Fireplace size is all important. Fireplaces that are bigger than necessary may smoke, and will remove a lot of warm air from the room.
So, while a large fire may look grand, it carries a hidden cost of warmth lost to the outside.
The rules on fireplace size are simple:
Ideally the area of the fireplace opening (height x width) should not exceed nine times the cross sectional area of the smallest part of the chimney/flue.
For example: If you have a flue that measures one foot by one foot, then the fireplace can be up to three feet wide and three feet high. This rule is more relaxed the bigger the flue, but the 9:1 rule almost always guarantees a successful appliance, all things being equal.
The smaller flues follow the rule much more tightly, and the legal minimum flue which has an internal diameter of 8 inches shouldn’t be matched with a fireplace bigger than 18 x 18 inches, which is about 2.25 sq feet.
For example: From this you can see that fitting a small pot to a wide chimney will radically affect its function, when a square foot flue (144 sq inches) terminates in a pot with an internal diameter of 8 inches (50 sq inches). As you can see it has dropped to a third, hugely reducing the bore and causing backpressure that will push smoke back into the room.
Even reducing a 9×9 flue to a 9″ internal diameter pot reduces its area by 25% and proportionately its ability to remove smoke from the appliance.
The 9:1 Rule
To help you visualise how this relationship functions, consider this:
For a fireplace, the amount of air entering it is proportional to its size, and hot air goes up the flue 9 times as fast. So in unit time, in a balanced fireplace and flue, 9 volumes of cold air enter the fireplace at speed 1. In the same time, 9 volumes travel up the chimney at speed 9. So, if the fireplace is 10 times the size of the flue, then in unit time, 10 volumes enter the fireplace and 9 go up the flue, but before the tenth can follow, the next 10 volumes enter, and the tenth from the last lot is displaced into the room, mixed with some smoke.
Hood Sizes and the TARDIS Effect
There is a problem with this that causes an awful lot of grief when people don’t fully grasp the concept of 9:1 and this happens with hoods, where installers often overlook the TARDIS effect, whereby something is bigger than it appears.
Consider a fireplace 5′ x 5′ connected to a flue that is 1′ x 1′. This is a ratio of 25:1 so we can confidently predict that it won’t function. A hood is fitted that measures three feet on a side and is three feet above the ground. This is assumed to create a fireplace area of 9sq feet (3×3=9) , but we actually need to take the sides of the hood into the equation, so instead the equation should read:
(3 3 3) x 3 = 9×3 = 27 sq feet.
In short, the hood is actually bigger than the fireplace it is in.
Another hood error is where the neck of the hood is smaller than the least portion of the flue, creating the identical problems to a pot too small for the flue.
Legal Minimum Size of Flue
Nothing, but nothing should be allowed to reduce any part of the flue below the 8 inch diameter legal minimum (for an open fire).