Jon Kerr

Forced Air Flue Questions

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Hi everyone,

Firstly, apologies as usual for my newbie questions.

I'm not trying to cut corners here, and I've done my research, but I'm not sure that the "recommended" solution is going to work for me for the reasons given below....

 

I'm in the process of building a new shop, and am now agonising over my flue....

My issues are as follows:

  • Height of the chimney. I want to keep this to a bare minimum which is obviously contrary to the "ideal", which is as high as possible! Im concerned about local planning permission and dont want to bother the neighbours.
  • Cost. I need to keep costs down (doesn't everyone?). I already have a couple of metres of twin wall 8" flue so it would be great to make use of that if possible.
  • Running flue through the roof worries me given the rainy british climate, and all the associated flashing, storm collars, etc are EXPENSIVE.

Having done my research on the "super sucker" hood, and taken advice of those on this forum, thats pointing towards a 12" single wall flue running straight through the roof and standing 4feet proud of the highest point of my building. I just don't think I'm going to get away with such a monstrous flue which planning permission or my neighbours.....

(For info- my shop is 3.5m x 5m, log cabin construction, with two windows and a double door which will be opened when forging.)

 

I did some more digging and found these images of gote's chimney. Unfortunately it looks like he's been inactive on this forum since 2017.

Can anyone give me any advice on this type of forced are flue?

  • Will this work with a side draft hood? Will 8" be acceptable in a forced are system?
  • Any guidance on the type of shop vac/blower needed?
  • Could I achieve the same with a shorter length of flue?
  • Can this style of flue terminate below roof level thanks to the forced air?
  • Any pros/cons of this style of flue? Any design guidelines?

I'm wondering whether I can do the same as Gote. i would keep a similar height of the initial pipe, but the horizontal outlet would only be 1m to the wall. Would this still work?

If it works, this would be perfect as it won't bother the neighbours, and I can use the 2x 1m lengths and 2x 45degree connectors of 8" flue that I already own.

 

 

Gote1.jpg

Gote2.jpg

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Consult your local building codes for information regarding flue terminations and penetrations of a structure built with adjacent combustible materials.  You may be able to "get away with" some of the options you are proposing, but in the event of a potential fire, if there were any questions regarding the installation your insurance may balk about covering any losses.  I don't know how things go in your area, but depending on whether you are burning coal or coke, the neighbors may object more to the smoke and smell than an unsightly stack rising the correct distance above your roof.  Note that the IMC lists 3' above the adjacent roof as far as I know, not 4'.

I have no information on the type of induced flow system that you show in the photos, other than theoretically it should work (in a fairly similar fashion to the linear induction that takes place with NA burners using high pressure gas flow to entrain combustion air).  Personally I'm more a fan of systems that don't require a loud blower to properly vent a hood (though this design keeps the exhaust fan out of the hot flue airstream at least, which is a defect in most other types I've seen proposed), and the super sucker design seems to work very well.  Please take a look at the original blueprints on the site from Uri Hoffi.  If I recall correctly he has his stack penetrating a side wall , then running  exposed and vertical on the side of the building

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Latticino- thanks for your reply!

My local building codes for a flue assume you are using a high temp log burner or similar. As such, they insist on a twin skinned (and expensive) flue.

Going by advice on this forum, thats completely unnecessary for a forge but I dont think theres any getting around it. I either build to code (twin skinned with specified distance to combustibles) or ignore the code and accept the insurance company might throw the book at  me in the event of a fire.

As it stands, I have 8" flue available to me already so if I can make that work somehow (rather than switching to single wall 12") then all the better in terms of satisfying local building codes.

 

When I say "Get away with" I'm more concerned about local PLANNING permission rather than building regulation. Its very unclear to me according to info online what the laws are regarding installing a flue to an outbuilding, and the allowable height of such a flue. I've asked the local planning office who refuse to help unless to pay for, and submit, a planning application. Very unhelpful.

