Filling old chimney flue with limecrete type mix?

[Lithgae]

Hi all,
Story is that chimney is being rebuilt at the moment by a stonemason. The stone is thin in places, around where the flue passes along the gable wall. A proposal has been made to:

1. Drop liners down the flues for the lower two fireplaces, needed for one which is going to be used, just in case wanted in the future for the other (I'm working on the premise that I won't want fires upstairs) and these are already closed off.
2. Seal off at top of fireplace builders opening
3. Pour a mixture of X (lime mortar/vermiculate or leca etc?) into the stack from the top which will set and bind everything together.

There are plenty of examples on the web of loose fill being poured into flues around liners for insulation reasons, but not so much about doing it with a wet mix for structural reasons, which is why I am asking if anyone has any experience of this? Just trying to evaluate the pro's and con's.

Secondly, if is a good idea in principle then what should the mix be? As an old building I am presuming that a breathable/flexible lime mortar mix would be better than pure cement. I also understand that perlite, vermiculite and leca are commonly used, from what I have read leca is standing out as the better option, and is used with lime mortar to make limecrete for floors, but I haven't seen anything about limecrete being used in chimneys.

Has anyone had any experience of filling a chimney flue in this way?

Regards,
Kenny.

 

[Zebra]

Hmm. Interesting. I don't know. Questions which occur are: will the liner react with the lime mortar? Will the liner ever be replaceable thereafter? Will the insulation around the liner be sufficient for any future fireplace, to prevent the hot gases damaging the flue? An alternative idea, would it be possible to pour the mix around a larger flue, then line it with a smaller one and pour vermiculite in between? Keeping the structural layer and the insulating layer separate.
As for the mix, I reckon I'd be tempted to buy a bag of various things and give it a go with dropping some into a hole or tube and seeing how hard it sets. I'd be inclined to try some NHL. I can't imagine the mix having much strength seeing as you won't be able to tamp it or compress it, but it can't hurt, as long as it's lime. I can see why you would want to avoid cement.

 

[noj]

Not exactly, but a PO did have the flues here filled with a poured liner - no consent so far as I tell (I guess it was http://thermocrete.com/technical-specification or something similar). Once it's done there's no going back but it does seem to work and there's certainly a track record for the sort of thing you're trying to do.

A very lightweight aggregate and a suitable binder should seal everything together and add structural stability assuming the very bottom of the stack can support the extra weight. Not heard of lime being used as the binder but it ought to work for a complete fill rather than a working flue - and what about hempcrete as an option?

 

[fernicarry]

Don't flexible liners have a lifetime of 25 or so years depending on the quality of the steel and what you send up it? Number chosen at random on the internet, not sure what sort of frequency of usage that's based on. So I guess you'd need to be confident that whatever you pour around it is well enough packed that it remains gas tight if or when the liner fails.

One of our flues is prone to water ingress when the wind is in the right direction and also lets a bit of smoke into the loft so would be interested in how you get on.

 

[Flyfisher]

The flexible liners mainly come in two stainless grades: 316 and 904 with 904 having the longest life but I don't know how they work out lifetimes or what the duty cycle would be. 316 types often have 10 year warranty and 904 types often have 25 years.

But given that they are 'corrugated' then I reckon it would be practically impossible to remove/replace one that had been surrounded by any form of solid mixture. If it must be done then Zebra's idea of a larger outer liner sounds like a good one. Might be a good idea to talk to a stove specialist, they will likely have come across all manner of chimney issues.

 

[TheForge]

I don’t have the details to hand but there is a system to do this with an inflatable ‘flue’ that is then deflated and removed after pouring once the mix has set. A retractable liner can then be installed.

 

[88v8]

If the concern is the thin stonework, and there is room to 'thicken it up' with a poured fill, then why not just thicken the stonework?

A poured fill is a one-way ticket. No renewing the liner. And there's no way to tell how much will really reach the bottom, or whether it will just form a blockage part way down so you end up with the top filled and the bottom not. Anyway, a mix that will pour won't add much in the way of strength. Especially, as has been said, if it's uncompressed.

I wouldn't exclude the possibility of fires upstairs. In our previous house we installed a gas-coal fire in the bathroom, it was lovely.
And you'd not only be depriving yourselves of that possibility, but also future owners.

Ivor

 

[Lithgae]

Hi Folks,
Sorry for my delay in replying. I don't seem to have been getting notifications, and as it was taking a while for my posts to be approved I stopped checking in on it. Hopefully below will answer most of the queries above.

I got in touch with Dupre minerals who supply Leca and vermiculite, and they recommended a vermiculite/cement mix, and had a standard procedure for this. I wanted to upload the PDF they sent me but this forum won't seem to accept PDF attachments for some reason. Basically it is a 6:1 Vermiculite to cement mix that is poured around the outside of the liner. The reasoning is just to firm up the flue further down to stop any more sand or stone coming down.

I was thinking of NHL, but the stone mason thinks that cement would be better, and this is also in line with the instructions from Dupre and what has been done in the past. It is such a weak mix that the normal reasons for not using cement don't apply.

Our plan is to use 8 inch liners, so that if needed a 6 or 7 inch liner could be pulled down the inside as a replacement, however if a flue was to fail there would be cement/vermiculite around it, and the original stone flue, so I don't see this as a major concern.

 

[88v8]

Well I suppose it won't do much harm. The problem with a loose liner - and the 8" is a good idea, as you say a 6" could be pulled inside it - is that the liner will flop around inside the flue, lean unpredictably against the side of it, so the thickness of the poured surround will be random and there is no certainty that the area of concern will be strengthened at all.

