Combustion air rules: interior closet water heater. 304.6.2 One-permanent-opening method. Why Top Vent?

[iOne]

I working on a retroactive permit situation / code enforcement on a water heater closet that looks like the attached image.
The closet is interior in a 200 square foot studio unit, the water heater is a 30k BTU flue vented gas tank water heater.
The closet has a latching door with no vents in the door.

I read the code incorrectly and specified a 10 square inch vent through the floor to the crawl space below.
Under 304.6.2 One-permanent-opening method (California Mechanical Code 701.6.2) looks like I screwed up. The opening has to be in the ceiling.

• My question is why?
• Would the ceiling mounted one opening vent perform better?
• Can I run a 3x4" or 2" round duct from the floor to within 12" of the ceiling and call it good?

The AJH inspector is calling for a ceiling vent, which I'd rather not do. The AJH inspector is not being very helpful or specific about why (he may not have really looked into it).

 

[ICE]

The opening shall commence within twelve inches of the top of the enclosure and communicate with the outdoors.

506.4.2 One Permanent Opening Method. One permanent opening, commencing within 12 inches of the top of the enclosure, shall be provided. The appliance shall have clearances of not less than 1 inch from the sides and back and 6 inches from the front of the appliance. The opening shall directly communicate with the outdoors or shall com-municate through a vertical or horizontal duct to the outdoors or spaces that freely communicate with theoutdoors (see Figure 506.4.2) and shall have a minimum free area of the following:


Note that it is combustion air....not makeup air.

 

[iOne]

The opening shall commence within twelve inches of the top of the enclosure and communicate with the outdoors.

Yes, thus the proposed tube from the floor to within 12".
I'm interested in any insight into why this might be the code: would a ceiling mounted vent somehow function better than a floor mounted vent?
As I learn, are the available code or subcommittee notes from when this code dated to?

 

[ICE]

As the code is written, there could be an argument presented that a crawl space freely communicates with the outdoors. You could also argue that since the opening of the duct commences within 12" of the top of the enclosure, code has been met. The fact that the illustration does not show that configuration presents an argument against all of that.

I for one am leery of gas fired appliances that are installed inside a dwelling. So I would reject it without some authority that I trust agreeing with what you propose. Truth be told, I think that the single upper combustion opening is a mistake.

 

[iOne]

So I would reject it without some authority that I trust agreeing with what you propose. Truth be told, I think that the single upper combustion opening is a mistake.

As a code inspector, on what authority would you reject it given 701.6.2 CMC fuel gas code?
Is there a physical reason to expect the top opening would work better than the bottom opening for supply combustion makeup air?

This is my proposed solution... anyone see an issue with it?

Note that the existing heater showed no sign of combustion distress prior to the 10 square inch opening: no soot, discoloration,
blowback, etc. Of course the flame looked good when the door was open, I was unable to observe the flame quality in the enclosed cabinet

 

[ICE]

As a code inspector, on what authority would you reject it given 701.6.2 CMC fuel gas code?

My apology…..I am not a code inspector. If you check my profile you will see that I am a professional driver.

I really have no business disagreeing with you. I would delete my post but that creates confusion.

As far as authority is concerned….well then Shirley you’ve heard about Tiger Code.

 

[ICE]

supply combustion makeup air?

?

 

[ICE]

This is my proposed solution... anyone besides the professional driver see an issue with it?

There I fixed it for you.

 

[north star]

$ ~ $ ~ $

iOne, regarding the "bottom opening" vs. the "top opening"
...one reason that I would at least consider is pest(s) intrusion.
Even if you had screening on the bottom opening, I would
be concerned about blockage of the opening.......IMO, another
reason would be that a "bottom opening [ duct ]" would
decrease the free area \ wiggle room \ working space around
the appliance.

< = < = <

 

[Ed Cooke]

As a code inspector, on what authority would you reject it given 701.6.2 CMC fuel gas code?
Is there a physical reason to expect the top opening would work better than the bottom opening for supply combustion makeup air?

This is my proposed solution... anyone see an issue with it?

