Thread: Clearance to Combustibles

I am sure to many of you this may seem like an elementary questions but I am training to be a home inspector and I took this pic from the boiler room of my friends apartment complex.

The vent connectors are of NG boiler and water heater and I was wondering what is the proper clearance to combustibles for this type of installation. These vents appear to be single wall vents. I was under the impression that 9" is required and for the boiler (vent on the right) it is barely 3/4" to the rotted drywall.

My 2 questions are, what is the clearance required for this type of installation?
When measuring clearance is drywall considered combustible?

In the picture the vent on the right is the boiler.

Thank you.

Similar Threads:

• B Vent Window Clearance
• Propane regulator clearance
• Gas Fireplace Chimney Clearance
• insualation clearance from draft hood
• Who needs 6" of clearance for single wall

Hi Joseph,

Here's some good info. for you...List of Fire safety Clearance Required from Combustibles for Single-Wall Metal Flue Vent Connectors

Drywall is not classified as "non- combustible" so clearance requirements must be met.

Thanks Brandon.

If this is suppose to be 9" clearance why would anyone install it so close to the ceiling, and how did it ever pass inspection. Furthermore how has it not started a fire. This is a large apartment building, it just seems like there is something I'm missing.

That's the first time I've heard of a 9" clearance for a single walled pipe but it may vary depending upon manufacturers. For single walled flue pipe, I look for a 6" clearance. For B-vent, a 1" clearance. Also, there should not be any insulation in contact with either type of flue pipe.

Originally Posted by Brandon Whitmore

Drywall is not classified as "non- combustible" so clearance requirements must be met.

I was going to ask about that, but I suspect that's because drywall is directly connected to something that is a combustible, and it isn't much of an insulator (if at all). So the wood behind the drywall would get pretty much as hot as if the drywall wasn't there.

Originally Posted by Kary Krismer

I was going to ask about that, but I suspect that's because drywall is directly connected to something that is a combustible, and it isn't much of an insulator (if at all). So the wood behind the drywall would get pretty much as hot as if the drywall wasn't there.

The paper facing on drywall is combustible.

Also, installing a piece of non-combustible material directly against a wood framing member or ceiling joist as you may see in many older houses is not acceptable either when the minimum clearances are not met. Heat can still transfer through to the combustible material by means of conduction.

The only explanation as to why somebody would install combustibles that close to the flue pipes is because 1) they don't know what they are doing and 2) don't understand the way flue pipes work.

Originally Posted by Joseph Stevens

When measuring clearance is drywall considered combustible?

Yes.

That is why Type B gas vent requires 1" clearance to gypsum board just like it does to wood.

I know this thread is a little old, but I called out lack of 1" clearance to a double wall water heater flue in a garage. The fire-rated drywall was installed right up against the flue.

The plumber just called me and said that it was OK because the drywall was fire rated. ? He said he even called a City of San Diego Building inspector who said this installation was ok, and that a metal collar was not required.

Am I missing something? Does anyone know of an exemption based on the drywall being 5/8" fire-rated?

Thanks.

The 1" clearance is an air space. You cannot back up a Redi-Mix truck and fill the space with concrete for ex. I don't care if this is asbestos millboard or Supermans's cape, it cannot be in contact. In fact, it will create a hot spot.

As for the drywall, it IS considered 'combustible' according to the definitions in NFPA 97 and thus NFPA 211 and also reflected by UL and ANSI. Even if you had a material having passed ASTM E-136, it STILL cannot invade that air space, which is required to cool the pipe.
The 9" clearance may appy if this is an unlisted gas appliance and not listed for use with B-vent. If this is a listed Cat I gas appliance then it is 6" for single walled connectors and 1" for B-vent.

Thanks for the quick reply. That is what I thought.

Is a metal collar acceptable? The way you wrote it, it sounds like nothing can bridge that gap. Because an air gap would be a breach in the firewall.

I'll be corrected if I am wrong, but that metal collar is not required, but is allowed.

What is required at a penetration is the firestop. The collar you typically see on the garage side is cosmetic.

I think this has been answered before, but can a fire rated caulk be used to seal this gap between the collar and B vent in the garage to help prevent the spread of fire? (separation wall).