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

I have only had 1 twin wall SS flue and it never needed cleaning. The rate of expansion and contraction cycles seemed to be enough to dislodge any deposits. Quality fuel and proper burn temps. produces very little chimney deposits anyway.

4.5m total height flue seems a lot for a single story structure, ask the installers to show the calculations (they are expensive enough without adding extra length). The height needed will depend upon surrounding structures. Flue height regs are easy to find online to do your own estimate, you might find guidelines on the manufacturers data or web site. And don't forget that they are v. easy to add height later if needed.

The the question of how much single wall flue - I would go for single wall up to 45cm from the ceiling then change to twin wall. There are restrictions on how close a flue pipe can be to combustible materials which if my ageing memory serves is about 45cm or 18". Check the flue pipe manufactures data - it should say. If I recall correctly the twin wall SS flue that I had had a limit of 50mm or 2" from combustibles. ( BTW I installed my SS flue DIY in a weekend) Whilst the outer of a twin wall flue gets warm it shouldn't go over 80 - 90 deg.C and usually a lot less. The design will (should) withstand a chimney fire (temps over 1000 deg.C) without the outer getting dangerously hot

<blockquote><cite>Posted By: Peter_in_Hungary</cite>Whilst the outer of a twin wall flue gets warm it shouldn't go over 80 - 90 deg.C and usually a lot less.</blockquote>

That's what I'd read, and experience confirms it. It does get jolly hot, but so does a hot water pipe, and no-one imagines they are fire risks.

I don't see how an uninsulated flue above the roof can waste heat - the heat has already left the envelope of the house, and has to get lost somewhere, from the end of the pipe if not the sides. But I can see that cooling the gases before they pass into the air might perhaps affect the updraught. But then the same must be true of uninsulated pipe inside the room, and I agree the amount of heat given off inside the room is enormous and very welcome at the moment!

Many thanks for all of your replies.

@Peter_in_Hungary – looking back I think it was only one installer who said 4.5m, the others who quoted did not give a length. That being said, I thought regs (or at the very least “good practice”) suggest 4.5m as the minimum for a flue length? Just checked the stove manual too. It states “not less than 4.5 metres” but then goes on to say “Alternatively the calculation procedure given in EN 13384-1 may be used”.

There are no combustibles to worry about near the flue – just plastered walls – so all good on that front.

@philedge – yep, your assumption is correct, I’m talking insulated rigid flue. I have asked the stove manufacturer and their opinion is that for a short run of flue (4.5m) we should have insulated twin wall from the stove top up. However, I do wonder if I spoke to a different rep from the company they might have said different. Plus, their advice on other matters has been rather hit and miss!

Yes, we will be doing all of those most important things like having insulated flue outdoors, burning decent seasoned wood, and using a thermometer. We have our other stove swept annually at the moment and the sweep has always been happy the amount of soot and the “quality” of it.

@Cliff Pope. That all makes sense. So is your opinion to have single wall flue up to where it have to change below the ceiling?

Thanks again everyone

<blockquote><cite>Posted By: greenfinger</cite>

@Cliff Pope. That all makes sense. So is your opinion to have single wall flue up to where it have to change below the ceiling?

</blockquote>
I suppose it depends whether you want the extra heat in the room or not. We find it very useful. The flue slopes at 45 degrees from the top of the stove above the kitchen worktop before turning upwards into the insulated flue to pass through the ceiling and the room above before joining the chimney itself. So there is a section of hot radiating pipe from about chest high upwards. The cats compete for basking on the hottest part of the worktop nearest the stove.

Traditionally a vertical flue would have a metal clamp with 4 or 5 swivelling rods for hanging damp tea towels, soaked socks, scarves etc for quick drying.