Chimney Provisions on the F-9A

Normally, JUCA builds the F-9A units with a 10" round stub connector. This is to make it compatible with any of the brands of Class-A Chimney which are available. The 10" size is for "standard-sized" door openings, and may be different for larger or smaller doorsets (see below). In recent years, most contractors and remodelers seem to prefer using Factory-Built Class-A Chimney over building a traditional entirely masonry chimney.

Metal Class-A Chimney

When we describe the pre-fab chimney as a 10" size, we are referring to the inside dimension. The various brands of Class-A chimney have different thickness walls. Many solid-pack brands have one-inch thick walls, making the outside diameter 12". A few have walls which are 1.5" thick, and there are some products with about 2" thick walls. Only one manufacturer still makes the once-popular Triple-Wall chimney which has a wall thickness of about 3.5", making its outside diameter about 17". When planning installation, these chimney sections also require a clearance to anything wood or otherwise combustible. Generally, this clearance is 1" to 2".

Masonry Chimney

On request, the JUCA F-9A series units can be made with a "pocket" on top to be compatible with various sized tile liners used in standard masonry chimney construction. If this is the plan you want to follow, just specify the size tile liner to be used, such as 13 by 13, 13 by 17, 17 by 17, or whatever.

Such chimney structure would have to be supported in a conventional manner by lintels bridging the body of the JUCA. If you would have supported the masonry on the JUCA, a potential problem is related to how strong the JUCA is! During heating and cooling, the metal structure of the JUCA expands and contracts more quickly than the masonry. The JUCA is built so strong that it could raise the tons of weight above it a fraction of an inch, which would potentially loosen some of the mortar joints between the bricks and blocks above. This would get more likely after many repetitions over years. So don't even think to do it that way! The installation instructions for the F-9A include directions on properly supporting the chimney structure.

The "pocket" on top of a JUCA F-9A made for a masonry chimney is designed to allow a tight but sliding connection where the differential expansion of the metal and the masonry are provided for. The very lower end of the chimney tile liner actually "hangs" inside the pocket and is sealed around its perimeter.

There is no extra charge for the F-9A to be built for a masonry chimney.

Chimney Size and Door Opening

A long accepted guideline for fireplace design is that the chimney area should be no smaller than 1/10 of the area of the opening of the fireplace. In the case of a JUCA F-9A, which is NOT designed to be operated without a doorset attached, this refers to the actual openable (moveable) area of the doorset.

For example, a traditionally wide-framed 36x28 doorset (with doors wide open) will usually have a clear opening of about 30" by 25" high. That's 750 square inches of opening area. The 10" round chimney has an area of 78.54 square inches, which complies with the 1:10 or better requirement. This ensures proper drafting.

Other size fireplace openings can require different sized chimney size (the corners in rectangular flues have little function):

Opening Area Range	Round Chimney Size	Tile Chim Size
640-785	10"	8" by 12"
785-950	11"
950-1130	12"	12" by 12"
1130-1540	14"	12" by 16"
1540-2010	16"	16" by 16"
2010-2600	18"	16" by 20"
500-640	9"
up to 500	8"	8" by 8"

O.K., so now you're considering a doorset which has an actual openable area of 32" by 24" high. That's 768 sq.in., so the standard sized 10" chimney is fine.

How about a 48" wide doorset which has an opening approximately 43" by 26"? The area is 1118 sq.in., which means that a 12" chimney is appropriate to make sure it draws properly.

Arch-Top Openings

How about a Flat-Arch 42" wide doorset, 27" high in the middle and 20" high at the sides, and super narrow frame? Calculating the area is a little more difficult. It is about 42x23.5 (the average height) or 987 sq.in., so a 12" is called for.

Double-Sided F-9AX Units

The chimney sizing on a standard F-9AX unit is normally the same as for a single-sided F-9A unit. This is based on a likelihood that under most usage conditions, only the doors on one side would be open at any specific time. That situation is probably also a good idea for a separate reason which has nothing to actually do with the fireplace itself. Some houses have been built with see-through open fireplaces which did not have any glass doors. Occasionally, the HVAC forced-air heating system for that house winds up supplying more warmed air to the room on one side of such a fireplace than the other. This might be due to improperly designed ducting or it might be due to the owner having closed the warm air outlets in one of the rooms. The room receiving more of the warm air from the furnace gets slightly "pressurized". In order to equalize itself, some of that air needs to get to the other room. One possible path for this room-to-room air flow could be actually through the fireplace openings! If a fire was going, its smoke could then be dragged out into the room, even though it was not the fireplace's fault!

If your expected usage of an F-9AX see-through fireplace is such that you expect to regularly have both door sets open at the same time, then you need to DOUBLE the opening area in the table above, which is certain to then indicate a much larger (and more expensive) chimney system.

More Thoughts

All the foregoing is a simplification of a subject that is affected by quite a few variables. For example, the normal guidelines are intended for chimneys that are of "normal" height of about 16 feet from the height that the air enters the firebox to the top of the chimney where it leaves. Some installations involve much taller chimneys, which generally draw much better. Often, a 25 or 30 foot tall, 8" chimney draws as well and performs as well as a 16 foot tall, 10" chimney.

Sometimes, unusual circumstances also affect how a chimney draws. In most of the USA, where winter winds commonly come from the west and north, a relatively short chimney on the east side of a tall house may draw as well as one 10 feet taller on the west side of the same house! Tall trees 50 yards to the west of a house can cause occasional turbulence that might occasional draft problems for a chimney, again, especially if the chimney is on the west side of the structure. Even the shape of the building and its roof can influence the performance of a chimney. Contrary to some currently popular beliefs, going a little larger is sometimes a good precaution if there's any uncertainty as to the appropriate size. (Some people in recent years have argued that it's better to REDUCE the size of a troublesome chimney to reduce the chance of winds blowing air back down the chimney. Unfortunately, that logic also reduces the opportunity for the smoke to leave in the first place!)

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