Single Wall vs Twin Wall flue for section inside house? Pros and Cons...

[Krome10]

Hi

Back again with a second wood stove query, if anyone would be kind enough to share their thoughts please...

We're planning to have a stove fitted into a single storey room. The stove has a 5" flue. The installers who have quoted said they think the flue will need to be a total height of 4.5m. It will be a straight run, no bends. If there were to be a single wall flue in the room up until where it has to change to twin to pass through the ceiling, the single wall section would be around 1.6 to 1.7m. The rest would be twin wall, although most of it would be outdoors.

We only burn well seasoned hardwoods.

From what I've learnt, the advantage of single wall would be more heat to the room. One installer said he thinks for each 1m of single wall flue you get 1kW of heat, which would be a great bonus. I'll also admit a bias - I think the stove looks so much nicer with the single wall 5" flue. I've only found a couple of photos online where the same stove has been installed with twin wall and to be honest I think it looks f-ugly. Also, more space on the stove top for a kettle etc would be an extra bonus.

However, I've also learnt that the advantage of twin wall is the flue maintains a higher internal heat, creating a better draw, which results in a stove that is easier to start, runs better, and a flue that doesn't block with creosote so much. (Although I should add that some say they have none of these problems with single wall).

It seems to be a very divided topic, with strong opinions on both sides. While I want the aesthetics and extra heat to sway me to single wall, I also don't want a stove that is tough to light and doesn't run well.

So over to you guys for some thoughts and insights...

Many thanks

 

[Jameshow]

Might be an issue if you burn resinous softwoods as the resin will condense on the cooler single wal but for hardwood I doubt it's an issue?

 

[niall Y]

I would go for single flue in the room, only switching to double flue at the transition through the ceiling and upwards, as has been suggested. You will get the benefit from the extra heat in the room coming from the flue. I don't think that the other considerations are necessarily worth worrying about for such a short length of single wall.

What is probably more important is easy access to cleaning the flue which has to be done regularly. The stove I have in the workshop is more easily cleaned from above by passing a brush on a rope down and pulling it into the stove. It is single wall throughout and needs cleaning every year.

The one indoors has a double flue that passes through two stories and can only be cleaned from below. This necessitates removing the metal baffle plate in the stove, which can only be done by first removing the insulating cheeks . These invariably fall into pieces and have to be replaced either in full or in part. So, I'm happy if I can get away with a proper clean of the flue every 2/3 years only.

 

[Eshmiel]

Best do without a wood burning stove as they damage health in pernicious ways.

But this to the side.

As you mention, a double-wall flue will keep the hot gases from precipitating out more tars and other gunges on the inside of the chimney, which deposits can become a chimney fire cause. Even dry hardwoods will eventually deposit such stuff if the smoke and gases cool too much before exiting the chimney top. Single skin chimneys cool rapidly as they heat up the house air.

Another danger of a single flue in the house, as I can attest from bitter experience, is that it can impart a serious burn if you inadvertently touch it. Fall against it in such a way that it takes a second or three to get off it and you'll probably be in hospital.

But a single flue can, as you say, impart an immense amount of warmth. I once rented a cottage with a tiny coke-burning stove in the cellar-kitchen. The chimney passed up through another three floors (single room per floor) and heated the whole house with not much coke. It was a 4 " diameter cast iron chimney so got very hot. It was this chimney that taught me about how well such a thing can sizzle a human. It also had to be cleaned about every three months, even though coke burns very clean.

 

[Phil Pascoe]

Might be an issue if you burn resinous softwoods as the resin will condense on the cooler single wal but for hardwood I doubt it's an issue?

Not really - the single wall is where it's hottest.

 

[Phil Pascoe]

I believe it's usual to have a sigle skin for the first length. Excuse poor photo.

 

[Spectric]

You have to comply with the HETAS regulations and the install is notifiable. We have just had one installed and used single wall from the stove into twin wall, the change from single to twin has to be 450mm below the ceiling but to make this transition more appealing you can use one of these from Dura flue,

https://www.directstoves.com/dura-flue-single-to-twin-wall-conical-adaptor-in-black-150mm.html

In fact I would use all Dura flue twin wall pipes as they are really good quality with sound connecting bands. Our stove fitters said that using single wall from the stove does allow more heat to radiate into the room, using twin wall you get no heat from the flue so lost heat. Our flue has a total length of 4.6 metres and has a good draw, there were no concerns over flue gas temperature and single wall does look nicer.

although most of it would be outdoors.

I assume you have a flat roof ?

We are in a bungalow and have only enough flue to clear the roof apex. The regs also covers these dimensions.

When looking for parts there are some really flimsy brackets around, we used these and can say they are really solid and well made

https://www.directstoves.com/dura-flue-roof-support-in-stainless-steel-200mm.html

 

[Eshmiel]

Not really - the single wall is where it's hottest.

The single wall is hot on the outside, where it loses heat rapidly (and burns the unwary toucher). With a double wall chimney, the great majority of the heat stays in the chimney, so the inner part is hotter than would be the inner side of a single skin chimney with the same fire beneath it. This being so, the double wall chimney will see less tar and other solids precipitate out of the gas and smoke on to the inner chimney walls than would be the case with a single wall chimney.

Double wall chimneys generally have insulation between the outer and inner chimney walls, which keeps the heat inside and prevents accidental chimney touchers from getting their person quick-fried. They also stop the chimney heating up any chimney-claspers or the 'oles that the chimney passes through in the house walls/roof.

 

[Krome10]

Many thanks one and all for the helpful replies

@Jameshow – just well seasoned hardwood here. If we ever were to use softwoods, I would make sure it’d been seasoned for a good few years, and would also make sure I burn it alongside hardwood.

On our existing stove, I often turn the stove up full whack when I put the last log on, in an attempt to clean up the inners of the flue

@niall Y – That’s good to hear. The more people who think single wall would be fine, the better! It’s a tough choice when it comes to form over function, and this single wall vs twin wall all-the-way debate has certainly got strong proponents in each camp.

The stove we’re planning to get has easy access for sweeping. Just have to remove the baffle plate, which can be done one handed and without having to remove any firebricks nor anything else first.

@Eshmiel – Thanks for the safety warning. It’s just the two of us in the house, no kids, and a dog that’s short arse enough that the flue will be well out of reach. The stove will be in a corner and has a reasonable depth to it. And the flue outlet is quite near the back of the stove. So with all that in mind, I feel the flue will be tucked well out of the way, and the stove itself is probably more of a risk.

Regarding your other points, they summarise the pros and cons of each position well. Which doesn’t make deciding any easier!

@Phil Pascoe – Yes, I agree, you see that a lot in stove installations. Tbh, I’m not that keen on the single going into twin below the ceiling, but still prefer the look of that compared to twin wall all the way

@Spectric – We will be using a HETAS installer. Thanks for sharing your experience (and for the links). Good to hear you’ve had no issues. So does your single wall go into twin wall 450mm below the ceiling? Do you have high ceilings or standard/normal?

No, we have a pitched roof.


Thanks again for all the help everyone.

Any further thoughts from forumites always welcomed

 

[Spectric]

So does your single wall go into twin wall 450mm below the ceiling? Do you have high ceilings or standard/normal?

Yes that is a HETAS requirement to do with single wall pipe and combustables, single wall has to end 450mm from the ceiling and become twin wall to transition through the ceiling, you used to get a very ugly transition before you could get the conical adaptor but it now looks ok. Our ceilings are normal height.