Stainless or aluminum chimney liner for gas water heater?

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Rayson Arce

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I just replaced our gas water heater with an AO Smith 40gal 40,000 but water heater. Once I installed it I noticed it wasn't venting properly so I replaced the old vent pipe with Type B double wall pipe, and checked the chimney which was clogged up as well. I cleaned it out, and after some research I believe I need to install a chimney liner. Since the water heater is the only thing using the chimney to vent. So 2 questions...what size of chimney is too big for a 3" flue pipe to vent into and assuming mine is too big do I have to use a stainless steel chimney liner or can I use aluminum? I'm sure you guys know the stainless ones are quite a bit more expensive and I want to make sure I do it right.
 

Dana

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You have a classic example of an "orphaned water heater" (google it, if you haven't already come across the term.)

For an atmospheric drafted water heater aluminium is fine. Stainless is only necessary at 85% combustion efficiency and up. Atmospheric drafted water heaters run about 78-82% raw combustion efficiency, and the draft hood ahead of the B-vent dilutes the exhaust , lowering it's dew point temperature to acceptable levels, as long as the stack velocity is high enough the exhaust doesn't linger and chill in the chimney, which is why chimney size matters.

The " ideal" chimney for a 40,000 BTU/hr ~80% combustion efficiency gas water heater with 5-10' of lateral run to the chimney with 15-25' of vertical to the top would be a 3" round.

Anything bigger than a 5" round or a 4" x 4" terra cotta liner would be oversized, susceptible to back drafting & condensation.
 

Rayson Arce

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You have a classic example of an "orphaned water heater" (google it, if you haven't already come across the term.)

For an atmospheric drafted water heater aluminium is fine. Stainless is only necessary at 85% combustion efficiency and up. Atmospheric drafted water heaters run about 78-82% raw combustion efficiency, and the draft hood ahead of the B-vent dilutes the exhaust , lowering it's dew point temperature to acceptable levels, as long as the stack velocity is high enough the exhaust doesn't linger and chill in the chimney, which is why chimney size matters.

The " ideal" chimney for a 40,000 BTU/hr ~80% combustion efficiency gas water heater with 5-10' of lateral run to the chimney with 15-25' of vertical to the top would be a 3" round.

Anything bigger than a 5" round or a 4" x 4" terra cotta liner would be oversized, susceptible to back drafting & condensation.

Thank you for the detailed reply. I have came across the term orphaned water and that is definitely what it is. Currently it vents into the bottom of the chimney but doesnt run up the chimney. Which I believe is causing it to linger like you said since theres not enough velocity in the big open chimney. If you say aluminum is ok to use I will go with that and save a couple hundred dollars. Thanks again
 

Master Plumber Mark

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You would be wise to go with a 4 inch liner.... the reason is some day someone
might want to increase the size of the

heater to a 4 inch flu and they cannot do it cause you skimped a little ..
I run into this all the time when someone wants a standard 75 gal heater...

the 4 inch would work fine and is a good compromise between going too small and going too big...

Personally , I would leave it alone, in my home I got a 75 gal heater going up a 14 inch x 8 tile roof and I dont
plan on ever fooling with it.......t
 

Dana

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Mark, really? Oversize the vent because some day in the future another owner might want a bigger burner?

The notion that spending more money for a bigger liner now on the theory that the next owner or the one after that might want to go with a bigger burner seems a bit silly. The next owner might want to buy a condensing unit with a bigger burner and side vent it too, but it would be silly to install a plastic vent "just in case".

The notion that that theoretical future owner "...cannot do it..." is also a bit silly- the person who wants the bigger burner can buy and install the bigger liner appropriate for the burner- it's not as if you're pouring concrete around the liner. Do you really tell the customer that they simply can't have the 75 gallon tank, or just let them know they need to replace the liner when they upsize?

You can of course do whatever you want with your 14" x 8" tile liner (I presume you meant, rather than roof?) but if it back drafts or condenses and destroys the mortar of the brick over time that's your problem to deal with too. Personally I'd at least test it for backdrafting, but clearly YMMV.

When it's time to replace the water heater I'm personally in favor of going with sealed-combustion/direct-vented equipment over atmospheric drafted, independent of burner size then sealing up the abandoned flue, which is a 24/365 outdoor air infiltration driver. Again YMMV.
 

Master Plumber Mark

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I really dont care what this fellow does,,,
the cost difference between a 3 and 4 inch liner is nominal so who cares??
if you are that much of a tight ass to go the bare minimum then you generally someday will suffer the consequences...

If he wants to install the bare minimum, that is his choice.....
I have had to deal with this issue many times with 60 and 75 gallon units
where the flu pipe going out the roof was only a 3 inch one........
I suppose you can install the unit and hope for the best or install a
power vented unit---

I have made people sign off on the larger installs stating
they will get the liner upgraded in the spring, to cover
my liability, .. but they never do....

and in many places, if you are upgrading , the code requires a minimum 4 inch flu on even a 40 gallon gas heater....if you are installing a furnace and taking out a permit they expect it.....

as far as my house goes, I wont lose any sleep over it..... and dont you
worry your pretty little head over my chimney either......:p:p:)




 

Dana

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I certainly won't be worrying this pretty head over the chimney in any event.

IMHO atmospheric drafted equipment is likely to be going away within the lifecycle of a water heater (or two). If this water year were more than 5 years old as opposed to brand new just installed I'd be recommending against going with a liner and applying the liner money toward a sealed combustion condensing or power drafted tank and just mothballing the chimney.
 
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