Note that the existing heater showed no sign of combustion distress prior to the 10 square inch opening: no soot, discoloration,
blowback, etc. Of course the flame looked good when the door was open, I was unable to observe the flame quality in the enclosed cabinet


View attachment 10736

I working on a retroactive permit situation / code enforcement on a water heater closet that looks like the attached image.
The closet is interior in a 200 square foot studio unit, the water heater is a 30k BTU flue vented gas tank water heater.
The closet has a latching door with no vents in the door.

I read the code incorrectly and specified a 10 square inch vent through the floor to the crawl space below.
Under 304.6.2 One-permanent-opening method (California Mechanical Code 701.6.2) looks like I screwed up. The opening has to be in the ceiling.

• My question is why?
• Would the ceiling mounted one opening vent perform better?
• Can I run a 3x4" or 2" round duct from the floor to within 12" of the ceiling and call it good?

The AJH inspector is calling for a ceiling vent, which I'd rather not do. The AJH inspector is not being very helpful or specific about why (he may not have really looked into it).

Ok, where do I start? The 1st thing, science has shown that air movement is essentially an upward movement, it does not flow downward. That is why the code is written in that manner.
2nd: A water heater is best when following the Plumbing code, not the mechanical code. Although a good amount of info does cross each book, the completeness is best served by the CPC, since you are in California.
Now, if there is a water heater in a closet without any combustion air venting, even if the existing seemed to be no issue, it is incorrect. A gas water heater may not be open to sleeping areas, and a studio of 200 ft. is a sleeping area. So, that door should be sealed as noted in CPC 504.1. Since it is a standard B-vent, use subsection (2) along with 1.1 and 1.2 for self-closing gasketed doors.
3rd: The combustion air would need to be from the exterior as the interior being a sleeping room is not approvable, hopefully by anyone. No matter which you choose, the 1 or 2 opening method, both require the high portion. If you cannot go through a wall to the exterior, your only option is through the ceiling/roof. The opening, commencing within 12" of the top of the room seems to be the most efficient.
Question, is there an attic space in which air communicates to the exterior? If yes, then that would be why the inspector noted the upper vent there. If not, and your photo is depicting a roof right above the room and no attic space, then you would be going through the roof.
OK, hope that will assist your efforts.

 

[iOne]

Question, is there an attic space in which air communicates to the exterior? If yes, then that would be why the inspector noted the upper vent there. If not, and your photo is depicting a roof right above the room and no attic space, then you would be going through the roof.
OK, hope that will assist your efforts.

There is no attic, it's a cathedral ceiling setup.
10 square inches is required (30,000 BTU/3000 = 10 square inches NFA),
A 2" pipe gives A = π r^2 ≈12.57 inches squared.

The straightforward pipe route is from the open crawl space below, to the 12" from the ceiling.
I'm not seeing anything in the code that would preclude that solution.

It's possible perhaps, but awkward to bring the same air in from the right hand side wall via a 2" pipe from
the room the water heater closet is in (a bathroom, included in the 200 square foot total of the studio).

 

[klarenbeek]

NOt sure exactly what the California code says about it but combustion air requirements seem to be pretty universally the same. The IFGC Specifically states that horizontal upper combustion air ducts shall not slope downward toward the source of combustion air, so your proposed scenario with the duct going up to the top of the closet is prohibited in the IFGC.
Like Ed said, in this case with it being a studio apartment the combustion air has to come from the outdoors. If the duct goes up, or the opening is below the burners and its colder outside, cold air is heavier than warm air. Its much harder for a natural draft appliance to pull cold combustion air up from a lower location than to let it fall from a higher location.

 

[klarenbeek]

Also, You may need to recheck your math. A 2" diameter pipe is only 3.14 square inches. You'll need 4" pipe to get 12.57 square inches. The IFGC also prohibits the duct from being smaller than 3" in any dimension, not sure about the CMC.

 

[klarenbeek]

And you can't take combustion air from a bathroom either.

 

[iOne]

The IFGC Specifically states that horizontal upper combustion air ducts shall not slope downward toward the source of combustion air

Ok, that's very helpful. Seems to be

IFGC 304.11 Combustion Air Ducts.
"5: Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air".

It also says something with definite tradeoffs "Ducts shall not be screened where terminating in an attic space". Hopefully that means the duct can (and should) be screened at the appliance side, just not at the attic side where nobody would ever clean or observe it.

