Chimney Connector on a B-3 Series Unit

Flue All JUCAs need a chimney to carry away the gases that result from the burning of the wood or other fuel. The connection to this chimney is a round stub connector, usually located on the top of the unit.

The necessary size of this connector (and the chimney system connected to it) depends on quite a number of things. Our units NORMALLY have a flue sized for a chimney that is average height, 16 feet. For most normal applications, that means an 8" flue (chimney) for a B-3B, B-3A, B-3N. A 6" flue is normally on the B-3J, B-3JN, K-3. A 10" flue is normal on the B-3D, B-3C, C-3, and other three-glass-sided units, and on the F-9A Built-In Fireplace models.

On free-standing JUCAs, the OUTSIDE diameter of the connector is EXTREMELY accurately that diameter. The tolerance is generally about 0.003". This accuracy is so the first piece of smoke pipe goes over the OUTSIDE of it, and is so tight that no leakage of gases or liquid creosote could occur.

There are some circumstances where a different sized flue is called for. For example, if the chimney is a lot taller, it draws better. Sometimes, a 25 foot tall 6" chimney will draw as well as a 15 foot tall 8" one. In such cases, sometimes customers choose to use an existing 6" chimney system rather than an 8" system, often to save money.

If this possibility is being considered for a B-3B, for example, please consider also having us make another change in the unit. Since we custom build each unit, it would not be any problem for us to build a B-3B unit with a (smaller) B-3J door. We call that unit a B-3B-6. Let me explain the thinking.

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Fireplaces are generally designed so that the area of the flue is about 1/10 of the area of the largest open door area. JUCA follows a modified version of this. The B-3B has a maximum door opening of 21 x 15 inches or 315 square inches. The B-3 chimney is 8&qyot; round, or about 50 square inches. This is more than the apparently appropriate 31 square inches. Why? It's because of how efficient the JUCAs are! Since JUCAs are so very good at extracting heat from the smoke, the smoke going up the chimney actually has less heat in it. Since it is not as hot, it has less buoyancy to WANT to go up the chimney! So it has a little less effective draft. To account for this effect, the JUCA B-3B has a 50 square inch chimney rather than the 31 that would have seemed sufficient.

In reducing the chimney to a 6", (28 with the normal-sized B-3B door opening, you might be increasing the chance of some smoke back-puffing out when you open the door. To reduce the chance of this, making the door opening also smaller (to 204 matches the reduction in the chimney area. Voile! A B-3B-6. Similarly, we could make a B-3A-6, a B-3D-8, or an F-9A-8, IF you are sure that the chimney is extremely tall.

Our Technical Series includes a very technical discussion of chimney design.

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