Ivor

 

[MatthewC]

I've been trying to work out why this leaves me a bit worried. I think it's the "doing something irreversible" which never sounds good on an old property if you're not sure that you understand the proposal fully. I just don't like the sound of a solid lump (of some considerable weight) filling the gap between the external wall and the liner - there is a danger of it spewing out under pressure as you put it in, and taking a lump of chimney with it!

I'd like to know why the stone mason recommends using cement instead of lime, and if you do use cement, why is it a weaker mix than normal? Will that be sufficiently durable when mixed with Vermiculite/LECA etc? Will moisture get in the chimney? If so, what's the result?

"The Forge" mentions above the possibility of an inflatable insert that could then be deflated once it's set - have you tried to investigate that?

Just can't get rid of that worried nagging feeling...

Matthew

 

[Lithgae]

Hi Folks,
It's true, there is no certainty about any of this, but there is also no certainty about what would happen if I do nothing.

I'm not sure that strengthened is the right word, the mix will be relatively soft and light compared to stone or concrete, because it will be mainly vermiculite, the cement is only acting as a binder. If this works as planned, and the mix is fluid enough, then it will spread around the liner and fill any gaps. Its true that it will be impossible to stop the liner for example sitting against the stone at a bend, but that's not necessarily a major issue. Most of the area of the chimney flue will still be air, inside the liner, and between the liner and existing flue will be the mix, if a stone is loose, rather than falling down the chimney and exposing the next stone it will be held in place. The recommendations from the manufacturer are to make sure the first batches going in are fluid enough to get all the way down.

This is a standard mix that has been used for chimneys so its not really an untried procedure. The idea of the inflatable liner is great, but no-one around here does it, and to be honest it doesn't do anything more than the flexible liners would do, its just a former for the mix around it.

I think the main reason that the Mason wants cement is that it will go off quickly, and because he doesn't think NHL is necessary in this case, and to be fair this is from a person that would never advocate putting cement on the outside of a building. The reason I am starting to be inclined towards cement because the established procedure uses cement, and to use NHL would be guessing at the equivalents. This is a procedure published by a company who have a reputation to be conscious off, in relation to their flagship product. The characteristics that normally make lime preferable to cement are vapour permeability and flexibility, but I did some research into this and found out that a sand/cement mix of say 8:1, has similar vapour permeability and flexibility to a 3:1 sand/lime mix. This will be a very weak vermiculite/cement mix, around 6:1, so it will be solid, but relatively light and vapour permeable. The accounts I have heard of this are that it sets a little like cork, solid, but with a sponginess.

I would imagine if water came in from the top it would come down the inside of the liners, path of least resistance, and evaporate due to the bottom to top ventilation, just as it would in a normal chimney. I would expect that, as a weak cement mix, it would be no less at risk to water than lime mortar or a weak sand/cement mix. In some ways this seems better to me than putting pure vermiculite into a flue, which is very common. Pure vermiculite would soak up and hold on to water, while vermiculite in cement is already bound. I am fairly sure that people make outdoor pizza ovens with vermiculite cement.

 

[88v8]

But.... your concern was potential weakness, right? And yet you now say it won't add much strength.
Hmmm.

This type of procedure is indeed not intended for strengthening. It is a means of lining a flue where the parging has failed and there is smoke percolation into upstairs rooms &/or the attic. It's generally used as an alternative to a flue liner, where it is wished to maintain an open fire as distinct from a woodburner.

As I said, it won't do much harm, but I fear a degree of muddled thinking is creeping in here.

Ivor

 

[Lithgae]

I can see why you think its muddled thinking, but I think its more of a case of me posting the first message when I was still a little unclear on why he was suggesting it, and have also done a lot of research on it since.

I didn't have a concern as such, the Mason made an observation and a proposal, and on later clarification the reasoning is to help hold any loose stone etc together, fill any gaps, strengthen is probably the wrong word but there will be a degree of this, in the same way that mortar is not strong but it holds stronger stones together. Over time stones loosen and fall inside the flue, they fall and hit other stones and loosen them, or the stone behind them where it fell out becomes loose and so on. I have seen the effect of this on opening some of the fireplaces, where all the stones and sand from the sandstone has fallen done. I would liken this to pointing or rendering the inside of the chimney. Personally I think that this will help stabilise and keep the inside of the flue clear, but anything you do or don't do carries a risk - I guess I will find out in 30 years time, or less! :mrgreen:

From the research I have done I wouldn't agree that this is only normally used as an alternative to a flue liner, the Dupre publication specifically refers to the mix being used as a fill around a liner, and for other voids where a weak, light, fireproof concrete, with insulating properties might be beneficial.

 

[88v8]

....to help hold any loose stone etc together, fill any gaps, strengthen is probably the wrong word but there will be a degree of this, in the same way that mortar is not strong but it holds stronger stones together....

Hmmm, well, OK, I can see how that might work if the mix is sloppy enough. Albeit one wouldn't apply mortar to dusty sooty joints.

One of the chimneys in our previous (1921) house had smoke percolation problems. Absent a small boy to send up there with a pointing trowel,
I suppose this approach may have merits.

Ivor

 

[urowho]

As far as I can see this is the sort of short cut, that is almost impossible to rectify, that will have future owners scratching their heads saying why on earth did they think this was a good idea (or much worse words to the same effect). It will work for 20 years and then for everyone else will it will be entirely fubar. Compare the costs of getting a dead liner out of a chimney filled with concrete out with some rebuilding now.

 

[Lithgae]

Urowho,
I think you have misunderstood what this 'concrete' is like. It is mostly vermiculite, 6 parts to 1 cement, it can be broken in two with your hands.

The costs for completely opening up all the flues would be huge, and risky, and if the flue is to be opened up anyway, then I don't see why that would be any more onerous to to do it in 20 years time if the vermiculite needs to be removed rather than now.