My worry here is that I put a hole in my roof for a flue, buy all the appropriate (expensive) weatherproofing, a neighbour complains to the council and I have to pull it all down.

If I install a 12" flue to the recommended height I'm not sure my neighbours, or the council, will be happy. That may be the way I HAVE to go, and just cross my fingers that nobody complains, but I'd like to explore other options.

I've looked at Uri Hofi's side draw hoods and they look great, but they do still have very tall chimneys to get the pressure differential and encourage draw. His photos on the blueprint are for a very tall building! Mine is only 2.5m to the top of the pitched roof.

I'll be using a coke forge so smoke should be less that charcoal/coal, right?

Anyone else got any thoughts/experience with a forced air flue system like Gote's?

 

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Chimneys are tall because that is how they work, not because its fashionable.  A fan can help draw the smoke into the flue, but it also helps to get the smoke away from your shop when its up  and out of the way.

If you have the proper permits for building it, you should be fine, and not have to worry about complaints later.

HVAC is a licensed trade, Many places allow a home owner to perform their own work, but if  you want to do the job yourself, you should know what you are doing.  If you need someone to explain it to you,  why should they have to train you for free? do you still do your work for free when some one doesn't want to pay for it?

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19 minutes ago, Steve Sells said:

Chimneys are tall because that is how they work, not because its fashionable. 

Of course. I'm not interested in fashion- only finding the best solution balancing performance, safety and my neighbours. I'm simply checking if theres an alternative as the one in the picture above looks very different and worked well according to the other thread.

20 minutes ago, Steve Sells said:

If you have the proper permits for building it, you should be fine, and not have to worry about complaints later.

In the UK (no idea about elsewhere) we can build to a certain extent under "permitted development". No permits needed, as such. My building, so far, falls under this. I'm only having trouble finding out what the rules are for flues on outbuildings, locally.

22 minutes ago, Steve Sells said:

HVAC is a licensed trade, Many places allow a home owner to perform their own work, but if  you want to do the job yourself, you should know what you are doing.  If you need someone to explain it to you,  why should they have to train you for free? do you still do your work for free when some one doesn't want to pay for it?

Indeed, although this forum is full of people generously sharing their hard-earned knowledge and experience for free. I'm only asking.

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IForgeIron represents over 150 countries, and you have already referenced information from Israel, Sweden, USA, etc It would be best to contact someone in UK (your location) for advice. A HVAC person should have some information you can use, as well as what you can and can not do or get away with. Blacksmiths in your area are another good source of information. 

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Jon: It's getting harder to make suggestions the way you keep jumping around, mostly to why they might not work. We don't live near you so we don't know. A HVAC, Code or such officer isn't likely to give a home builder much advice on how to build a stack. If you follow their advice and it turns out there was a factor you didn't know about or you did it wrong and (to pick the worst case scenario) you burn down the neighboring orphanage. Guess who will be named in the law suits? Of course the amature  builder will be on the hook but when it comes out he did it how a professional told him, that professional will be on a bigger sharper hook. 

If you're going to go with a fan boosted flue you want one that creates a vacuum inside the shop. Positive pressure in the flue WILL cause any leaks to eject smoke. Negative pressure in the flue will cause any leaks to draw room air into the flue. That make sense?

The reason flues are taller than the building is to keep them above the turbulence zone where pressure fluctuates and flow directions change. You can see this kind of turbulence when you look at a stone in a stream, as the water flows over the stone it forms a horizontal and very sloppy vortex on the down stream side. This exact same thing when a breeze flows over an obstacle like your: house, shop, fence next door, neighbor's house, etc. large trees don't make strong flows as they're largely flow through so the turbulence isn't a vortex.

