Boiler manual contradicts gama sizing table - who to trust?

ForumMember

Our duplex has two Weil-McLain Gold CGa Gas-Fired Water boilers and two gas water heaters to one chimney.

According to the gama sizing table, we have 220k btus and a 28' chimney so we'd need a 7" liner to vent.

However, in the boiler manual under vent requirements it says this:

which is confusing. I assume the last sentence is meant to say plus 50 percent of vent outlet area on each additional appliance. Each boiler vent is 5 inches, each water heater vent is 3 inches, so that is 5+2.5+1.5+1.5 = 10.5 inches.

Which do we trust??

On the same page of the same manual under the heading When removing boiler from existing common vent system (which we are not doing but still seems like relelvent info) it says: Any improper operation of common venting system should be corrected so the installation conforms with the National Fuel Gas Code, ANSI Z223.1-latest edition. Do I need to buy that gas code for $94 to find my answer?

When I called Weil-McLain today they said they couldn't give me technical advice and said whatever the manual says I should follow the manual. He was very short and unhelpful but I guess I could call them back and ask more specifically about the meaning of the words in the manaul.

Feels like a conundrum. I used to like that wine.

ethicalpaul

why do you think the manual is wrong?

KC_Jones

That manual says area, not diameter.

By area it would come out to 8”.

Jamie Hall

You are confusing area with diameter. Don't. @KC_Jones has it. And which do you trust? You go with the larger of the two — in this case, you need an 8 inch circular flue.

ForumMember

Thank you! This is helpful! I was confusing area with diameter. You saved me a lot of stress and running in circles.

How did you guys get 8? My math says I need a liner with diameter of 6.82 inches

Heres's my math:

Boiler radius is 2.5 = 19.6 sq inches is the area of the circle coming out of each boiler.

Water radius is 1.5  = is 7.1 sq inches is the area of the circle coming out of each water heater

19.6 + half of 19.6 + half of 7.1 + half of 7.1 = 36.5 square inches total area

Diameter of a circle with area 36.5 square inches is 6.82 so 7 inch liner. Same as gama sizing table.

KC_Jones

Apologies I took full size of water heaters, don’t would be what you stated.

EBEBRATT-Ed

1 5" flue= 5 x5 x.7854=19.635 square inches

1 3" flue =7.0686 square inches

so 19.635 +(.5 x 19.635) + (.5 x 7.0686) + (.5 x7.0686)=19.635 + 9.8175 +3.5343=3.5343=36.5211 square inches

7" x 7" x.7854=38.4846 would be the size so 7" pipe

I would question if this is large enough since you have 2 large appliances of = size.

taking the tow boilers at full size=39.27 +3.5343 +3.5343=46.3386.

8" would be 8 x 8 x.7854=50.2656 square inches. But that is just my opinion

Since it is a liner the mfg. of the liner would be who I would go to for sizing. The height of the chimney also has a big impact on sizing. The type of liner corrugated or smooth make a difference as well. The 28' height is helping you.

Don't forget that the page of the manual you printed says the calculation is the "minimum required.

All things considered 8" would be my pick.

The diameter squared x .7854 is a lot quicker than fooling with a radius etc.

The area of any circle drawn within a square of the same outer dimension is 78.45% of the square dimension area