The internal diameter of a flue system is one of the most crucial measurements that needs to be considered when planning your installation. Go too large and a draw may never get going or worse, it won’t fit down your chimney. Go too small and you restrict the flow of gases, cooling them down causing all manner of performance and safety issues.
Fortunately, this isn’t left to chance and the diameter your flue system must be is dictated by two things; Firstly, the diameter of the Single Wall stove pipe will match that of the outlet collar on the stove (eg. 5” outlet collar needs a 5” stove pipe). Secondly, from this point onwards the Building Regulations are very clear on the size you must be in for the remainder of your flue installation no matter if it’s Single Wall, Twin Wall or Chimney Liner:
“Closed appliances of up to 30kW rated output burning any fuel 150mm diameter or square flues having the same cross sectional area”
This clearly states that you are required to attain a minimum internal flue diameter of 6” (150mm). But what if the outlet on your stove is smaller than this? Not to worry, this is quite often the case, and you are simply use an Increasing Adaptor to get from 4” or 5” stove pipe to 6” Twin Wall or Flexible Chimney Liner.
There is, however, an exception to this regulation for most modern stoves – If your wood burner is DEFRA Approved or EcoDesign 2022 Ready, then you can install a smaller 125mm (5”) internal diameter flue system:
”Closed appliances of up to 20kW output which: a) burns smokeless fuels or b) is an appliance which meets the requirements of the Clean Air Act 125mm diameter or square flues having the same cross sectional area”
This permits a minimum flue diameter for a DEFRA or EcoDesign Ready stove of 5” (125mm) and does not require the increase to 6” due to the cleaner burning nature of contemporary stoves. Please click here for our full range of DEFRA Approved stoves.
Below are a few basic rules for your flue pipes:
- The diameter of your stove outlet is the minimum diameter your flue system can be
- In no circumstance should you ever reduce the diameter of your flue system as it travels up
- If you have a flue system already in place and do not wish to change it, that will influence the stove you can install due its outlet collar diameter
- The diameters quoted in the Building Regulations are minimum diameters only
- If your stove outlet is 7″ or 8″ in diameter, then your flue system will have a 7″ or 8″ internal diameter. Will this fit down your existing chimney?
- Open fires require a minimum 8″ (200mm) diameter Chimney Liner, typically connected to a Gather Hood
- DEFRA and EcoDesign Ready stoves burn cleaner and / or burn smokeless fuels which leave less soot and tar deposits hence the reduced diameter flue required
I am installing a open log fire in an inglenook fire place and I am not sure on the flue diameter the flue length will be 6.5m long
Minimum flue diameter for an open fire is 8″ (200mm). We supply our Helix Premium Flue Liner and all compatible components in 8″ diameter in the link below:
https://www.glowing-embers.co.uk/ChimneyAndFlues/FlexibleChimneyFlueLinerFittingKits-UnderConstruction/8200mmFlexibleFlueLiner
Are there any benefits to a larger flue diameter? We have a 7” single wall coming from the wood burner, and this increases to 10” double wall (10” internal diameter) at ceiling height, through the first floor and then out through the roof. Would it be better running 7” all the way? Thanks
There is absolutely no need to increase from a 7″ flue system (already particularly large and only required if your stove features a 7″ diameter outlet) to a 10″ Twin Wall system! 7″ internal diameter flue all the way will be sufficient.
Hi ,is there any way I can connect a 5 inch flue liner to a 6 inch outlet as 5 inch is all that will fit down the chimney, I would be very grateful if you could get me out of this situation
Joe
Unfortunately, as your stove features a 6” flue outlet, that is the minimum diameter flue system that can be used. The flue diameter cannot be reduced as this may have an adverse effect of the draw within the flue and therefore the efficiency of the fire. I would suggest perhaps purchasing a fire with a 5” outlet collar, or even discussing this with your HETAS installer who may be able to advise on an alternative route for your flue.
That does depend on the brand/manufacturer. 5″ is used as an industry standard term for the internal diameter but this can measure 120mm, 125mm, 130mm as you have seen. Each brand will show you what their pipe measures precisely and ours are all 125mm.
Is there a benefit to increasing a flue from single 5″ to 6″ twin wall before it goes through the roof? I’m asking because the installer that quoted for our wood burner is recommending this is done.
The only reason it would be required to increase from 5″ single skin pipe to 6″ Twin Wall flue is if your stove is not DEFRA approved (and has a 5″ diameter outlet). In that case it is a building regulation requirement to attain a minimum 150mm internal diameter for the flue system. Otherwise, you are fine to stay in 5″ (125mm) flue the entire way (if DEFRA approved).
I have small multi fuel/log burner system
3kw only going From 3 to 4inch not Defra approved on a boat
Would be good idea go to a bigger twin skinned pipe of 5 inch
The smallest internal diameter Twin Wall we supply is 5″ (125mm) for compliance with HETAS and Building regulations. You can convert to this using a 4″-5″ Increasing Adapter (Single to Twin).
Hi I have a 150mm dia stove outlet but my brick chimney reduces half way up down to 100mm by 250mm so a 150mm flue can’t slide through it . It is currently an open fire place I was hoping to put a wood burner in for better efficiency but I’m thinking it can t be done, A flue liner ovaled to fit if force down I’m thinking would not be a good option
Is this a can’t be done situation?
Unfortunately, I am afraid you are correct. You cannot disfigure the shape of the liner in this way as you risk damaging the seams and thus voiding the warranty and the strength of the liner. You would be able to have the stove but you would need to look at the option of installing a Twinwall flue system with the stove instead.
Hello
I have a sawdust and wood burning stove in my large workshop that has a single skin 6 inch flue going through a corrugated metal roof 3m up.The flue currently goes through a high temp silicon collar should the flue be changed for double skin insulated one. If so is there a fitting to go from single to double whilst retaining a 6 inch external diameter. I really don’t want to have to replace the whole flue!
Albert
I would highly advise you do indeed convert from your single skin stove pipe to twin wall insulated flue a minimum of 150mm before passing through that roof. This can be done using the 6″ Single to Twin Adapter (available in stainless or black finishes) and a run of 6″ Twin Wall flue through the flashing and up to a rain cap to terminate the system. Wall Brackets of a Roof Support can be used to stabilise the system.
I have a 4″ opening on my Defra approved stove. Is it okay to therefore use a 4″ flue pipe?
You can start in 4″ stove pipe but then need to increase to 5″ diameter for the Flexible Liner or Twin Wall flue. This is easily achieved using a 4″-5″ Increasing Adapter (to Flex or Twin Wall).