Filling chimney flue for structural reasons?

[Kaymo]

Hi all,
I had a recent post about opening up a fireplace but this is only partly related so I thought it best to open a new thread. Hoping that someone out there has some experience of this?

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).
2. Seal off at top of fireplace builders opening
3. Pour a mixture of X (lime/cement/vermiculate 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.

Regards,
Kenny.

 

[Kaymo]

Anyone have any thoughts on this?

From what I can find out vermiculite is much lighter, but absorbs water so might be tricky to mix and prone to problems if moisture gets into the the flue. Leca is heavier but doesn't absorb moisture, and might be a bit easier to use and mix.

 

[stuart45]

If you use a wet cement/vermiculite mix for a flexible liner you won't be able to pull it out and replace it when it's finished.
That mix is for clay liners which are longer lived and the mix stabilizes the liners.
With a flexible liner a loose mix is used.

 

[Kaymo]

Thanks Stuart, I was working on the premise that a mix of leca (clay balls) and lime mortar would create a solid external liner, with the steel liner effectively acting as a former to cast the clay/leca liner. If the flexible liner eventually failed, there would effectively be a clay liner behind it, and the original chimney flue behind that. A good quality 904 liner should also have a 25 or 30 year guarantee.

 

[stuart45]

The 25years does depend on burning the correct fuels, and things like not leaving it on a slow burn all night. I have seen liners wrecked after a few years if not treated right.
However what you propose might work OK as long as you make sure there are no voids in the flue mix.
It is a similar method to the perlite concrete flue liners using an inflatable centre.

 

[lostinthelight]

Could mixing lime mortar with leca make a non absorbant product do the opposite?.
litl

 

[Kaymo]

Thanks Stuart - I don't do overnight burns, too paranoid!

Thanks Lost - Leca and lime mortar are mixed to make limecrete floors, used a lot in retrofits to old houses, so while it will be breathable it shouldn't be absorbent.

I've emailed the manufacturer to ask if they have any comments about mixing it with mortar, will update the thread with any feedback I get.

 

[lostinthelight]

Thanks Stuart - I don't do overnight burns, too paranoid!

Thanks Lost - Leca and lime mortar are mixed to make limecrete floors, used a lot in retrofits to old houses, so while it will be breathable it shouldn't be absorbent.

I've emailed the manufacturer to ask if they have any comments about mixing it with mortar, will update the thread with any feedback I get.

I will shortly be laying limecrete floor but on the advice of Ty Mawr lime in Brecon, will be laying it on a bed of foamed glass which serves as a non absorbant DPM.
litl

 

[Kaymo]

Just updating the thread with some of the feedback I got from a supplier about possible mixes for this application for anyone that is interested. I've put this on hold myself at the moment as there are a few unanswered questions for me about how to ensure that the mix will completely surround the liner, liner sizes and related stuff, but I will update again if I go ahead to show how I got on.

Composed of a mixture of 4-10mm uncoated Leca® clay aggregate and NHL5 mixed at 3:1 (Aggregate : NHL) and then gently compacted. Compaction reduces the mix by approx. 10%. The drying times of the mix varies due to humidity, using for example Hanson NHL curing time is appx 24 - 48 hours. In certain warm conditions, it may be necessary to lightly mist the mix so that it does not dry out too quickly.

I have also spoken with our technical team in regards to using Lime Motar with Micafil Vermiculite (this product is appx x3 times lighter than Leca) and they cannot see an issue with this at all for your property. I have attached further information on using Micafil for your chimney. You would need to try to keep the mixture as dry as possible (but enough for it to bind together). Use the Lime Mortar in the place of cement on the attached document for the correct ratios.