Rendering inside chimney breast for multi fuel stove use - advice?

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Looking to see that we don't end up making another howler. Currently we have a multi fuel stove installed in to a fairly shallow recess in the chimney breast.

We've had damp issues and as such everything has been tanked using Sovereign Chemicals products and IIRC the finish at that point was 'Renderlite', this was then boarded on top of using 'Magply' as we were advised to use "A1 rated non-combustable boards". Painted with standard Dulux emulsion paint as per advice online and the stove then installed.

Problem is that the gap between the stove and the rear wall is barely 10mm. More like 5mm-7mm. The minimum advised from the manufacturer is 50mm. I was in touch with HETAS who also say Magply is not recommended.
The effects are that at the immediate rear the paint is now flaking (we're 2 years in). The boards 'pop away' from the walls. Even the sides pop slightly and then when everything cools they 'settle down'.


I was in touch with the guy who installed it who agreed that the stove is too close to the board and that they now no longer board out the recess. Instead they render it. He did warn that this may crack a little. They add fibres for strength he says. So this is his take on it - come back out, cut the boards out and then render the inside and refit the stove.


2 questions at this moment:

1) How would you tackle it with a render approach? From what i've read he would/should need to throw lime in to the mix (what kind? Hydrated?) Anything else? What sand should he be using - standard building, plastering, sharp? I'm very wary about it cracking and also how bad it's likely to crack. Hairline is one thing, half of it falling off is another.

2) Would you put a bend in to bring the stove away from the back wall? As said, the recess is quite shallow so that will actually bring the stove out beyond the face of the chimney breast a little.


If you need any more info, pictures etc then just ask.
 
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These are the solutions -

1 - Follow the minimum clearances specified by the wood burner manufacture.
2 - Always lime mortar or fire board or brick the recess, plaster will fall off.
3 - Make sure the flaunching on the chimney stack and pointing is in good order.
4 - A Colt Top cover on the chimney pot will stop rain entering the stack.

Deviate from those and you will enjoy problems.
 
Regards your point 2:

I spoke with the guy who installs fires and he says he uses lime and rendering sand. I have a small knowledge of the materials so asked him whether he uses building sand, plastering sand or sharp/grit sand. I didn't even finish the question & he said rendering sand & that building sand is no good for it. Says the guy he uses either uses lime or fibres for strengthening. I told him everything i'd read said lime was a must & he said that's what they'll use.

Does that make it 'lime mortar'? Or is there anything else other than lime and rendering sand to make it lime mortar? Sorry, i said i have a brief knowledge of the products but i don't know what goes in to a mix totally :)



On a side note, what do you mean by fire board because i serve customers all the time who mean different things when they say 'fire board'. Fireline plasterboard - the pink stuff? Magply which is a cement type non combustable board, viroc board perhaps?
 
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Plastering sand / rendering sand. it's between building sand and sharpe sand. I don't know the proper name for the boards but it's an 8b4 sheet, say 9mm thick, very dark grey and best to cut with an angle grinder/stone saw.

The problem with an 8b4 sheet, you have a lot left if you just need to do one fireplace.

So I normally just build them in brick or render them out with Lime Mortar.
 

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