main vent size

dobro23

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.

BobC

A single Barnes and Jones bigmouth vents at the same rate 4 Hoffman 75"s do and one 75 costs as much or more than one Bigmouth does.

That 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

dobro23

thanks bob. maybe i will order a few and submit the bill for reimbursement from my company. what is the formula for figuring out the cubic feet in the pipe and how fast to vent it? so i can determine my own vents in the future.

BobC

You want to vent the air from the mains as fast as you can without tripping over dollars to pick up pennies. So if we want to vent that 5 cuft main we would like to do it in a minute or two,.

The 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

Paul48

https://heatinghelp.com/systems-help-center/new-resource-post-9/

The 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

dobro23

thank you paul. the charts in the link you passed along are excellent and very helpful.