# Do you need to use twin flue pipe on wood burner ?



## flanajb (13 Dec 2011)

Still doing the sums on how much it will cost to put heating into my garage.

Am now thinking of just putting a wood burner in as I submitted a building notice last year which included the electrical wiring of the garage and the installation of a wood burner.

The wood burner itself just needs to be a 6kw relax stove ~ £200, but the expense seems to come from the flue pipe and brackets, especially when you start using twin walled flue.

I suspect that from a safety perspective you ideally should use twin flue throughout ?

Just trying to understand this, as single flue is £12/metre and twin flue is ~ £80/meter. So very quickly it gets expensive.


Thanks


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## BigShot (13 Dec 2011)

I can't remember off hand when the regulations changed, but in the last year or so the regulations for fitting wood burners were tightened a bit. Previously you didn't need a fixed vent for anything less than 5kw. Now you do for most (all?) burners.
You may want to check the current regulations in case it changes your plans.

As I understand it, use of twinwall is mandated when you have combustible materials within a certain distance of the flue. (Something like 3x the diameter of the pipe.)

How big is your garage? 6kw will kick out a LOT of heat. Depending on things like how well insulated it is, how big the doors are and so on I'd be considering something smaller - but I have no experience of woodburners in a shop, only for heating rooms in the house, and 5kw makes things really very toasty when it's in full swing in a space roughly the same volume as a 2 or 3 car garage. Again though, that's not in a garage with a garage door on it.


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## henton49er (14 Dec 2011)

Flanajb,

I have just had a woodburner installed at home and was advised the following by my installer:-

Any stove over 5kw output needs to have a fixed vent to the outside (not sure of the area required but it varies with the stove's rated output - my 8kw fire needed a vent of two 40mm diameter holes, covered inside and out by a mouseproof mesh and vent cover). Under 5kw no fixed vent is needed.

The use of a twin wall flue is to prevent water, soot and other resins etc condensing on the inside of the flue due to the temperature difference between the flue and the room. The gap in the twin wall flue is filled with vermiculite or similar. A single wall flue with a woodburner will very quickly corrode with all the gunge that will adhere to it and attack the stainless steel from which they are usually made. I do not know if this is a requirement to comply with Building Regs or just best practice.

Hope this helps,

Mike.


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## Modernist (14 Dec 2011)

You certainly need twin wall where you are close to combustible materials such as going through a roof or ceiling but you are OK with vitreous enamel stove pipe from the stove to within 300mm of the ceiling. In fact it is an advantage as you get a great deal of additional heat from the stove pipe. Adapters are available between single and twin wall and I also recommend going up a diameter into the twin wall.

I used this lot recently who were very helpful and quite reasonable

http://www.fluesupplies.com/


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## Digit (14 Dec 2011)

A single skin pipe will give up heat to your shop, but in the process it will cool, once the gases cool below a certain point they will cease to rise up the flue pipe and 'blow back', thus a twin skin can be longer than a single before fan assist is required.
As for single skin giving more heat into your shop, having recently installed a small stove in my home, I strongly suspect that 6 kW will cook you!
Wheras heat energy is heat energy a 6 kW wood burner radiatess heat in all directions and the 'comfort' effect is much greater than you might think, in a wood working shop I would also want some form of barrier around it that can prevent flamable materials falling against it.
If it's any help I have three metres of S/H 5 inch here you can have if you can arrange collection.

Roy.


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## BigShot (14 Dec 2011)

henton49er":3ht48ptf said:


> Under 5kw no fixed vent is needed.


I'm almost certain that's no longer the case. I've got the relevant approved document here and will have a look in a short while.
We didn't need a vent when our 5kw burner was put in, but the regs changed soon after and I'm pretty sure the vent below 5kw rule went by the wayside.

I'll post back soon.


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## Digit (14 Dec 2011)

Going by the list of materials for insulating his garage Bigshot I would suggest that if he needs 6kW to heat it it's that draughty that any vent would be superflous! :lol: 

Roy.


