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 as an example. Most radiant floor systems are set to a temperature of 70^oF to 74^oF. Recall that in the first part of this series, a hot air baseboard emits air at 120^oF. 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 temperate of 0^oF and hot air washing up the wall at 120^oF we have a temperature differential of 120. With radiant heat we have a temperature differential of only 72^oF (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^oF but 72^oF (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!

We all want to save energy. Whether your main reason is to be environmentally friendly (“green”) or just to save money, we all recognize the importance of not wasting energy.

Did you know that the very nature of radiant heat can do that for you?

In conventional heating systems (hot air systems such as furnaces or baseboards) first use energy to heat air. The air is then blown around (forced air systems) or left to convect naturally (baseboard systems). Note, too, that blowing air around requires additional energy to operate the fans or blowers.

Typically these systems are placed on outside walls where the heat loss occurs – and in particular you’ll find them concentrated directly under windows. That makes some sense since windows are a source of relatively high heat loss.

Heat Loss Explained

I need to get a little technical here to explain how heat energy is lost in a home or building. We’re all familiar with the concept of insulation and that more insulation is “better”. But how does this actually work?

When we speak of heat loss through a building material, we’re speaking specifically of conductive heat transfer. It’s important to understand that heat travels to cold. So when we have a warm room, heat will naturally want to migrate to cold. That’s why we insulate the exterior of our buildings (walls, floors, etc.) – to slow the rate at which the heat in the warm side will transfer to the cold side. The more insulation we have (or more accurately the higher the R-value) the slower the heat can transfer through to the cold side. More heat stays on the warm side (and you have to re-heat less often) over a given period of time. And that’s a good thing. Graphically you can see the warm air rising above the baseboard heater with a large part of the heat escaping to the outside before it gets a chance to actually heat the room. That energy simply goes out the window!

I’m going to toss in a few calculations here to demonstrate what this all means. As I stated earlier, the R-value of a building component is a measure of the resistance of the flow of heat. The other thing we need to know is the difference in temperature between the warm side to the cold side. The greater the temperature difference, the faster heat will flow. In other words, the colder it is outside, the more heat we will lose in a given period of time.

To demonstrate this mathematically, let’s say we want to calculate the heat loss through an exterior wall and compare that against a window. Let us assume that the air temperature coming out of a convection heater (or air vent) is 120^oF, the R-value of the wall is R-20, R-2 for the window and the outside temperature is 0^oF. I’m using imperial measurements here since the majority of my audience is familiar with it. I’m also using typical numbers you’ll find for R-values in relatively cold climates to give a fair idea of what most of my readers can expect to encounter.

So the math goes like this: 1 square foot of wall with an R-value of 20 and a temperature difference of 120 = 1/20*120=6 Btu/h. So 6 Btu per hour will find it’s way to the outside. Let’s do the same for a window: 1 square foot of window with an R-value of 2 and a temperature difference of 120 = 1/2*120=60 Btu/h. So 60 Btu per hour will be lost through the window. Astute readers will note that the heat lost through the window is 10 times that of the wall. That stands to reason, of course, since the R-value of the wall is ten times greater than the window. And that’s basically how it works from a conductive heat loss point of view.

Phew! I know the physics of what happens can be a little dry, but it’s important to understand the underlying principles. The main thing to take away from this discussion is that hot air traveling up your exterior wall can have a significant heat loss before the warmth can actually benefit the occupants of the room. Without that knowledge and understanding the next part of this article won’t make much sense. And you really won’t understand why radiant heat can save energy.

Let’s move on to Part 2 of this series, shall we?