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How Radiant Heat Saves Energy (Part II)

Posted on 2013-11-14 07:04:48 PST

In the first part of this series I described how conventional hot air heating systems lose energy through conductive heat loss. In Part 2, I’ll describe how radiant heat differs from hot air systems and how that translates into energy savings.

Radiant Heat is Different

Unlike hot air systems which use energy to heat up air, then move that air around to warm objects and people, radiant heat transfers its energy directly to objects and people. There’s no middleman. That’s why on a cool day when the sun pops out from behind a cloud you instantly feel warmer. Radiant heat warms you directly (at the speed of light) and there’s no waiting around for the air to first warm up.

Radiant heat is also very different in that the operative temperature of the system (the actual average temperature of the radiating surface) is generally significantly cooler than a hot air source. Let’s take a radiant heated floor or ceiling as an example. Most radiant floor systems are set to a temperature of 70°F to 74°F. Recall that in the first part of this series, a hot air baseboard emits air at 120°F.

wall__window_ceiling

Recall, too, that the rate of heat loss is directly affected by temperature differential – the difference in temperature between the warm side and the cold side. With an outside temperature of 0°F and hot air washing up the wall at 120°F we have a temperature differential of 120. With radiant heat we have a temperature differential of only 72°F (taking an average between 70 and 74).

Revisiting the Numbers

In the first part of this series I worked through the numbers for a hot air system. To summarize: hot air washing up an exterior wall lost 6 Btus of energy per hour through an R-20 wall and 60 Btus of energy through an R-2 window. Let’s now look at the numbers for radiant heat. The math for 1 square foot of wall is as follows: 1/20*72 = 3.6 Btu per hour. For a window it is: 1/2*72 = 36 Btu/hour.

Compare 60 Btu/h versus 36 Btu/h and you’ll see that hot air systems will lose 167% more heat through a wall or window than does radiant heat! That’s 167% more energy going out through the wall or window during the heating cycle.

Not the Whole Story

A truly astute reader will notice that once the hot air system is off (because the thermostat is eventually satisfied and thus shuts the system down) the air temperature at the wall will no longer be at 120°F but 72°F (on average). That’s very true. When the room air has reached the desired temperature, both systems will lose heat at the same rate.

So one cannot simply state that hot air systems always use 167% more energy than radiant heating systems. That would be overstating it. The two systems eventually (theoretically) will equalize for a portion of the heating cycle and thus have equivalent losses. The major thing to note is that while the hot air system is on, the heat lost through the wall and window (and thus not available to heat the room) is significantly higher than that of the radiant heating system.
And that’s only one way in which radiant heat saves energy.

In subsequent posts in this series I’ll describe other ways in which radiant heat is more energy efficient than conventional hot air systems. Stay tuned!