Thread: Combustion Air and Ventilation Openings

Occasioanally, I run across a gas appliance (boiler, furnace or WH) exhaust vent installed inside the upper (12" below ceiling) vertical air opening metal duct.

The smaller exhaust connected to the gas appliance in effect becomes a "solid" blocking the vertical air opening.

Example: 4 inch diameter exhaust vent inside an 8 inch diameter air opening.

Doesn't this in effect reduce the required square inches of free air area by 25%? Note: For any appliance greater 76000 Input BTUH this would no longer be sufficient and require a 10 inch diameter air opening.

I write it up: Recommend a licensed HVAC contractor perform an inspection of the heating system and perform preventive maintenance as stated by the appliance manufacturer.

Had one inspection where the CO levels were high because the air openings and exhaust vent were undersized for the boiler and water heater connected to the shared exhaust vent. (Second HVAC contractor corrected after the first one ignored the requirement... like carrying a gas and co detector)

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Originally Posted by Charles Smith

Example: 4 inch diameter exhaust vent inside an 8 inch diameter air opening.

4 / 2 = 2 squared = 4 x 3.14 = 12.56 square inches.

8 / 2 = 4 squared = 16 x 3.14 = 50.24 square inches.

The MINIMUM opening size is 100 square inches.

The 4" duct is a NO-NO, and the 8" duct is a NO-NO, forget even putting the 4" duct inside the 8" duct.

10 / 2 = 5 squared = 25 x 3.14 = 78.5 square inches

12 / 2 = 6 squared = 36 x 3.14 = 113.04 square inches.

MINIMUM round duct size is 12 inches - and that is *with nothing* inside it.

Also, another problem shown in your photo is the height of those duct.

One must be within 12" of the floor, one within 12" of the ceiling ... and they are not.

If you wanted to install one duct within the other, then you would need at least a 12" round duct inside an 18" round duct to leave enough in the outer duct to meet the 100 sq in requirement.

The smaller duct inside the larger duct in the picture is not an air opening vent.
It is the exhaust vent that will be connected to the boiler exhaust.

The lower 12" above floor (deck) air opening was not installed yet. This is new construction.

Appreciate the feedback, Jerry!

Originally Posted by Charles Smith

The smaller duct inside the larger duct in the picture is not an air opening vent.
It is the exhaust vent that will be connected to the boiler exhaust.

So, if I have this right (now) ... the inside pipe is the Type B gas vent for the boiler?

And the outside pipe is the upper combustion air opening?

In which case you go back to my calculations based on your example (not know whether or not your example was representative of what is actually there) and you see the even the outside pipe itself - even if there was nothing in it - is still too small for a combustion make air opening.

Of course, I could be misunderstanding the whole thing again and your example may have been just that ... an example ... and not representative of what is in the photo. In which case I must say to forget everything I've stated (except for the calculation based on your example) and then ask for you to specify what sizes you are referring to.

Originally Posted by Charles Smith

The smaller duct inside the larger duct in the picture is not an air opening vent.
It is the exhaust vent that will be connected to the boiler exhaust.

Charles,

Could they be using the smaller inside vent to exhaust combustion gas, the larger
duct just to provide the proper distance from combustibles as it's only purpose?

It is an example of what I have run across here in New Mexico.

Yes, in some instances it is the seperation from combustibles.

However, often it is the upper air opening (12" from ceiling).

In any case, if it is the air opening; the B vent pipe inside is effectively obstructing air flow and as Jerry calculated the outer pipe has to increase proportionally.

It appears the HVAC's are trying to save $$$ and minimize the number of penetrations through the roof. Two instead of three... 1) B vent double lined, 2) air vent upper and 3) air vent lower.. I often see it done with the three.

Ran across this www.link for vent calculations House of Craig

Charles,

Recipes and a COMBUSTION AIR calculator

Sweet


Thanks

Originally Posted by Charles Smith

In any case, if it is the air opening; the B vent pipe inside is effectively obstructing air flow and as Jerry calculated the outer pipe has to increase proportionally.

Charles,

Whether the above is the case, or is not the case (with the B vent inside the outer pipe), the outer pipe does not need to increase proportionally, it just plain and simply needs to be increased to a minimum of 12" round pipe *with no be vent in it* just to meet the minimum size (which could need to be larger for more total Btu input).

Then, if they choose to install a B vent within the outer pipe, then the outer pipe really needs to be larger.

One problem is, though, they are not even meeting minimum opening size.

It appears the HVAC's are trying to save $$$ and minimize the number of penetrations through the roof. Two instead of three... 1) B vent double lined, 2) air vent upper and 3) air vent lower.. I often see it done with the three.

The two combustion air vents do not need to go through the roof, for combustion air, a ventilated attic is considered "outside air" and the combustion air pipes only need to extend a minimum of 6" above the top of the insulation.

Now, if the attic is insulated with Icynene on the underside of the roof sheathing, yes, that complicates the issue and those pipes for combustion air would then need to go through the roof.

Originally Posted by Charles Smith

Had one inspection where the CO levels were high because the air openings and exhaust vent were undersized for the boiler and water heater connected to the shared exhaust vent. (Second HVAC contractor corrected after the first one ignored the requirement... like carrying a gas and co detector)

Are you referring to the ambient air level of CO Charles?

There might be more going on there than just combustion air opening problems.

Did you take any combustion readings in the appliances before the draft hoods and any draft readings in the flue?

Hey charles I'm going to take the easy way out. This looks like new construction and what I believe we are looking at is an insulation shield to maintain clearances of 1 inch. The contractor may come back later and run a hi/lo in attic too meet combustion air requirements. Assuming attic is ventilated.