Common Issue: Lack of combustion air.......

[Brandon Whitmore]

My class today dealt with combustion air requirements. Not one of the examples we worked with had adequate combustion air, and we were told that a very large portion of the houses out there have inadequate combustion air. Just something you may want to watch for.

For anyone who actually cares (it's in the code book)......

You need 50 cubic feet of combustion air for every 1,000 btu's

Easiest formula= Total btu's x .05

That will let you know if you have enough combustion air. It not, it's considered a confined space, and needs air brought into that space.

If anyone cares about any of the calculations to figure out how large the combustion air openings need to be, let me know.

[Jim Katen]

Of course, in Portland, they'll just let you cut a supply register in the confined space.

[Jim Katen]

Off topic. Have you seen this yet?

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[Darren]

Brandon;

Just curious, did the discussion/calculations include closets that contain gas clothes dryers?

[kurt]

Of course, in Portland, they'll just let you cut a supply register in the confined space.

Don't they require two vents, one high and one low?

[ghentjr]

Whenever I went into a basement or other room containing an air system the first thing I looked at was the hinge side of the door. Dirt streaks gave away the lack of sufficient air to the space.

[Chad Fabry]

Of course, in Portland, they'll just let you cut a supply register in the confined space.

Don't they require two vents, one high and one low?

I think he meant heat supply register

[Brandon Whitmore]

Off topic. Have you seen this yet?

Nope-- very interesting.

[Brandon Whitmore]

Of course, in Portland, they'll just let you cut a supply register in the confined space

Do you mean that they'll let you add a register/ opening leading into another room? Tecnhically, a register is not allowed since it can be closed.

The combustion air section of the 2011 ORSC has been modified heavily, and a bunch of restrictions have been removed. (can now be pulled from a bedroom, etc.)

[Brandon Whitmore]

Just curious, did the discussion/calculations include closets that contain gas clothes dryers?

I'll double check today, but am positive that they should be included in the calculations.

[hausdok]

Jim will correct me if I'm wrong; but I think what he's saying is that when there is a supply register in that confined space the space becomes pressurized with heated air, thus eliminating lack of combustion air due to size as an issue.

I've been looking at combustion air availability and calculating volume and opening sizes since I got into this gig. In many of the homes where the area had insufficient volume there'd been supply registers in those confined spaces. I can't remember ever seeing anything that says it's OK to not have sufficiently sized openings into the area as long as there is a supply register there; so I typically write the issue up anyway and simply point out that there is a supply register in the confined space and explain that if they keep that register open the space will be pressurized with lots of air anyway.

One place where I see this combustion air issue a lot is when the furnace and water heater are installed in small garages and those garages don't have any openings to the outside other than what leaks in around the overhead door. I'm still seeing it in the occasional new home. Some builders automatically assume that there will be enough leakage around the overhead door to provide sufficient combustion air and some are completely oblivious to the issue.

One thing that this formula doesn't take into account is the stuff stored in these spaces. If the basement or garage is also a storage space the actual in-use volume is way below what's required by basic rule-of-thumb calculation. I've always wondered how they account for several thousand cubic feet of airspace occupied by cars and boxes and such.

I always include every gas-burning device in that space in my calculations; and, if there's an exhaust fan or a clothes dryer (gas or electric) in that space that's within 10ft. of the open combustion chamber or draft diverter of a furnace or water heater, I've always considered the fan or dryer as the equivalent of a return air opening. Sure, it only depressurizes that area when the washer or dryer runs, but if it depressurizes it enough to cause the flue of a device with a permanently lit pilot light to backdraft it's possible that the device might establish a new exhaust path and not reestablish a draft in the exhaust vent once the dryer shuts down. If that happens things can get dicey.

Sometimes there's lots of evidence that an inadequately sized area that doesn't have any combustion air supplied to it has been an issue and sometimes things appear to be great despite the lack of required volume or openings. It's very much a case-by-case basis thing.

ONE TEAM - ONE FIGHT!!!

Mike

[Mike Lamb]

I recall if you get the air from outside you need 25% of the indoor requirements? But that's more typical for multi-units with a common boiler.

Kurt was talking about this I assume:

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[randynavarro]

I constantly cite lack of combustion air in garages all over the Puget Sound. It's most common here to install the furnace/water heater combo in the garage.

I'm dumbfounded as to why builders/HVAC folks aren't hip to the usually very clear, printed in black and white, instructions included with the manuals that are hanging right on the furnace.

[Chad Fabry]

I constantly cite lack of combustion air in garages all over the Puget Sound. It's most common here to install the furnace/water heater combo in the garage.

I'm dumbfounded as to why builders/HVAC folks aren't hip to the usually very clear, printed in black and white, instructions included with the manuals that are hanging right on the furnace.

