Does anyone know anything the flow of gases?

Gazz · World Chat Champion Joined: 19 May 2009 Karma :

Does anyone know anything about the science involved where the flow of gases are concerned?

To put things into perspective, I'm ripping out an old open fire at home and replacing it with the same fire but plastering the area around the fire.

The previous occupant had an elaborate stone surround which was build outwards away from the wall which left a considerable hole for the gases to go up.

After my plastering has been done, if I was to put the same surround back in situ it would be closer to the wall; thus reducing the size of the hole for the gases to go up.

The other half is concerned that due to the reduction in the hole size, fumes won't be able to get sucked up the chimney and some would come back into the room.

I however, think that it may flow better than it did before because the chimney would be pulling the fumes upwards because the pressure would be greater than it was before.

We kinda want to get this correct first time, because a lot of money has been spent so far and we really don't want to have to rip it all apart and start again.

** on a side note, I've read a few Wikipedia pages on "Stack effect, flue-gas stacks" and there are a few mathematical equations on them but we are just really looking for a yes or no answer at the whether we are going to get a room filled with smoke the fire time that a fire is lit.

***pics available on request if you cannot understand my description on the situation well enough.

P. S. - the gap before would have been roughly 18"x4", but if we go ahead with original plans will be reduced to 18"x2.5".

Riejufixing · World Chat Champion Joined: 24 Jun 2018 Karma :

Pics. Also, remember what the area of an 8" flue liner is.

Edit: And I suppose it will annoy you if I say "consider a multifuel stove"....

stinkwheel · Bovine Proctologist Joined: 12 Jul 2004 Karma :

Open fires draw better if you obstruct the opening. We used to get ours lit for years by holding a sheet of newspaper across the top half of the fireplace to make it draw.

Pretty sure either your chimney will draw, or it wont. If it wont, it's about the pot, not the hearth.
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Using the flow rate equation:

Q = C*A* √(2*g*h* (Ti-To/Ti)

where:
Q = flow rate in m3/s
A = Flue cross sectional area.
g = acceleration due to gravity (9.81m/s2)
h = Height of the flue.
Ti = Temp inside, initially at room temp rising as flames get hotter.
To = Temp outside, & cooler in winter.

Area for larger flue - 18" x 4" = 0.04645152m2
Area for smaller flue - 18" x 2.5" = 0.02903220m2

Assuming height of 8 mtrs for typical two story building.
Assumed 150 degrees K for inside temp, but I'm sure it would be hotter at times, my flue lazered up at around the 100 degs mark, but was on a low burn.

Gives a flow rate of 0.340m3/s for the original flue,
and a flow rate of 0.212m3/s for the smaller version.

You're thinking of using a flue opening that gives you 63% of the original flow rate.

Would I use it like that, NOPE.

I built my own log burner, and in the early stages of development I thought would be a great idea to get as much heat out of the flue as possible, not realising it is the difference in temperature that creates the lift (hence how hot air balloons work) needless to say it caused issues and a smoky building. I've also noticed on particularly windy days, the flow rate on mine isn't quite enough to overcome the gusts, and makes for a smell building.

Also, worth noting is if the room is airtight, then there won't be any airflow out up the flue, no matter how hot it gets.

I'm hoping one of you guys will run over these numbers to confirm (or not).

Just my tuppence worth.

Gazz · World Chat Champion Joined: 19 May 2009 Karma :

So, what you are saying is that: the bigger the hole at the bottom, the better?

I was thinking that if the hole was too big then it would be like trying to pour oil down a funnel (upside down if you get what I'm saying). And the flow would be restricted.

Regardless, I'll get some photos this afternoon to try and clear this up before I end up making a balls of it.

P. S. The original fire brick surround has been dismantled by one of our Eastern European neighbours so it's kinda guess work on what the fire looked like under the bricks as I wasn't at home at the time that it got dismantled. (pics should make things clearer).

