Fire problems!

[jenrob]

Please can anyone help??? We have got a 5ft inglenook fireplace. We have got a 55cm grate and an 85cm hood, as advised. There is also a 125mm flexible flue liner already in place in the chimney. The fire has a good draw yet it still smokes?

Any ideas gratefully received.

Thanks

 

[Penners]

First question: do you really mean a 125 centimetre flexible flue liner? Or did you mean 125 millimetres?

Is this an open fire? If so, (and if you meant millimetres) I don't think your flue liner is big enough.

 

[jenrob]

Hi

Yes it is a 125 millimetre flue liner (sorry!). It was here when we bought the house and they had a log burner in place. We do want an open fire - do you think this will be compatible?

Thanks

 

[Penners]

No, I'm afraid that's unlikely.

Some years ago I had a neighbour whose chimney was porous (smoke leaked into the upstairs bedroom). So he installed a steel hood in the chimney above the (open) fire and a flexible liner like yours (actually I think it might have been 150mm diameter).

Result? No more smoke in the bedroom, but the room with the fire filled with smoke.

He called in a fireplace specialist, who told him that the liner simply wasn't big enough to carry away all the smoke from a normal open fire. So he took out the liner and (at considerable expense) had the chimney properly repaired. That solved the problem.

That's probably not the answer you were hoping for - sorry!

 

[middi]

Also it must be remembered that open fire, and log burning stoves , must have adequate flow of fresh air into the room otherwise chimeys will not be very effective and will smoke.

It wasn't so important in the bygone era of drughty windows, doors and purpose buit air inlets.

Nowdays we tend to seal ourselves in our treble gazed enviroments.

 

[Mark]

I suspect that the size and shape of the chimney have an effect as well. One of our fireplaces smokes badly until it gets going, unless we open the window!

Looking at the fireplace its a large old opening which has been closed in to make a more 'normal' size opening. But inside the chimney brest and above the opening is a large void and the chimney goes up from one corner of the void. My suspicion is that the chimney and the void are full of cold air and provide an impediment to the smoke from the fire rising up the chimney - so it comes into the room instead. Opening the window seems to create enough extra draught to get things going. The alternative (which I've done in very cold weather / snowing etc) is to put a bit of firelighter on a shovel and hold it up into the void above the fireplace, once the void is warmed up the fire will light OK.

I seem to remember that the Buildinjg Regs part J say that 6" is the minimum diameter flue to use for woodburning so I dont think it is really big enough even with a small stove. 5" is rather small especially for a large fire.

I agree with the previous comments - The flue isnt big enough but I also wonder if the size & shape of the opening isnt helping. How far above the grate is the hood? Does it have a fire curtain? If so can you start the fire with the curtain shut ?

Have you tried lighting a very small fire in the hearth, if so how did it go?

 

[Nemesis]

We had a fire which would only draw once the chimney was warm - smoke filled room until then.

By chance, we had to renew the chimney pot, which was shorter than the other on the stack. We replaced it with one which was slightly taller and now the fire draws so well it burns away rapidly!

On a previous house, for complex reasons the rebuilding of a stack meant it was slightly lower than the previous (by one course of bricks). What was a fire which always drew became one which filled the room with smoke until the chimney was warm.

It's a black art!

It may be that the hood is the wrong design ... all manner of things. It may be that a specially designed cowl or pot helps.

Sadly, there is a no simple answer.


http://www.erringtonreay.co.uk/company_web/er/story4.htm

 

[Lime]

It's a black art!

Not really.
The problem for most of us is understanding the venturi effect that occurs in chimneys and any tube/pipe with a restriction.
At the restriction the air speed increases and, contrary to most peoples' expectation, this causes the air pressure to fall at this point.
This drop in air pressure is made use of when designing chimneys for a good draught.

In most successful fireplaces there is a restriction placed in or around the chimney opening to the fire.
In old open fires this is seen as a bulge at the top of the fireclay fireback.
It has the effect of increasing the speed of air flow at the restriction which causes a lowering of air pressure at that point.
Because there is a slight lowering of pressure, air and otherwise escaping smoke is drawn into the chimney from the room.