 

[Ed Cooke]

There is no attic, it's a cathedral ceiling setup.
10 square inches is required (30,000 BTU/3000 = 10 square inches NFA),
A 2" pipe gives A = π r^2 ≈12.57 inches squared.

The straightforward pipe route is from the open crawl space below, to the 12" from the ceiling.
I'm not seeing anything in the code that would preclude that solution.

It's possible perhaps, but awkward to bring the same air in from the right hand side wall via a 2" pipe from
the room the water heater closet is in (a bathroom, included in the 200 square foot total of the studio).

So hopefully by now, you see the bottom vent is not complete unless an upper vent is included. The only difference is a one vent method used 3K BTU and the two opening, upper and lower, calculates @ 4K BTU. I would say the W/H door being solid and gasketed and venting through the roof is pretty much your option. And the calc for sq in is as noted, 12.56 for a 4" pipe, and 3.14 for a 2". For Sh__s & Giggles, a 3" id 7.07.

 

[iOne]

venting through the roof is pretty much your option.

We're in a heating climate (no A/C usually) so roof penetrations are energy losses. We'll try to get a vent placed out a side wall at 12" down.
A challenge has been the City inspector is not particularly familiar with the codes, so this discussion has been of help.

The exact same unit ALSO has a gas range, with range hood, again in the 200 square foot space:

 

[iOne]

And, it turns out, there's a second fuel appliance in the 210 square foot unit: a gas range.

Per 304.5.1 Standard Method. The minimum required volume shall be 50 cubic feet per 1000 Btu/h of the appliance input rating.

5000 BTU/hr +
9500 BTU/hr +
9500 BTU/hr +

12000 BTU/hr +
13000 BTU/hr (oven)
TOTAL 49,000 BTU/hr

(Total 49000 btu/hour) / (1000 btu/hour) = 9 * 50 cubic feet = 2450 cubic feet required
2450 cubic feet / 210 square feet = 11.6 foot ceiling height.
The unit does not have 11.6 foot ceilings.


This will probably end up as an induction stove instead.
The AJH is saying it's impossible to meet the standard with any sort of vent: I'm tempted to try and disagree. But at the same time it's probably not worth it....

 

[iOne]

And, it turns out, there's a second fuel appliance in the 210 square foot unit: a gas range.

Note: I was brought in to the unit, constructed without permits, to retroactively apply for permits.
That's how all this came up. I started with obvious safety hazards which were mitigated, and am now now to the more technical code issues. The unit is so leaky the actual hazard seems entirely remote.

 

[ICE]

And, it turns out, there's a second fuel appliance in the 210 square foot unit: a gas range.
View attachment 10760
Per 304.5.1 Standard Method. The minimum required volume shall be 50 cubic feet per 1000 Btu/h of the appliance input rating.


(Total 49000 btu/hour) / (1000 btu/hour) = 9 * 50 cubic feet = 2450 cubic feet required
2450 cubic feet / 210 square feet = 11.6 foot ceiling height.
The unit does not have 11.6 foot ceilings.


This will probably end up as an induction stove instead.
The AJH is saying it's impossible to meet the standard with any sort of vent: I'm tempted to try and disagree. But at the same time it's probably not worth it....

The code used to limit the available volume for the purposes of calculations to an eight foot high ceiling. I don't find that in the current code. I always wondered why that limit was there and now that it is gone...my curiosity is piqued.

 

[iOne]

The code used to limit the available volume for the purposes of calculations to an eight foot high ceiling. I don't find that in the current code. I always wondered why that limit was there and now that it is gone...my curiosity is piqued.

Does anything prevent addition of venting, for such a situation? Holes in floors are not good for energy, but...

 

[steveray]

A hole in the thermal envelope is not allowed by the energy code....Shirley Cali has one of those.....

N1103.6 (R403.6) Mechanical Ventilation​

The buildings complying with Section R402.4.1 shall be provided with ventilation that complies with the requirements of Section M1505.4 of the International Residential Code or International Mechanical Code, as applicable, or with other approved means of ventilation. Outdoor air intakes and exhausts shall have automatic or gravity dampers that close when the ventilation system is not operating.

 

[Enrgxprt]

I didn't have time to read all replies... This might have been mentioned already...
I have noticed a few gas equipment manufacturers require venting as per NFPA31 even though that's geared for oil...
Did you look in the installation instructions ?