4.5m Flu Height? Why so tall?

[Andehh]

We are fitting a stove in a bungalow, so are looking at the height & angles required.

 

The stove flu (fitted approx 800mm up the wall) will need to go up 600mm ish then angle 45 degrees to clear a lintel in the wall, then 45 degrees again to go straight....and straight up through the roof. All ok - total internal flue length of around 2m.

 

The problem is we will need to then have about 2.5m of flu above the flat roof to hit the height requirement from the supplier! They have then said for every 45 degree angle you need to add 0.5m to it to compensate for the 'air flow' restriction. They couldn't explain why, just explained 'required for a proper burn'.

 

I (engineer by profession, but not fire related) cant see the reasoning behind a 4.5m need for a chimney, nor the extra metre of height (so 3.5m above roof height!!!) to ensure it works properly. 

 

Now, to complicate things.... we designed in a dual sided stove to benefit two rooms, which meant we had to go to 9kw - far more then we will ever truly need in a well insulated house, but 'because ambiance' its what we chose to do. This means rarely will we burn it truly hot (I am aware of deposit build up etc, so we will need to be extra careful on what we burn & the temperatures we use it at.)

 

Stove: 

Stovax Riva Studio 2 Duplex Cassette Woodburning Stove | Stonewoods

 

 

Can anyone explain the need for such a big flu? What does the final 2m+ of the flu achieve in air flow dynamics that the first 3m+ doesn't? 

 

 

 

 

[Carrerahill]

14 minutes ago, Andehh said:

We are fitting a stove in a bungalow, so are looking at the height & angles required.

 

The stove flu (fitted approx 800mm up the wall) will need to go up 600mm ish then angle 45 degrees to clear a lintel in the wall, then 45 degrees again to go straight....and straight up through the roof. All ok - total internal flue length of around 2m.

 

The problem is we will need to then have about 2.5m of flu above the flat roof to hit the height requirement from the supplier! They have then said for every 45 degree angle you need to add 0.5m to it to compensate for the 'air flow' restriction. They couldn't explain why, just explained 'required for a proper burn'.

 

I (engineer by profession, but not fire related) cant see the reasoning behind a 4.5m need for a chimney, nor the extra metre of height (so 3.5m above roof height!!!) to ensure it works properly. 

 

Now, to complicate things.... we designed in a dual sided stove to benefit two rooms, which meant we had to go to 9kw - far more then we will ever truly need in a well insulated house, but 'because ambiance' its what we chose to do. This means rarely will we burn it truly hot (I am aware of deposit build up etc, so we will need to be extra careful on what we burn & the temperatures we use it at.)

 

Stove: 

Stovax Riva Studio 2 Duplex Cassette Woodburning Stove | Stonewoods

 

 

Can anyone explain the need for such a big flu? What does the final 2m+ of the flu achieve in air flow dynamics that the first 3m+ doesn't? 

 

 

 

 

I am going to guess, based on logic taken from plumbing and ventilation, that for each 45° bend you are adding a restriction in the flue, a restriction which I assume slows the passage of combustion gases, therefore adding more height allows a good tall straight run to create some vertical acceleration of the gases and thus create a good draw. 

 

Having seen plenty of stoves in cottages with all sorts of bends and joints, and terminating just above roof level, I cannot see that the extra 3.5m is really needed, but maybe on paper that is indeed what you need - ask them for the flue calcs and get them to show you the workings of it all. 

 

I also find a lot of these so called "experts" have misread regs and guides or want to sell you more than you need sadly. As an M&E consultant myself I see it all the time. 

Edited February 9, 2023 by Carrerahill

[Conor]

It takes approx 3 to 5x the diameter of a pipe for a fluid (or gas) to return to its stable (laminar) condition after a restriction. So if it's a 150mm flue, then allowing 500mm for flow to settle is bang on. BUT it's not strictly cumulative... If you have two or three bends / restrictions you still just need the 3-5x, (maybe up to 10x to be safe) diameter length to "settle" after the last point. 

