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Radiant Heat VS Convection Heat

Q: We've been shopping for a wood stove, and have noticed that some are called "Radiant" stoves and some are called "Convection" Stoves. In order to make a buying decision, we need to know the properties of the two types, and which would work best for our situation. We asked the clerk at the stove store what the difference was, and he didn't know. Do you?

Sweepy A: Simply stated, radiant stoves heat objects, and convection stoves heat air. Here's details:

Radiant energy travels in straight lines from the source of heat, and is absorbed by any surface it encounters. As the surface absorbs the radiant energy, it heats up. Think of warming your backside around a campfire and you'll have a pretty good picture of how radiant energy works: the air surrounding you might be 0 degrees, but the radiant energy passes right through it until it hits you (the object), and when it does it warms you up. When you take the campfire and put it inside a stove, the energy radiates through the surfaces of the stove in the same fashion, heating the objects in the room. Eventually, the air in the room is also heated through contact with the hot stove and surrounding heated surfaces, and a comfortable room temperature is achieved. Because walls interfere with the flow of radiant energy, radiant stoves are best used when the area to be heated is one open space.

To get a mental picture of how convection heat works, think of a forced-air furnace. The combustion chamber in the furnace is surrounded by a heat exchanger which harnesses the radiant energy from the fire, transferring it to the air contained within. The super-heated air then delivers the heat throughout the house via a forced air ducting system. To create a convection stove, simply mount an outer shell around the firebox, leaving an airspace between the firebox and the outer shell. The shell will reflect radiant energy back at the firebox, causing the air in the space between to super-heat and rise out openings in the top, drawing cooler air into the convection space through openings at the bottom to be heated in turn. Blowers are sometimes incorporated to increase the flow of air through the chamber. As the rising air carries away heat, the amount of radiant energy that penetrates the outer shell is significantly diminished, while the production of heated air is proportionately increased. This makes it easier to move the heat out of the room and into the rest of the house, by circulating the heated air. For this reason, convection stoves are the best choice when you're trying to heat areas where the radiant energy can't travel, like other rooms or stories in the house. Because of the reduced heat radiation through the outer shell, convection heaters can often be placed closer to combustibles than radiant heaters.

The individual walls that form the firebox of a stove tend to focus the radiant energy at their centers, and the heat radiates straight out from that point. This is one reason cast iron is so often used to make radiant heaters: the raised designs cast into the plates create curved surfaces that diffuse the straight-line radiation, making the stove more comfortable to sit near. Perhaps the ultimate example of radiant stove design is the traditional cast iron pot-belly stove: its round shape and ornate castings provided even heat radiation in every direction, without creating any "hot spots" in the room.

Plate steel stoves tend to generate uncomfortably intense radiation from the center of each firebox wall, because there are no raised areas or rounded shapes in the flat plates to diffuse the focused radiant energy. Fortunately, plate steel stoves lend themselves particularly well to the attachment of convection shells to reduce the intensity of the heat radiating into the room. This is why most convection heaters are made from plate steel. Note that this reduction of radiant transfer doesn't affect the net heat output: the heat is still delivered to the room, in the form of heated air.

Soapstone is an ideal material for a radiant stove. Because soapstone will only warm to about 500 degrees no matter how hot the fire inside is, soapstone stoves create a very gentle radiant heat, and are easier to sit near. It might seem that some of the heat from an 800+ degree fire must be lost if the stone only lets 500 degrees through, but that's not the case: the heat is stored in the stone, which will continue to radiate at comfortable levels for hours after the fire has gone out (long after a cast iron or plate steel stove will have gone cold).

In practice, most of today's stoves are combination heaters. In other words, most radiant stoves have built-in convection tubes or chambers and/or at least one convection panel (a rear heat shield, for example), and most convection stoves have one or more direct radiant surfaces (typically the top plate and/or loading door). When looking at combination radiant/convection heaters, You'll want to choose a model that is primarily the type that will work best in your situation.

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