PrincessMommy Posted March 13, 2019 Share Posted March 13, 2019 I'm hoping someone here can help me with this question. We have a fireplace with two chimney caps. One is a dummy flue - it goes at least 1/2 way down the chimney. The other is for the wood-burning fireplace that we are converting to a vented gas fireplace insert. We also have a serious problem with spalling and water in the chimney. The previous owner never put a cap on the chimney 😡 We had to break the damper in order to get it open and clean out the debris that had accumulated. Fortunately, no animal carcasses. The chimney company that cleaned the flue and put the cap on in prep for the insert suggested we fill the dummy flue with concrete and fix the spalling. I'm trying to do research about filling a chimney with concrete but can't find anything associated with dummy flues. I don't want to make a bad problem worse. Quote Link to comment Share on other sites More sharing options...
Carrie12345 Posted March 13, 2019 Share Posted March 13, 2019 I’m not sure I understand what the advantage would be. Does the guy not trust his own work putting a cap on??? 2 Quote Link to comment Share on other sites More sharing options...
Pen Posted March 13, 2019 Share Posted March 13, 2019 I clicked because I have also had chimney issues. One old chimney was roofed over. Another cleared out and capped, filling the area between metal flue and masonry chimney with something that would not burn but that would help retain heat was suggested, but not concrete. I don’t know how concrete in an unused flue would help? I am sure in our flues it would not have been a benefit. What is your flue made of? And where does it end? I’m picturing the concrete going down and flowing out into some space below...? I probably can’t picture the situation accurately in any case. Maybe get a second or third opinion from other chimney people in your area? 2 Quote Link to comment Share on other sites More sharing options...
PrincessMommy Posted March 13, 2019 Author Share Posted March 13, 2019 One of the reasons given was because it would deal with the obvious moisture issue that has caused the spalling. The flue ends probably where the other chimney's damper is. I actually don't know what is at the end - brick? Wood? Dunno. That is a good question to ask. I think the issue is, if I cap it off without filling it there is air and moisture trapped in there that doesn't escape easily. Also, because brick and mortar is somewhat porous, there could be more issues with moisture down the road. But, concrete seems so extreme - and not easily fixed if it's the wrong decision. Ugh. Quote Link to comment Share on other sites More sharing options...
Pen Posted March 13, 2019 Share Posted March 13, 2019 By “flue” do you mean a tube of some material? If so, what material? And where is the spalling? The outer chimney brick or concrete or...??? How is a tube of wet concrete supposed to cure and dry better than just moisture? If there’s a working flue in same chimney, would not that warm and dry the other flue? Quote Link to comment Share on other sites More sharing options...
Anne Posted March 13, 2019 Share Posted March 13, 2019 I would totally get a second opinion on this. I live in earthquake country, so ymmv, but I would be concerned about drastically increasing the weight load in a space that was not designed to carry it. Anne 3 Quote Link to comment Share on other sites More sharing options...
PrincessMommy Posted March 13, 2019 Author Share Posted March 13, 2019 (edited) 7 minutes ago, Pen said: By “flue” do you mean a tube of some material? If so, what material? And where is the spalling? The outer chimney brick or concrete or...??? How is a tube of wet concrete supposed to cure and dry better than just moisture? If there’s a working flue in same chimney, would not that warm and dry the other flue? The spalling is about the top 10 feet or so. The previous owner added to the length of the chimney when he added a 2nd floor about 30yrs ago. It looks like that is the part that is spalling. But, I can see water damage down the length of the chimney brick. The chimney was not lined previously. It looks like the inside is masonry- but the house was built in 1960 - so I don't know what they were using then. The outside brick/mortar. Yes, there would be warmth generated by the other flue with the vented gas. I don't know if it would dry it. I live in a humid climate. I think I will get a 2nd opinion. I'm also worried about the extra weight from cement. Edited March 13, 2019 by PrincessMommy 1 Quote Link to comment Share on other sites More sharing options...
PrincessMommy Posted March 13, 2019 Author Share Posted March 13, 2019 Wanted edit but decided to just put this here. It looks like masonry on the inside - at least around the firebox. Quote Link to comment Share on other sites More sharing options...
Pen Posted March 13, 2019 Share Posted March 13, 2019 I don’t see how the concrete in flue would help. It might even cause wetness to gather there if condensation is an issue in your climate. I would think water is going to soak into brick from rain and humidity from the outside unless it is sealed and has a cap with large overhang. 1 Quote Link to comment Share on other sites More sharing options...
KungFuPanda Posted March 13, 2019 Share Posted March 13, 2019 Isn't concrete also porous? It seems like this could be solved at the roof level. 1 Quote Link to comment Share on other sites More sharing options...
geodob Posted March 14, 2019 Share Posted March 14, 2019 A cubic meter of concrete, weighs 2.5 tons. Their might not be adequate support, for the extra weight of the concrete? 1 Quote Link to comment Share on other sites More sharing options...
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