Slow Combustion Fireplace Flue Rusting Internally?

Just had a similar experience with my flue. Mine is completely stainless steel and rust wasn't the issue for me. I found two areas where the water was getting through. My ridge capping was letting water through onto the sarking. The sarking of course was breached by the flue. I discovered the ridge capping is poorly designed and is not formed to divert water flow and instead depends on the pointing cement. This was causing water to run down the outside of the flue. The other leak was a problem with the flues 'hat'. It was a minimal design, just a disc and a shroud. The wind was driving rain through this and into the flue's surrounding heat shield. Water would then flow down the flue and also through the internal ventilation holes.
You will need to get up on the roof and remove the flue's 'hat so you can look down inside the flue. I am assuming the flue's roof flashing is good and this was my first area of suspicion. If your roof is tile like mine, you may need to lift tiles as well. It may be necessary to hose the area up there and have a second person observe precisely where water comes in.

thanks for the reply.i remember being in the roof before and there's no sarking. can just see the bare underside of the tiles from memory...

Many years ago I ventured into the roof space for reasons unrelated to the pot belly stove that was installed in the house.

In short, I had a look at the flue whilst I was up there and poked my thumb straight through it! I then proceeded to remove the whole thing there and then - just tearing the steel (rust was all that was really left) with my hands and pushing the bits out onto the roof. Within a few minutes it was gone apart from having to get up on the roof and remove the bits that hadn't already rolled down into the garden.

Had I not done that, pretty soon it would have been the whole house that was gone due to fire.

This was an ordinary steel inner flue (not stainless) and a galvanised outer. But if you've got bits of metal falling down then I'd be extremely concerned about the safety of it based on my past experience.

a valid concern that makes sense. As I indicated, the top cap on mine lets in rain even though it is meant to keep it out. Sounds like yours might be doing the same thing and could be rusting things up.

Originally Posted by satorumusashi

Also, large pieces of metal 5x5cm a mm thick have been falling down the inside of the chimney and I'm wondering if this is the chimney core rusting and degrading (and should I be worried)

Are there things I can inspect or that I should be looking for?

Sounds like time for a new one. Get it inspected by some one who knows what they are doing.

Plenty of house fires caused by worn out flues

A bit off topic, but if you burn a fuel containing sulphur (coal or fuel oil) and condensation occurs in the flue (or rain gets in) then it will rust like you wouldn't believe.

In short, the sulphur in the exhaust gases plus water leads to the formation of sulphuric acid. And having sulphuric acid on mild steel is a sure fire way to destroy it pretty quickly due to rust.

This won't be an issue if you burn wood, but if anyone is burning coal then it's something to be aware of especially if the coal is high in moisture (eg raw brown coal) or is burnt in combination with a higher moisture fuel (eg green wood).