[ProDave]

Not "advice" but the little WBS in our static caravan worked fine with a total of 3 metres of flue, two 45 degree bends right next to the stove.

 

[saveasteading]

2.5m above the roof will be unstable , so you would need 3 guy lines fixed to the roof.

I agree with Conor. The amount of heat is what will restore smooth flow.

I hadn't heard of the 5 x dia rule of thumb to return to stable flow, but it seems right based on drainage observations.

I would fit what you need for building regs. If it doesnt work you can later add to the top.

There are cowls to assist flow and prevent back-draughts, and i would have that rather than the very long pipe.

[Temp]

Is this going in an extension? How tall is the rest of the house?

 

A more serious problem can occur if the flue is lower than some parts of the building. In some wind directions you get a down draught in the wind shadow of the building. 

 

The normal recommendation is 1m or so taller than the highest roof if possible (rarely is). 

Edited February 9, 2023 by Temp

[nod]

You need to be very careful 

Someone I worked with installed one on a canal boat which went badly wrong He survived but his wife and children didn’t

He worked as a gas engineer 

[Andehh]

Thanks for the posts all! 

 

It's a flat roof new build, so we will go up approx 1 to 1.5m, from roof anyway. Fairly exposed location so will need to look at the chimney pot that goes onto the flu.

 

We will have a clear vertical run of approx 3.5m from the last 45deg bend, which is where my "how much faster/more could the air need to accelerate" query came from. I'll insulate the pipe more just to keep it extra warm though. 

 

We have heat alarms in both rooms, and carbon monoxide alarms. Also a dozen fire alarms throughout rest of the property, Inc corridors between bedroom area and where the fire will be. 

Edited February 10, 2023 by Andehh

[PeterW]

Could always increase the diameter of the flue to compensate for flow rates however it’s not as easy to calculate - any way you can make the 45° into 22.5° as that’s classed as near vertical and less restrictive 

[Andehh]

Thanks Peter! I will raise the 22.5 option with them. I believe our flu will need to be 8" or 10" due to the size of the stove.

 

I have been looking around for a flexible insulated ducting, but they don't seem to exist.

Edited February 11, 2023 by Andehh

[limecc]

Our 5" liner is only 4m long, was not necessary to use insulation wrap but I did in preference top messy vermiculite.

 

In the few weeks since it was signed off, I've never had a problem with draw and I've two 30° bends. Incidentally gas velocity is higher with a smaller diameter, I imagine a 6" or greater flue would hold a larger plug of cold air when trying to light a fire and be more susceptible to condensation and downdraft.

 

My main issue is fumes driving down the roof in certain conditions annoying neighbours. I was going to try a spinning cowl but purchased a WindKat instead. Will see how it goes.

 

P.S. I think it will be hard to burn a smaller fire in a larger stove because you need correct firebox temps to get good efficiency and get the gases away from the roof in cold and still weather. Just my take.

 

Edited February 19, 2023 by limecc

[SteamyTea]

A flue works by having a lower pressure at the outlet than the inlet.

The pressure difference is caused by temperature related density changes.

Flow rates are affected by the ratio of cross sectional area to circumference ratio, which changes greatly during a bend.

Flow rates, or more accurately, mass flow rates, are complicated to calculated as increased velocity can actually stall the system.

 

Much easier to just put a fan heater in.

 

[limecc]

On 19/02/2023 at 11:04, SteamyTea said:

Much easier to just put a fan heater in.

 

Imho it's easiest to stick with the guidelines.

If the stove has a 6" outlet stay with that size all the way to the cowl, 5" stay 5" etc and no more than four bends (max 45° each) per chimney. 4.5m is typically the minimum length. If a rear outlet then there's a max distance of 450mm before the 90° bend and you'd have 2x45° left.

 

http://www.bfcma.co.uk/siteFiles/resources/docs/WoodBurningMultiFuelinResidentialProperties06.02.23.pdf

Edited February 21, 2023 by limecc