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Can I ask about insulating/ventilating a pumice lined chmney on an external wall (I was going to revive an earlier thread but couldn't add the attachment) The attached detail shows the construction of the chimney (except that it is built on a gable end wall) It was designed to take a wood burner but I have been considering fitting a balanced flue lpg stove (due in part to concerns about air leakage - see Leaky Stove thread) which would make it redundant. I thought I would put a cap (designed to prevent rain ingress but allow some air movement) on the chimey pot, seal the bottom of the flue and fill it with polystyrene beads or vermiculite. However - would I need to allow ventilation from the bottom of the flue? If I do then much of the advantage of a installing a lpg stove is lost. Thanks
It is generally recommended to ventilate an unused chimney, but you should ventilate it with outside air where possible. Could you fit a vent pipe to the bottom of the liner & connect this to a duct through the external wall?
This will stop the main heat loss due to stack effect. And it could be insulated & boxed in to reduce conducted heat loss. It has the additional advantage that its easily reversible if you change your mind in the future or the next owner prefers a wood burner.
I cannot see why it couldn't be filled with say polystyrene beads top to bottom. Then there would be less danger of internal condensation in the old flue (as it would be warmer or as warm) then than in any other part of the wall and so long as rain could not enter from the top all would be well. qed