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I’m looking to add a chimney pot to my brick fireplace chimney to help improve the draft, as the chimney height is barely even with the roof 10′ away and is lower than the ridge. The pot will add about 3′ of height. Both the base of the chimney pot and the clay flue liner are 13″ square. What is an acceptable way to install one of these? The flue liner extends about 4″ above the concrete cap (6″ above the top course of brick). Is it better to cut the liner down to the level of the brick or add 2 courses of brick to bring it up to the level of the liner? What material should I put between the new concrete cap and the chimney pot to allow for expansion and contraction? What should I use to seal the pot to the flue liner?
Paul on Long Island
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Paul,idealy I would like to see the pot first but I believe what I would do is-------
Remove the existing mortar cap.Remove the top section of flue. cut a new section so that when reinstalled the top edge of the flue will be several inches below the top of the brick work( how far below would have been determined by what I saw of the pot). Install the pot.A couple of pieces of rebar and some hardware cloth will hold the wet mortar cap in place untill it sets up---often you don't even need the re-bar and the hardware cloth.A couple wraps of styrofoam sill seal that sometimes goes between the foundation block and the bottom plate of a wall will isolate the mortar cap nicely. once the cap has set up you can cut that foal seal back and caulk the joint easily.A furnace flue may not heat up and expand enough to crack the mortar cap,but a fireplace flue probably will without being isolated.
*"A furnace flue may not heat up and expand enough to crack the mortar cap,but a fireplace flue probably will without being isolated."Interesting point. I've never seen info about relative flue gas temps for wood fires v. furnaces.Obviously, there will be tremendous variation depending on fire size / furnace capacity, efficiency, flue size, length, position (outside wall v interior) etc.Do you have any info on this? Enquiring minds want to know!
*I was curious about this too when building my chimney. My flue liner is 18" x 20" and there are 12 sections of it to the top (24 feet total). I was more concerned about water penetration from rain than cracking from heat that far up.Nonetheless, I poured a mortar bed in place and left a 1/8" space around the bed to liner interface. I then caulked the daylights out of it.Mine has been in place just over one year and I'll have a lift on site soon and be able to check its condition after a season of use.Mike
*Thanks for the advice. The bottom 6 inches of the chimney pot tapers inward, so I would imagine the brick must be brought up to a level somewhere within this section. Just FYI, the existing mortar cap on my chimney had no gap for isolation and was cracked badly. The age of the chimney is around 25 years and this happened recently, so it does take some time. Another brick chimney on the house with the same construction that carries flues for the boiler and an unused fireplace is fine. I guess Steve is correct about flue temperature of a fireplace being higher than one for a boiler.
*Bob,I do a lot of repair work on older homes. This provides an excellent education in what can go wrong over time and, reversely----what works well.I can't provide you any published data----just direct observations from the zillions of older roofs I have climbed.I am not saying this WILL happen,just that I have observed a trend.Here is another detail I have noticed.The outside of a flue tile is glazed and seems to hold up well in weather. The end or top of the flue is often unglazed and porous and I think that might contribute to soaking up rain,freezing and spalling of the top of flues.I find alot of flues that seem pretty solid from the brickwork downwards,but have totally crumbled from the mortar cap upwards.
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I'm looking to add a chimney pot to my brick fireplace chimney to help improve the draft, as the chimney height is barely even with the roof 10' away and is lower than the ridge. The pot will add about 3' of height. Both the base of the chimney pot and the clay flue liner are 13" square. What is an acceptable way to install one of these? The flue liner extends about 4" above the concrete cap (6" above the top course of brick). Is it better to cut the liner down to the level of the brick or add 2 courses of brick to bring it up to the level of the liner? What material should I put between the new concrete cap and the chimney pot to allow for expansion and contraction? What should I use to seal the pot to the flue liner?
Paul on Long Island