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 Clearance between direct vent combustion intake and DWV vent on roof?
Author: DougsterB (CA)

Is there a minimum separation on a roof between a combustion intake air vent and a DWV stack vent? I am also concerned about the kitchen stove fan exhaust that also terminates nearby via a "code cap".

According to Inspectapedia there are "critical concerns with combustion air contamination for heating appliances".

However I can't find the codes that cover my potential contamination sources.

This is for a planned condensing tankless water heater utilizing a direct vent configuration where both intake and exhaust pipes (either two separate pipes, or a concentric configuration) are terminated just above a sloped roof.

San Jose, CA

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 Re: Clearance between direct vent combustion intake and DWV vent on roof?
Author: packy (MA)

go to the manufacturers website, find the installation instructions and refer to the section on venting. there will be a drawing of a building with measurements. it is all spelled out there.
the appliance must have "product approval" in order to be legally sold in your area. with no "product approval" in your area you can not install it.

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 Re: Clearance between direct vent combustion intake and DWV vent on roof?
Author: DougsterB (CA)

packy, my question came up after studying the manufacturer's vent clearance diagrams because they just use stock diagrams, and these, like the codes apparently, do not cover my question. I spent a lot of time on search engines and also checked a few other manufacturers literature with the same inconclusive result.

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 Re: Clearance between direct vent combustion intake and DWV vent on roof?
Author: North Carolina Plumber (NC)

Then I would treat it as a window , 10' horizontally.

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 Re: Clearance between direct vent combustion intake and DWV vent on roof?
Author: packy (MA)

your gas inspector will pass or fail your installation based on manufacturers recomendations.
as i said, the manufacturer goes to your state plumbers board with an engineers report showing how their product was tested and the results. the plumbing board then gives them permission to sell their product in your state.
now, you buy one of their products and install it according to their specifications. the inspector looks at the job and says "that vent is too close to the exhaust". so you produce the installation instructions and show the inspector the diagram with their measurements. your measurements fall within their guidelines. since the appliance has recieved product approval by the state and you install acording to their directions, the inspector has no choice but to approve it.
remember it is all about liability.
you may think it is bad, i may think it is bad, the inspector may think it is bad but the controling authority being at the state level has studied and approved the appliance IF IT IS INSTALLED ACCORDING TO MANUFACTURERS DIRECTIONS.. which is is.

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 Re: Clearance between direct vent combustion intake and DWV vent on roof?
Author: DougsterB (CA)

The manufacturer’s installation manual indeed covers many common types of clearance questions. For some scenarios it gives specific numbers. For others it simply puts an asterisk on the diagram alerting us to the scenario without giving a number and instead deferring to local codes.

But the manual says nothing regarding combustion air intake port clearance from:
- a DWV vent
- a ktichen stove exhaust
- a dryer vent exhaust
In other words there’s no diagram, no distance, and no deferral to local codes.

The manual does give specific clearances for what some folks might consider to be similar exhaust/intake interference scenarios. E.g. for multiple units with concentric direct vents arranged side by side, the min clearance OC is “between 18 and 36 inches or greater than 72 inches”. I can see that this is a situation where they would have done some testing. They probably didn’t specifically address e.g. DWV vent clearance because it’s not even discussed in the codes, from what I can see (I checked NFPA 54).

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 Re: Clearance between direct vent combustion intake and DWV vent on roof?
Author: packy (MA)

doug, as I said.. it is all about responsibility.
if an appliance is installed according to manufacturers directions and it blows up, the state or city can not be sued for approving the installation.
the state does not test every companies gas water heater, the companies either have engineers test them for safety or have an independent testing agency do it.
the state then accepts the appliance as safe. the liability then falls on the testing authority.
MA does not run appliances and check the flue gases to see where they go. someone else does that and certifies the results..
again, all about responsibility
inspectors at any level don't give a damn if the instructions say 6 inches apart or 60 feet apart. all they care about is that you follow recommendations.

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