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dobro23
Member Posts: **63**

replacing a steam boiler on Wednesday and have a question with the main vents. the house has only 1 main that goes completely around the perimeter of the basement. I estimate it to be about 100 feet of 3 inch iron. there are a total of 9 radiators attached all with 1-1/4 iron. it is a 1 pipe system. we always replace the main and radiator vents but i'm sure they are undersized every time. the only vent my company seems to be able to get easily is a hoffman #75. this forum seems to recommend Barnes and jones and gorton more often than not but my question is how many Hoffman #75's should i expect to use at the end of this 3" main? and just for knowlege, how many big mouths? the boiler will be a burnham megasteam 396, 2 risers, dropped header configuration tied into the main with a 3 x 2 eccentric coupling. p404F with subtractive differential. main set for 1.5 or 2 and differential set for 1.

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## Comments

5,101That 100ft of 3" main has a lot of air (4.9 cubic feet), you can start with 8 Hoffman 75's or go with 2 B&J Bigmouths. Either one will do the job in my opinion.

You can buy the bigmouth at the store on this site. or from Amazon.

Bob

Vaporstat with a 12oz cut-out and 4oz cut-in

3PSI gauge

635,101The B&J is rated for about 2cfm so one will take ~2.5 minutes and two will take ~1.25 minutes. These times don't include whatever time it takes to warm that 100ft of pipe up so it doesn't just condense all the steam getting it all up to temperature. The initial run will be slow because we have to get all that piping from cellar temperature up to steam temperature but subsequent firing will be working with pipe that is already pretty hot (assuming pipe is insulated) and every minute you can shave off the boiler run time adds up over a heating season.

You are balancing the one time cost of the vent to the continuing cost of fuel.

Bob

Vaporstat with a 12oz cut-out and 4oz cut-in

3PSI gauge

4,492The volume of a pipe is calculated the same way as the volume of a cylinder or tank (v = area of base x height), where height is replaced by length of the pipe.

V, cu ft = 0.785 x (Diameter, ft)2 x pipe length, ft

or V = 0.785 d2 x l

where d= inside diameter (ft)

and l = length of pipe (ft)

Example: A pipe has an inside diameter of 2.5 feet and length of 120 feet. Find the volume.

V = 0.785 (2.5)2 x 120

V = 0.785 (6.25) x 120

V = 4.90625 x 120

V = 588.75 cubic feet

63