I'm going to sream...

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One flueless fire coming up.
Chances are she won't be able to complain about that one.
 
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One flueless fire coming up.
Chances are she won't be able to complain about that one.


Chances are she will.
"Why do I have to have that dirty great vent kit put through my lounge wall?"

I say go electric.
 
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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"?

If she wants 10 inches, give her 10 inches. Sounds like 7 inches aint gonna satisfy this customer! ;)
 
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I dont know if that OP lady is still reading anything, probably not.

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.

The draw of the flue is largely as a result of the hotter flue gases having expanded and are therefore lighter in weight.

The purpose of the liner is to stop the gasses cooling down on the cold inside of the chimney.

Sometimes the anular space between the liner and the chimney is filled with insulating material to reduce the heat loss further.

Funny how the basics sometimes get forgotten!

Tony
 
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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 am going to wade in here with a Physics Masters, here goes.

Yes a cross sectional area in general will cause more air to be drawn through it, in terms of cubic metres per minute. However the air speed will be lower. That last bit is critical as far as I understand it, as if the air speed is not sufficient it will allow heavier particles such as soot and carbon dioxide to sink back down the flue and escape into the room. In this respect a smaller cross sectional area will cause the air speed in the flue to be higher.

It should be noted however the "draw" of the chimney will depend on many other factors than just the cross sectional area. So things like shape i.e. is it round or square; how smooth the walls are; how many if any bends are in it; to name but a few.

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.

I will add to that, I recently lifted a floor board in an upstairs bed room, which has a flue from the lounge downstairs running through it. I found a large pile of ultra fine soot. Fortunately I have not used the fire in the lounge since I moved in and won't be now till it is replaced and the chimney lined.

As someone else pointed out if the manufactures instructions say it requires a 7" liner then you cannot get a "Gas Safety" (what was a Corgi) installation certificate without it, which makes the whole argument rather pointless if you ask me.
 
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this was flying about ages ago, its a studio edge cassette fire which has an extremelly large gather hood fitted on top. The hood has to be connected to the flue with a length of 7" liner to ensure a smooth transition for the flow of gases up into the throat of chimney. Plus once installed the fire is impossible to remove so anything falling down flue will drop into fire. Its 90% building work but not £1000's
 
D

dextrous

jabuzzard - thanks for the clarification and am glad I wasn't a million miles from being correct. It's been nearly 30 years since I did my physics o level, so detailed accuracy was always going to be a bit questionable.
 
D

dextrous

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.
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.
Sometimes the anular space between the liner and the chimney is filled with insulating material to reduce the heat loss further.
Tony
Didn't they used to pour vermiculite between the old liner and the void, which has the insulative properties you refer to?
 
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Exactly!

However, I have to disagree with Buzzard who imagines that the "draw" of the flue depends on wind across the top!

If that was the case then the flue would not work at all on every still day and poison everyone in the house.

I realise the younger people dont remember the days of smoke coming out of chimneys and going straight up on still days.

The draw of the flue is primarily caused because the flue gases are hot and therefore lighter and therefore rise upwards. They also continue to rise after they leave the terminal or top of the chimney.

Tony
 

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Fail to see the need for being rude. It has frightened the poor girl and she has not posted since as a few have jumped down her throat. It is only fair to be fore warned and fore armed before letting someone carry out work. With so many so called experts out there no wonder the poor girl is confused.com

A few posters here have a perceived idea of how a flue/ chimney works.

OP. Read the manual and see what the requirements are. If the manual says flue liner required, then it has to be fitted (though few fires I have fitted have not required that- but then I do not fit many fires, only service them).

Normally a chimney has to be swept prior to fitting a gas fire.

If the chimney is routed externally (you can see the chimney brickwork side of the house), chances are it will need to be lined as flue gases from a gas appliance could cool too quickly and condense with possibly spillage of flue products at the fire.

Check the gas registration card before you allow anyone to fit the fire. Ensure the fitter is registered to fit fires (check reverse of his gas card)

Friday previous was called to fit a fire installed by an 'expert'. Builder was commissioned to build an extension adjoining the lounge. Homeowner wanted the builders opening in the lounge closed. Opening made in the extension to catch the said chimney. Chimney was 'checked' by the 'expert' and fire fitted which started spilling from word go.

Turned out the chimney was indeed healthy but cavity between the fire and the chimney was still open. Result. POC going up the cavity not the chimney. Some other coc*ups- two core flex fitted where three core HT flex required, 8mm copper pipe (no sleeve) use and too long (WP 16mb) and no seal on flanges when the LFE was fitted to wall (though with cavity open seal would have been useless any way)
 
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Exactly!

However, I have to disagree with Buzzard who imagines that the "draw" of the flue depends on wind across the top!

I did not. All I did was correct some physics about cross sectional area and amount and velocity of the flue gases.

This would hold true whether the "draw" was caused by Bernoulli's principle or differential gas densities as you suggest.

The reality is that both have an effect. When a gale blows Bernoulli's principle will dominate, on a still day it will all be down to differential gas densities. At other times it will be a mix of the two,
 

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