A tall flue clears turbulence while increasing draw to the inlet, a side draft hood for conversations sake. The reason they work so well is the inlet opening is relatively small and close to the fire so it gets hotter and the smaller the opening anything flows through the faster it goes. This is why you see horizontal flames being sucked into a side draft hood. You want the shortest possible horizontal run you can arrange. Horizontal runs just let the flue gasses cool without providing draft, (horizontal smoke stack BAD)

The one Gote built is a horrible design and wouldn't work at all without a forced draft. All the vertical is BEFORE a much longer horizontal section so the area it's cooling fastest is under positive pressure, leaks smoke. To make it work at all he had to use a T to change from horizontal to vertical flue outside the wall and put a strong blower under the downward pointing arm of the T. The blower had to have a nozzle that extended above the junction to keep from blowing smoke back into the shop. 

The T and blower outside his shop is the ONLY thing that makes it work at all. Don't do that!

Also, the taller the flue the less smoke your neighbors will smell though coke let alone charcoal aren't very stinky. Charcoal makes sparks to you may need a spark arrester to meet fire codes.

Who to ask? Maybe take a local fireman to lunch and JUST talk? Do NOT tell him you're building one or ask too specific questions. Just chat and bring up the subject. He can NOT render a professional opinion without being at risk of being liable if something goes wrong. He can however just chat about things, maybe even make sketches. 

Ask friendly leading questions about general fire stuff. Say why your great aunt Betsy used to talk about how her old stove smoked at a flue seam. Ask why flues have to stick up so high. etc. etc. Talk about what interests him too, his kids, hobbies, etc. You're just having lunch and a friendly chat. 

Another problem you're making for yourself is doing it on the cheap. Every project is time and money, less money means more time and visa versa. Not wanting to spend the time to research regulations and the reasons for doing things just so means you'll have to pay someone else to do it for you. That's just life with which we all have to deal.

Frosty The Lucky.

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Thanks for the detailed post as always Frosty.

Sorry to be a PITA with these threads. :( Its difficult without knowing anyone locally who has any experience. I'm trying to design and build a shop, forge, flue etc without around 3hrs total using a professional setup and its really difficult..... but I cant continue with the hobby till I resolve it. Everyone's advice is invaluable and greatly appreciated. I'm not trying to cut corners- I'm trying to do my research to improve my designs, ensure safety and yes, to an extent, save cash where possible.

Your comments regarding the pictured design tell me everything I need to know.

I'll go with the standard 12" flue and ensure I meet local building regulations.

Thanks,

Jon

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 Jon, read through this as well.  a LOT of the same questions with answers plus the way I did mine. 

   Todd

 

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It is pretty obvious posts that your first stop is with your city/county officials and present them your ideas. 

When you know what you have to do, come back for advice on what we may think is the best.

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On 5/16/2019 at 4:07 PM, anvil said:

It is pretty obvious posts that your first stop is with your city/county officials and present them your ideas. 

When you know what you have to do, come back for advice on what we may think is the best.

Indeed- I have approached them and been told they dont respond to enquiries.

You either have to submit a planning application (not applicable since I'm trying to operate under permitted development), or consult a local 3rd party company for advice (again, hundreds of £).

 

As I said I'm going to go with the hofi style 12" flue- at least it adheres to the guidance on places like anvilfire in terms of draw when conbined with a supersucker. The only concession I'm considering is whether I can use adjustable length flue so that I can raise and drop the chimney height whenever I use the forge, to keep my neighbours happy.

 

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5 hours ago, Jon Kerr said:

I have approached them and been told they dont respond to enquiries.

What a dilemma!  Flue size is good, and i have zero experience with an adjustable flue.  Good luck

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Your best bet on information if you are still looking is to search online for the code book for your location, you will be looking for residential code not out building or forge or anything else since we are a anomaly within the system, if you adhere to the residential code and permit it so one has a right to complain if you just adhere to the residential code you should be safe then take the advice of these other gentlemen and increase size accordingly to make it work right. If you lived in my area I could help more with code especially over lunch, also the local library should carry building code books, they are hard to read and find the right info so good luck

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