Greetings again from Sydney, Australia. I am sorry to bother you again, but the construction of our Rumford fireplace is turning into a saga. They builder came to fix the smoking issues last week with a new smoke chamber and a bigger flue (310mm diameter), but found the smoke chamber did not fit, and, using your instructions that the smoke chamber needed to be about half the volume of the firebox, with gently sloping sides and no restrictions flowing into the flue they are going back to try again. I am definitely a good advertisement for buying the Superior Clay components instead of trusting an architect to get it right. Our fireplace has an opening to the room of 1270mm x 700mm. This opening at the top you can see in the photo above is 660mm at the front, 450mm at the back and 120mm from front to back. Is there a ratio I can use to figure out what the area of this opening should be? Thank you for all your advice so far, it is very much appreciated. Best wishes, Jane Rowe Jane, I'm sorry you’re having so much trouble. Generally Rumford fireplaces have an opening which is about square - as tall as they are wide - an in our experience the hearth is more important than the width. Having said that, the flue area, by code, needs to be about ten percent of the opening area and we design our throats to be 1/20th of the opening area. So an opening of 1270mm x 700mm would require a flue with a cross-sectional area of 88,900 sq.mm. (889,000 ÷12=74,083) or a round flue with a diameter of 310 mm.

And the throat should have an area of at least 44,450 sq.mm. so your throat with an area of 66,600 sq.mm is fine. All of these dimensions and ratios depend on smooth internal flow through the rounded throat and from the smoke chamber into the flue. So the new smoke chamber flows smoothly into he new flue without the abrupt turbulence-creating reduction in the first picture you sent me below. If that’s the case, I think your Rumford should work fine. Don’t forget the pressure tests, however. Excessively negative indoor air pressure can cause any fireplace to smoke and that’s easy to test and easy to fix. To begin open a door or window to make sure you have draft in the cold fireplace before you light a fire. Then, if you find you need the extra make-up air you cn find a better more efficient way to bring it in that opening the door. The size of the flue and the transition from the smoke chamber to the flue are the problems. Sorry about the damper which was clever. Maybe it could still work if bigger but care must be taken to avoid restriction or turbulence. Best, Jim Buckley

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