Gazz · World Chat Champion Joined: 19 May 2009 Karma :

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Islander · World Chat Champion Joined: 05 Aug 2012 Karma :

If you want to find out if it draws, put a smoke pellet in the base and light it. If it draws well cold then it'll be even better hot. Thumbs Up

Generally speaking, chimneys are designed to have two air columns inside them which is why they have a sloping ledge at the rear. Hot gases from the fire ascend at the front, cooler air descends at the rear and 'spills' off of the sloping ledge. You don't need a huge open front to get a decent draw.

For illustration, I have an inglenook fireplace in my living room which is huge. In the middle I have a small 5kW multi-fuel stove. The throat of the chimney is closed off with a register plate and the hot gases go up a flue pipe and through a 6" liner. The remaining space in the flue is filled with vermiculite for insulation. All of the air for the stove comes through three slots at the front approx 2 x 1 cm each and those are controlled by a sliding damper. My chimney draws extremely well and to get a roaring fire in the stove I hardly have to have the damper open at all.

In short, it's all about the flue Smile

Gazz · World Chat Champion Joined: 19 May 2009 Karma :

Gazz · World Chat Champion Joined: 19 May 2009 Karma :

Riejufixing · World Chat Champion Joined: 24 Jun 2018 Karma :

Gazz · World Chat Champion Joined: 19 May 2009 Karma :

The problem that I am having at the moment is that by the looks of things the previous owner has built that old fire further away from the wall than it needed to be (the thickness of the chrome surround) which was placed in front of the back boiler. We did not know this until the fire was dismantled.

We also never knew that half of the actual fire (the metal reddish coloured bit) was hidden behind the surround.

And if you look at the top picture, it looks like the hole that we now have was covered in bricks before.
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I've bought the exact same fire (in matt black) but I dont know if I'm going to have to build the surrounding bits away from the wall to replicate the previous owners set-up exactly to keep the hole the same size.

It is possible to fit the fire surround (chrome bit) around the back boiler with a little bit of modification (hammering the top out a few mms and cutting away the inner lips) but once you do this, the damper comes in contact with the chrome surround and basically closes off the opening coming at the front. Whereas beforehand, there was a small gap (same size as chrome surround) even when it was pulled out.

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In addition to this, I don't know exactly how the hole in the wall at the front (if looking directly from the front) was covered by bricks (if it was completely bricked over or the bricks left a gap behind them for the smoke to go up behind them or not) because I wasn't there at the time it was dismantled.

Surely if the hole is there with no bricks in there, the original designers of the house/chimney intended it that way and should be left as free-flowing as possible: so basically if brick need to cover it, then its best to leave a gap behind them.

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Ideally I want everything to be pushed back, (smaller hearth, etc) but I don't want to reduce gaps that need to be large enough for the volume of smoke to pass through.

**One final note:

It wasn't just the fire that was the deciding factor in changing all of this. Although you cannot see it in the first photo I'll try and describe it the best I can: but basically that whole wall was covered in those large bricks, both alcoves were covered in it. We couldn't buy any bigger tv than what we have because it wouldn't fit on the section of marble that was left.

There was a triangular hole which looked like it had been made with a hammer to allow the tv aerial through.

There was pump under the marble which thankfully we have never needed to gain access to.

And there was a rectangular hole made out of the brickwork which I'm assuming had a VHS player in it at one time, but was now left empty and caused draughts.

Riejufixing · World Chat Champion Joined: 24 Jun 2018 Karma :

Gazz · World Chat Champion Joined: 19 May 2009 Karma :

Riejufixing · World Chat Champion Joined: 24 Jun 2018 Karma :

I am a little worried about your nice new plasterwork. If you are going to remove the old fire, and fit a new one of the same sort, how are you going to access the pipework?

Gazz · World Chat Champion Joined: 19 May 2009 Karma :

Riejufixing · World Chat Champion Joined: 24 Jun 2018 Karma :