dextrous, when you say 'vent' are you talking about the opening at the top of the flue? I'm talking about the size of the flue itself. I've just spoken to a chimney expert with 30 years experience who says that if I have a 10" square flue, then that will create a bigger 'draw' than a 7" diameter liner. Surely that sounds right? 10" is bigger than 7"?
However, with regard to coming to terms with the physics of it, if you had a vacuum cleaner with say a 4" diameter hose, it would need a much more powerful engine to operate it that is needed for a 1.5" hose to create the same effectiveness of suction. Thus, conversely, for example, a windspeed of 10mph over the flue opening will not create as much draw through a wider opening compared to a narrow opening. Now, although there is a natural conclusion that would imply that the 7" opening could be narrower still, you have to take into account the actual volume of gases that need to be drawn.
I can understand your feeling of the manufacturers being in cahoots with installers and each other, but I feel that you are in error in this instance. One of the other reasons for stipulating a flue liner as necessary is the safety aspect. Over the years, gases created by coal and wood fires, coupled with exposure to the elements will have weakened the mortar within the chimney; as well as other possible hairline cracks due to natural movement. If the only thing being burnt was still coal/timber, then any escaping (through the brickwork and possibly into rooms) would smell and be less dangerous chemically anyway. However, since carbon dioxide and monoxide are not nasally detectable, and are far more toxic, the instance of gases leaking into a room (especially for example a bedroom) would be potentially life threatening.
Tony, I was merely correcting her o level physics understanding, and reinforcing the point that mi's need to be adhered to for sound reason.Rather oddly with all these "expert" replies, only the one from Namsag mentioned the real reason that a liner is usually required but he did not properly highlight it.
Didn't they used to pour vermiculite between the old liner and the void, which has the insulative properties you refer to?Sometimes the anular space between the liner and the chimney is filled with insulating material to reduce the heat loss further.
However, I have to disagree with Buzzard who imagines that the "draw" of the flue depends on wind across the top!
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