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Topic Title: Open fire - draw calculation?
Topic Summary: Forced ventilation?
Created On: 04 October 2012 08:24 AM
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 04 October 2012 08:24 AM
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SherlockOhms

Posts: 325
Joined: 05 April 2011

One for the clever ones amongst us.

I have a large open fire that draws like a jet engine on re heat!

In front of the hearth are three 40mm dia vent pipes feeding fresh air from external to the house. These are clearly undersized as the draw from the fire causes a roaring gale through the house.

My question, is there a simple way to estimate/calculate the requirement of the chimney based on size/combustion requirements of wood etc?

I think the way ahead is to force the vents with a fan so I need to guestimate the fan size?

Thanks in advance,

S.
 04 October 2012 09:47 AM
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KFH

Posts: 216
Joined: 06 November 2010

I can't help with the calcs, but do you need a damper of some sort on the chimney. With the draw you are experiencing surely most of the heat generated is going up the chimney. In my experience chimney design/sizing is one of the black arts. From the fires/stoves I have seen/used 3 X 40mm should be more than adequate.
 04 October 2012 10:23 AM
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SherlockOhms

Posts: 325
Joined: 05 April 2011

There is a large chain operated damper across the full width of the throat. It's very difficult to modulate. Very easy to fill the room with smoke!!

Experimentation with forced ventilation I think?

S.
 04 October 2012 11:41 AM
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jj4091

Posts: 56
Joined: 20 July 2007

"If instead of being carried away, the smoke comes back into the room. it could be due either to a blockage or lack of ventilation. Building Regulations recommend 50% of thethroat area be provided as free ventilation area for open fires. In most cases thiswill equate to approximately 13,000mm (20sq ins). Care should be taken when siting this vent to ensure the incoming air does not cause discomfort to occupants of the room. For further advice contact The Solid Fuel Association."
Not sure if this helps or not but I can't really see what difference forced ventilation would make, what is it like with the room door closed?
 04 October 2012 02:43 PM
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AJJewsbury

Posts: 11587
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It's a while since I played with open fires, but it seems to me that the problem is that the draw is too great, rather than the replacement air too small (presumably the idea is to heat the room, rather than the outside). I recall being told off as a child for leaving the damper open after lighting the fire (open to light, close once it's established if I remember correctly). Sounds like fixing the damper so you can adjust it properly would be a better bet.
- Andy.
 04 October 2012 04:31 PM
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OMS

Posts: 19764
Joined: 23 March 2004

Perhaps the other point would be the air supply side - I suspect the pipes have been sized empirically to provide adequate oxygen at a know concentration in air for complete combustion of a fuel at zero excess oxygen. Adding fan forcing to them won't help.

The flue need a constriction (the damper) to control the air through the combustion processs that ends up coming out the top by stack effect.

Learn how to set the damper - don't fan boost the supply air would be my advice.

Or bin the idea, chuck in a liner and buy a modern clean burn multifuel or wood burning stove

regards

OMS

-------------------------
Failure is always an option
 04 October 2012 10:49 PM
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OldSparky

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surely forcing more air in to the room would increase the draw?

i agree with above sort out the damper.
 05 October 2012 08:47 AM
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broadgage

Posts: 1314
Joined: 07 August 2007

I would consider replaceing the open fire with a modern solid fuel closed stove.
These need less attention than an open fire, burn much less fuel for a given heat output and have a readily adjustable draught for optimal combustion.

With todays concerns re the enviroment it is desireable to reduce fuel use when reasonable.
Even if you are burning carbon neutral logs, every ton of logs saved is another ton for someone else who will hopefully burn less fossil fuel as a result.
 05 October 2012 11:28 AM
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SherlockOhms

Posts: 325
Joined: 05 April 2011

Indeed but, with every passing day, lifes pleasures seem to become fewer and fewer.

You can't beat popping the old feet up in front of a roaring pile of well seasoned oak.

The environment is going to have to take one for the team on this!

S.
 05 October 2012 03:52 PM
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broadgage

Posts: 1314
Joined: 07 August 2007

What is wrong with putting up the feet in front of a log burning stove ? at least as good as an open fire, and no risk of an errant spark burning ones slippers.
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