The installers also do not read the instructions for roofing, engineered lumber, trusses, garbage disposers, garage door openers, water heaters, windows, weather resistant barriers, vent free or vented appliances and joist hangers. My incredulity stopped being incredulity and moved to completely jaded about three years ago.

[kurt]

I recall if you get the air from outside you need 25% of the indoor requirements? But that's more typical for multi-units with a common boiler.

Kurt was talking about this I assume:

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Yup.

Dryer closets also need makeup air, not just combustion air. Any calcs should include makeup air.

Yeah, no incredulity here either. We all have businesses because no one reads instructions.

[Erby]

My life is made of pointing out goofs such as this. Don't work to hard to eliminate your job!

[Brandon Whitmore]

I recall if you get the air from outside you need 25% of the indoor requirements? But that's more typical for multi-units with a common boiler.

That's correct if using the 2 opening method.

If there's 2 openings (1 within 12" of the top/ one within 12" of the bottom), then you need 1 sq. in. per 4000 btus. There's the option of using one opening as well, whereas you need one sq. in. opening per 3,000 btus.

Just don't forget to take into account that wood louvers are only factored in to allow 25% air through, and metal only 75%.

[Brandon Whitmore]

One thing that this formula doesn't take into account is the stuff stored in these spaces. If the basement or garage is also a storage space the actual in-use volume is way below what's required by basic rule-of-thumb calculation. I've always wondered how they account for several thousand cubic feet of airspace occupied by cars and boxes and such.

Funny you say that Mike.... we just discussed that today.

[Brandon Whitmore]

Jim will correct me if I'm wrong; but I think what he's saying is that when there is a supply register in that confined space the space becomes pressurized with heated air, thus eliminating lack of combustion air due to size as an issue.

Ahh-- makes sense. Although, I would argue that this is not allowed since the register can be closed, and that the min. opening size is required to be a min. of 100 sq. in. net free area (NFA).

[randynavarro]

I constantly cite lack of combustion air in garages all over the Puget Sound. It's most common here to install the furnace/water heater combo in the garage.

I'm dumbfounded as to why builders/HVAC folks aren't hip to the usually very clear, printed in black and white, instructions included with the manuals that are hanging right on the furnace.

The installers also do not read the instructions for roofing, engineered lumber, trusses, garbage disposers, garage door openers, water heaters, windows, weather resistant barriers, vent free or vented appliances and joist hangers. My incredulity stopped being incredulity and moved to completely jaded about three years ago.

I guess I like to highlight the HVAC because the manual is usually right there--on the furnace.

When I write this issue in the report, I'll usually say, "Provide combustion air for the furnace just like the picture shows in the manual hanging right on the furnace."

I wonder if anyone gets the point. . .

[Jim Katen]

Jim will correct me if I'm wrong; but I think what he's saying is that when there is a supply register in that confined space the space becomes pressurized with heated air, thus eliminating lack of combustion air due to size as an issue.

Ahh-- makes sense. Although, I would argue that this is not allowed since the register can be closed, and that the min. opening size is required to be a min. of 100 sq. in. net free area (NFA).

The idea was that, when the furnace was running, it would pressurize the room, providing way more combustion air than necessary. I argued that, if there was a gas water heater in the room (as there often was), then it would have inadequate air when the furnace *wasn't* running. I also asked uncomfortable questions about what effect this practice would have in terms of depressurizing the rest of the house. They just rolled their eyes.

[Brandon Whitmore]

They just rolled their eyes.

And of course they did so in writing......

[hausdok]

I had this today.

Water heater and furnace in a garage with a volume of just about 4000 cubic feet. Did the calc. Together, they need 5300 cf. Tightly gasketed overhead door - bottom, sides and top. On the outside of the garage immediately adjacent to the alcove they are in, through-wall vents on the exterior wall perfectly sized within a foot of the garage ceiling and floor. On the inside of that same wall - solid drywall. Some numbnuts during the build - the drywall was definitely 100% original - covered over both vents with drywall. Some horse's ass with teeth inspector probably completely missed it.

ONE TEAM - ONE FIGHT!!!

Mike

[John Kogel]

Nice catch, Mike. I imagine the homeowner was mystified as to what those things were on the wall outside.

What about pulling combustion air down from the attic through a 4 or 5 " pipe? If soffit vents are plentiful, are there any negative issues with that?

[Mike Lamb]

What about pulling combustion air down from the attic through a 4 or 5 " pipe? If soffit vents are plentiful, are there any negative issues with that?

No. The IRC shows different ways this can be done.

M1703.3 Attic combustion air.

I usually see a 2 pipe configuration around here but I guess one pipe is OK, too.

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