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How Radiant Heat Saves Energy (Part I)

Posted on 2013-11-14 07:02:46 PST

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.

wall__window_baseboard

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°F, the R-value of the wall is R-20, R-2 for the window and the outside temperature is 0°F. 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.