Venting gas water heater through existing metal chimney

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Old 09-20-20, 01:17 PM
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Venting gas water heater through existing metal chimney

Hey ladies and gents,

I live in Maryland and I'm new to the forum.

I have a 1940s cape cod style home with wihat must be the original chimney for water heater and gas furnace. Im moving the water heater into the attic to make space in the 1st floor. Attic never drops below 40-50F as it sits right above the entire living space.

Id like to avoid another roof penetration for the 3 inch water heater flue but using the existing chimney. Can I cut into the metal chimney and route the 3 inch flue into the existing chimney with a tee? The chimney is lined with something (fiber wool for all I know). OR am I better off with a new flue through the roof?


Chimney as passes through attic.


Bottom of chimney inside house (living space) as 3 inch water heater vent pipe and copper furnace vent pipe enter chimney,
 

Last edited by asanchez999; 09-20-20 at 01:34 PM.
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Old 09-20-20, 03:22 PM
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Welcome to the forums.

I am not the pro in this forum but I have never seen that type of flue before.
My opinion is.... that is not a proper flue pipe.
It looks like standard HVAC ducting.
That would mean it's not double insulated to contact flammable services.

Possibly a member passing thru can add some info.
 
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Old 09-20-20, 04:56 PM
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While I've seen galvanized pipe used as a chimney flue before, it requires significant clearance from wood and any flammable material. My understanding is that any non-insulated pipe requires 18" clearance. This clearance can be reduced by using double-insulated ducts (B-vent), things like fire-rated boards, etc.

So first, I'd question the safety of that flue, that doesn't have much clearance other than some insulation.

Secondly, you can't tap into a vent and have 2 appliances using one flue (unless they are connected together at the same level. If there's ever a clog or issue, you'll mess with the updraft and one will inevitably vent into the house.
 
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Old 09-21-20, 08:52 AM
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Thanks for responding guys!!!

That flue is not hollow. Its not duct either. MUCH thicker and filled with something.

What I realized after I posted was that I need double wall (b vent) when putting a water heater in an unconditioned space and the existing flue is single wall vent and its against code to connect B to C vent so I have no choice but to perforate roof as I don't want to be messing with the internals of that old chimney. Its old and who knows how fragile.
 
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Old 09-21-20, 11:19 AM
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Ah, the way you describe your existing flue makes more sense. And good call making a separate flue/penetration on your water heater.

My understanding is that you should keep at least 16" horizontal separation between the flues to prevent any possible backdrafting.
 
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Old 09-28-20, 02:13 PM
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Thanks for your answer Zorfdt. Yes 16 inches apart and certainly enough room for the flashing not to interfere with the adjacent chimney's flashing.

After much research and back and forth with Amerivent (thanks Karen O.) tech support I've learned that they feel the SELKIRK guide is the definitive guide on venting (http://www.selkirkcorp.com/~/media/s...tion-guide.pdf) and as per that guide, 5 feet is the minimum vertical (only) height for B-vent with 6 feet recommended and that it doesn't matter if its conditioned or unconditioned space. And of course how high you have to stick up out of the roof penetration is of course dependent on slope and proximity to vertical surfaces and the roof itself, not including other combustion vents.



This gentleman also has some nice pictures and explanations. He doesn't advocate for no-caulk vent flashing boots but according to Oatey they are B-vent rated. (I'm gonna use an Amerivent storm collar above that Oatey no-caulk vent pipe boot as well) with some high temp silicon sealant as additional water proofing.
Reply to comment | Blue Palmetto Home Inspection

Also, don't use the generic vent cap Home Depot sells from Amerivent. Use the lock on kind you can get from Amerivent online.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
 

Last edited by asanchez999; 09-28-20 at 02:30 PM.
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Old 09-28-20, 07:07 PM
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Thanks for letting us know how you made out and for the additional information on venting.
 
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Old 10-05-20, 11:43 AM
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Youre welcome. Here are some pics. I accept all constructive criticism!!!

I still need to secure the vent stack. Its only 5.5 feet total but I still wanna secure her. Hence the metal plate hanging out in the middle of the vent stack.









 
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Old 10-05-20, 11:56 AM
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Looks good!

I'd run a 2x4 between the rafters and then bracket the vent pipe to that. Nice and simple.

It's also hard to tell, but the PEX pipe needs to be converted to metal at least 18" from where it goes into the water heater. You may already have that, but the PEX can melt in proximity to the vent.
 
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Old 10-05-20, 06:30 PM
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It does look good. So strange to see a water heater in the attic.
I see you have it sitting in a pan. Good move. Don't forget the drain needs to be connected and it wouldn't be a bad idea to put a water sensor alarm in the pan.
 
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Old 10-12-20, 07:29 AM
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Thanks for the tips guys.

Zorfdt I do have 24 inch flexible copper lines attached to 90 degree brass elbows. They are covered in insulation so ya cant see them.

BTW I never know if the insulation can go right next to the draft hood, in other words all the way to the top of the water heater.

I am gonna run a 2x4 between the rafters and strap down that flue vent.

PJMax, yes I agree. Attic was definitely not my top choice but there were zero options that didn't involved sticking it in an inhabited room. There is no basement.

And yes the pan drain was connected and routed to a near a sump pump in the crawl space. You cant see it in the photos by I have a WaterCop hooked into the feedline with 2 sensors lying in the drain pan.

I'm thinking about some sort of larger waterproof surround around the entire unit should a line pop outside the dripline of the aluminum pan but not sure if its practical or possible considering the combustion air intake. If anyone has any ideas I would appreciate it.

I also have a CO and explosive gas sensor near the heater and a remote thermostat. The two pipes in the attic will be wrapped with WRAP ON heat tape and insulated should the temp ever drop to near freezing upstairs which is not very common.
 
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