Here’s an undisputed fact:
Radiant heat provides greater warmth and comfort than convection (forced air) heating. Ask anyone who has experienced radiant heating in their home or commercial buildng and they will tell you there is simply no greater feeling of warmth and comfort.
Here’s another fact: we all want to save energy.
Many of us want to be environmentally friendly (“green”) and all of us want to save money. We recognize the importance of not wasting energy in achieving those objectives.
Radiant heat’s very nature delivers warmth and comfort as well as energy savings!
In conventional heating systems (hot air systems such as furnaces or baseboards), energy is used to heat the air. Air is then blown around the building, requiring additional energy to operate the fans or blowers.
Convection systems are often combined with the ventilation and air conditioning system and attempt to blow heated air near the ceiling. This approach is self-defeating, as we are fighting the laws of physics and hot air rising.
One key issue in heating is heat loss – the loss of heat through the walls, doors, windows and ceilings.
Heat Loss Explained
We’re all familiar with the concept of insulation and believe 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 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. 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!
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 0oF.
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.
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.
That’s how it works from a conductive heat loss point of view. Physics can be a little dry, but it’s important to understand the underlying principles.
The main thing 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.
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.
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 F to 74 F. Recall that in the first part of this series, a hot air baseboard emits air at 120 F.
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 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
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 with convection heating.
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
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.
How much energy can radiant heat save you?
Contact us today for a free project assessment!