Filling around balanced flue?

Any suggestions as to the best thing to fill around a balanced flue for a boiler? Its likely that the hole in the wall is not going to be exactle the same size as the flue and therefore needs something to fill up the gaps. As I understand it, the outer part is for fresh air with the inner part being the hot exhaust. If we used expanding foam would we need anything special due to the temperature (like Fire Foam), or is the outer part not very hot? Any other suggestions other than expanding foam?
Thanks
David
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I used expanding foam: it has proved entirely satisfactory. The outer flue tube does indeed not get very hot. I'd go with the foam if I were you unless one of the professional heating engineers on this newsgroup says otherwise.
Andy.
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andrewpreece wrote:

I too use ordinary filler foam. Seems fine on the 2 boilers I used it on after a 2 years. However, do not let the set foam be exposed to sunlight. In my case the flues had a rubber collar on the outside so this wasn't a problem..... actually as the boilers were rear flue entry there was probably no need to fill round the flue at all :-)
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Not sure that I'd see any reason not to use plain ordinary mortar to fill the gap. Small bag of pointing mortar from wickes or the likes ought to do it?
Others may have reasons why not, though....
-- Richard Sampson
email me at richard at olifant d-ot co do-t uk
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Blimey, well hope you're right: I used ordinary mortar last weekend on mine(there was a bloody great hole left, which needed a bit of bricklaying to fill it anyway), followed by ordinary plaster right up to the flue to finish off. Seems fine to me so far; as far as I'm aware the outer part of the pipe isn't particularly hot.
David
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