Building a chimney with lightweight concrete?

[xLnT]

Hi,

I have been searching for someone who has built a chimney with lightweight concrete, but it seems difficult to find information about it. This might be because it wouldn't be suitable, but I would like to know why.

We are now at the decision point about what material to use for construction. We were thinking of using a liner with vermiculite around it. I thought that 600x200x150 would be suitable to build with. It withstands 3N/mm2, but the question is why it would be unsuitable?

I am quite sure that the fire safety aspect is okay, as it's often sufficient to have 100mm of lightweight concrete between the liner and a combustible wall. So, what do you think, hit or miss?

 

[Stefan N]

When working on chimneys back in the day, we used Isokern a few times, kind of like what you're considering. It worked well and was quick to set up.

 

[xLnT]

When we worked with chimneys back in the day, we used Isokern for a few, kind of like what you're considering. Worked well and went up quickly.

Ok, so you think it would work well with lightweight concrete covers?
However, we will shift the chimney 25-30cm sideways in the cold attic. Maybe it should be plastered as well, bare lightweight concrete feels unnecessary up there.
Before we go through the outer roof, we'll build with bricks, since we want it in brick up there (another chimney built with brick).

 

[Stefan N]

The question is whether it has to be a type-approved system? Check with the chimneysweep before you do anything else.

 

[xLnT]

Talked to simab (Sotarna i Malmö AB) today, and the person I spoke to said that lightweight concrete couldn't be used for chimneys.
When I tried to ask him to elaborate why, he briefly responded with "temperature during a fire," whatever that means.
In our case (with an insert pipe and vermiculite around), the outer material's impact in a chimney fire should be minimal.
Or if he meant a fire in the house otherwise, I don't know, but that would be even stranger.

I'll call around a bit more and to more people tomorrow to get a broader understanding.

 

[joakim_j]

You've already read through my thread, so you know what I concluded. Unfortunately, I think it's that simple; chimney sweeps do as they please, it's as simple as that. The only way to get around it is to use a type-approved system or to do it the old way and brick with masonry (even if it's surely less safe than your solution).

You could compare the fire data for lightweight concrete with brick and show it to that chimney sweep, but I hardly think it will change anything.

 

[xLnT]

After talking to 6+ chimney sweeps today, I received mixed messages. Someone mentioned that it wasn't possible with lightweight concrete because it couldn't withstand the heat (he didn't specify which heat or go into details). Some replied that they weren't quite sure since they had no experience with it but recommended type-approved solutions like isokern and similar ones. They would also work with displacement (it turned out that the maximum displacement was 16cm).

BUT, I managed to get hold of the chimney sweep (at least one of them) who acts as (one of two) representatives for the municipality I'm building in, and he said there was no problem at all. Which felt especially comforting, we discussed the solution in quite detailed execution. He was well aware of the distance to the wooden structures that will be around, etc. So it feels hopeful and I think I will go with that approach and build it with lightweight concrete.

Quite interesting that there doesn't seem to be more concrete regulations they can refer to, it seems more about what the chimney sweep feels comfortable vouching for. So for my part, it now depends on him not leaving or the municipality not switching companies.

 

[jonaserik]

If you use lightweight concrete, it is very important that the verkuliten properly surrounds the smoke pipe and has a thick layer against the pipe. If lightweight concrete is exposed to high heat, it will crack, pulverize, and disintegrate.

 

[xLnT]

If you use aerated concrete, it is very important that the vermiculite properly surrounds the chimney pipe and has a thick layer against the pipe. If aerated concrete is exposed to high heat, it cracks and pulverizes.

With a cavity of 300x300 and a 150mm pipe, you theoretically get at least 75mm between the pipe and the aerated concrete at the narrowest point. According to the chimney sweep, that was okay.
What is your opinion?

 

[jonaserik]

Should be fine, but what I'm thinking about is your lateral movement, it's easy for the distance between pipes and the frame to become smaller. If all angles/degrees match with the pipes and the frame, then it's good.

 

[jonaserik]

Perhaps it's possible to assemble the pipes and frame in a sequence and push down the vermiculite as needed.

 

[xLnT]

Maybe it's possible to assemble the pipes and framework in sequence and push down the vermiculite as you go.

That's the idea, that's what we did with the one we built with bricks.

 

[joakim_j]

Quite interesting that there doesn't seem to be more concrete regulations they can refer to, it seems more like what the chimney sweep feels he can vouch for.

There are a lot of standards referenced in BBR. However, they cost money to read, but they are the ones the type approvals are based on. However, I don't think the chimney sweeps directly have any knowledge of them, most go by type-approved constructions or constructions they have already seen, and nothing else is OK. You seem to have found a gem!

Are you planning to have just one flue liner in the chimney? I would like to have (at least) two; it becomes even harder to convince someone then.

 

[xLnT]

There are loads of standards that are referred to in BBR. However, they cost money to read, but they are what the type approvals are done according to. However, I don't think the chimney sweeps directly have any knowledge about them, and most rely on type-approved constructions or constructions they have already seen, and nothing else is OK. You seem to have found a jewel!

Are you planning to have just one liner in the chimney? I would want to have (at least) two, then it becomes even harder to convince someone.

Yes, we will only have one liner. We have two fireplaces in different parts of the house, so each has its own chimney.

 

[Dan_Johansson]

Why lightweight concrete, and not lightweight expanded clay aggregate?