiant beats hot air by 3 percent (savings due only to temperature difference). And since the MRT is so much higher in a radiant system, our comfort is never sacrificed by lowering the air temperature to 65 degrees F. In a hot air setback, all the surfaces cool down to 60 degrees F. When you wake up and the setback warms the air to 70 degrees F, the objects in the room are still 60 degrees. Thus the MRT is much lower and we feel uncomfortable. Isn’t comfort what we’re selling? Aren’t we providing comfort and efficiency with a radiant heating system?

We still haven’t looked at the efficiency of the power plant (furnace/boiler) and the transfer system (ductwork/piping). There are plenty of studies that have addressed this issue and have found that water heat transfer is more efficient than air heat transfer. Please don’t call me biased. There is a place for everything. You do get a two-for-one deal with ductwork (heat/AC), but I wonder if you wear flippers on the construction site and swim in the ocean with boots on? I feel the right system should provide the right comfort.

Personally, it makes sense to me. A radiantly heated environment provides the ultimate comfort for humans and offers the highest energy savings.