# Precast Crown for furnace/HW tank venting



## Howard (Mar 22, 2011)

The old mortar wash crown on the vent stack was replaced with a nice precast from a local company.  Chiseled off the mortar wash, cleaned up the top course of brick and applied some Behr Waterproofing to the brick and the clay flue.  Used some adhesive caulk to install a 30lb roofing felt bond break then let it set up overnight with some bricks for weight to allow the adhesive to bond securely to the brick.

The most difficult portion of this exercise was getting the 70 lb precast crown up to the roof.

I applied a few beads of silicone to the felt as an anchor for the crown and, with the aid of a helper, placed the crown over the clay flue.

The crown-to-flue pipe gap is approximately 1/2", which was filled with 4 layer turns of closed cell backer rod and a generous bead of silicone applied to the top of the backer rod and flush with the top of the crown.

Some ThoroSeal was brushed on the crown to fill some of the surface imperfections and give the crown additional water shedding capabilities.

A picture of the crown underside reveals a casting groove which runs on all 4 sides and acts to shed any water that makes its way over the outer crown edge and migrates back toward the brick chimney.  Any water droplets that make it to the underside cannot traverse the groove and simply drop off.

The stainless cap keeps out the rodents and prevents water from entering the clay flue.

BTW - This chimney was once the vent stack for an oil fired furnace ( the reason the clay flue is blackened ), which sprung a boiler leak back in 1995.  I replaced it with a high efficiency (90% +) Vailliant boiler using hot surface ignition, modified for LPG and using a motorized vent damper.  Now if I can only get the Taco 571-2 zone controls working correctly.


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## begreen (Mar 22, 2011)

Looks good. Though I wonder about the use of closed cell backer rod in this application. Is it rated high temperature?


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## Howard (Mar 22, 2011)

If this was a chimney for a wood burning fireplace the backer rod would need to be rated for high temp.  Since this application is only for a vent flue for a gas fired hydronic boiler (HW baseboard heating) and a gas fired domestic HW tank, that consideration was unnecessary...but nice to see you were thinking outside the box.

The 30 lb roofing felt would not be a candidate for a wood burning fireplace chimney bond break - which is my next project...a pour-in-place 2 1/2 foot X 4 foot crown.  For that one I plan to use cement board as a bond break and a high temp closed cell backer rod and/or some high temp silicone caulk.  These products can be found at:  http://www.bestmaterials.com/Backer_Rod.aspx


High Temp backer rod - http://www.bestmaterials.com/Backer_Rod.aspx#HOT-ROD Backer Rod

High Temp Silicone Sealant - http://www.bestmaterials.com/detail.aspx?ID=16738

I've used Best Materials as a source for a number of construction and repair projects.  They have some really good products and will answer most e-mail questions within 24 hours.

And, as a final note:  ALWAYS allow the crown to "float."  The thermal characteristics of brick, clay, and concrete are very different and something will eventually give way when the expansion / contraction rates are not considered.  And no wire mesh!  Use rebar.

To quote Adrian Monk, "You'll thank me for it later."


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## begreen (Mar 22, 2011)

OK. I was confused because this was posted in the wood burning forum. Moved to the gas forum.


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