With this restriction is the need to have a free-flowing supply of air to the fire, a hot fireback and hot fumes in the chimney to cause lift to produce a successful open chimney.

Hoods and funnel-like chimneys rarely work well unless the fire/chimney is very hot and/or the wind is in the right direction because they lack the restriction.
Similarly with free-standing baskets as they usually lack the hot fireback.

 

[Nemesis]

Actually I still think it is a bit of a black art, and there are rarely simple answers to a cure. So many variables with individual buildings, and trial and error isn't always inexpensive. I do understand the venturi effect, Count Rumford's principles, etc etc.

My fire hasn't a restricted throat as it's a Georgian hobgrate without that type of fireback. A new pot of only a couple of inches higher cured the smoking, it wasn't something we had considered when we replaced.

The other chimney had a Victorian fireplace with a 'modern' fireclay back. The only thing which altered was one row of bricks was removed from the stack. A chimney which worked well suddenly smoked dreadfully.

 

[biffvernon]

The problem for most of us is understanding the venturi effect.

Not just Venturi, there is also Bernoulli to take into account.

 

[Nemesis]

The link doesn't work... is that height to size ratio?

Or am I thinking of something else...

PS it does now, ta!

 

[jenrob]

well i see we have caused a debate!

Thanks for all the replies - i think we will start by opening the window, then look at removing the liner which is too small, and by the sounds of it, throw it all out the window and install a wood burner!!

In seriousness though, we will try with no liner and report back.

Thanks again

Jen & Rob

 

[Mark]

thats probably a bad idea - unless you know that the chimney is in perfect condition ?

How big is the chimney ? even if you can get a 7 or 8 inch liner doen theres no guarrantee that youll get a decent result.

I would go with a wood burner. Check the regs you might be ok with 5 inch - as long as the burner only has a 4 or 5 inch flue - you cant put the stove into a smaller flue. Off the top of my head you would need one with a 4 inch flue if you have a rear flue connection 5 inch might be ok with a top connection - assuming reasonably straight chimney.

 

[jenrob]

Morning all,

Well i opened a window, and still it smoked. I then raised the grate by about 3 inches, and it smoked when i started the fire lighter, then as the chimney got hotter, it stopped! I then added some small bits of wood, and it seemed ok, but then the more smoke there was from the fire it wasnt all being taken up the flue.....

What is the best thing to do? We want an open fire.

Thanks

Rob

 

[Mark]

I think you need a bigger liner.

What sort of wood are you burning though ? Is it good seasoned hardwood or wet pine ? Better dry hardwoods smoke less than unseasoned softwoods.

How big a fire did you build - you could probably experiment to find a size of fire that will work with the size of flue that you have - it just might be a bit small.

All this fluid dynamic theory is OK but it deals in ideal fluids in non-turbulent flow. As you've seen part of the problem is getting the flow going. Once you get the air moving in the chimney things get easier.

 

[Silas]

Hi Jenrod

We have a very similar size inglenooke with hood dog and grate to the one in your cottage, we also had a similar smoke problem, we got our chimney sweep to look at it.

He said that the flue was a bit on the narrow side, he suggested a cowl could be installed on the chimney pot to improve the draw. He also recommended burning good dry wood in combination with smokeless coal, as with sappy wood a narrow flue could soon get a build up of soot and sap thus increasing the risk of a chimney fire. He also suggested starting with a small fire to warm the chimney up.

The combination of these measures has worked for us, particularly burning a small amount of smokeless coal with the wood, once going the fire keeps a more constant slow burn and we just add a small log every 20 mins or so to keep the fire gently crackling all evening.

 

[Nemesis]

It could also be worth considering buying a firebox type of fire to place in the hearth, or building a similar structure from brick, with a fireback. in order to improve efficiency.

http://home.clara.net/steve.thackery/stoves/chimneys.htm


[Open Image] [Hosted by PhotoGalaxy.com]


We had similar probs with a large fire opening, and opted for a Franklin type stove which can be used with open doors. It's very good and as the metal also heats up - much more efficient than simply a dog grate.


http://home.clara.net/steve.thackery/stoves/history.htm

 

[Penners]

What is the best thing to do? We want an open fire

As I said above - your flue liner isn't big enough.

 

[Nemesis]

Yup.