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## henton49er (14 Dec 2011)

BigShot":8t612isf said:


> henton49er":8t612isf said:
> 
> 
> > Under 5kw no fixed vent is needed.
> ...



Bigshot,

I have no persoanl knowledge - just advising what my installer told me. This was an installation in September 2011 so is quite recent and I was definitely told that under 5kw no fixed vent is needed (installer could be wrong of course!!).

Mike.


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## Digit (14 Dec 2011)

Same here, same date, same info.

Roy.


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## BigShot (14 Dec 2011)

OK... effective since 1st of October 2010:

In modern (relatively airtight houses) you need 300mm² of ventilation per kilowatt up to 5kw and then 850mm² for any rated output beyond 5kw.
For even more airtight houses you need 850mm² of ventilation per kw of rated output.

In older houses it depends on how well sealed against drafts it is. Full double glazing, filled cavity (or solid) walls, draught excluded doors and letterboxes, controllable trickle ventilation, sealing around pipes, cable entry, loft hatch and suspended ground floors and the like are likely to lead to needing to meet the above requirements too.

It's not as simple as it once was (when there was none needed below 5kw) it's not necessarily the case that with low output stoves you don't need permanent ventilation.
If you've improved your home to reduce heating bills pre-installation of wood-burners, there's a chance it needs permanent ventilation no matter the rating.

It can be that the whole house is very air-permeable, but the room with the stove is relatively airtight as a result of sealing and double glazing. In that case permanent ventilation is needed.
The relevant appendix (F) in Part J says if in any doubt you should fit the permanent ventilation or seek specialist advice to ascertain the air permeability that applies to the room/appliance in question.

(It's been a while since I needed to read Part J... not simple stuff!)


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## Digit (14 Dec 2011)

> not simple stuff!



Tell me about it!

Roy.


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## flanajb (14 Dec 2011)

Given all the hassle associated with installing a wood burner, I think I will stick to my original plan of putting a cheap 2nd hand LPG boiler into the garage.


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## Chrispy (14 Dec 2011)

Slight change of problem for you to think about, I burnt through two twin wall flues in three years, they are only rated to a certain temp sorry can't rember what, now I have had made up the first 2m in 10mm mild steel pipe 150mm dia I hope that lasts a bit longer! but at least it make a lovely extra radiator.


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## BigShot (14 Dec 2011)

flanajb - anything you put in the garage that burns anything will have regs attached.
If you can get a HETAS (I think) fitter sign your wood burner off without vents you're all set (and frankly I think a woodburner in a workshop is a fantastic idea - you're not going to be short on fuel!).
If the vents are in the right spot you shouldn't get drafts (and if you do you could cover it in such a way that an insurance adjuster wouldn't know in the case of a fire - hint... I've heard a rumour that wood is flammable and may burn away in case of a fire - but I didn't tell you that - I also hear that as long as the vents are clear when an insurance adjuster takes a look they wouldn't be any the wiser - but I didn't tell you that either).

The only reason I bring the ventilation issue up at all is that if you add a burner (LPG, Wood, whatever) or whatever to your house and it doesn't have building regulations approval - you may have an issue with your home insurance if the worst ever happens.

If you have no intention of having it regularised anyway I'd say it doesn't matter a jot. If you're went with the 4kw burner and didn't get smoke coming into the workshop there's enough ventilation and you've got no trouble (unless the worst happens and you need to make a claim on your home insurance, that is.)


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## les chicken (15 Dec 2011)

I have a Relax R2 in my garage/workshop, it is situated about a foot away from the outside brick wall with a straight flue up through the roof. The first two parts are single skin vitriol and the last piece galvanized with a chinamans hat. The connection through the roof tiles is with the available tile. Make sure the flue is equal between the rafters.
I am actually on my second stove as I have been using this type of wood burner for about 7 years, the first one rusted at the flue joint after about 5-6 years due to a bad joint at the roof tile.
Burns sawdust, off cuts and logs and heats the workshop up in about twenty minutes from cold. I would not be without it as there is a nice dry heat and also keeps a cup of tea or coffee nice and hot, also very useful for burning private papers.

Les


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