Breakdown of CFA in one-pipe parallel system

Fizz

Front: supply main with 133.5 attached rad; 31' of 11/2" and return main(steam filled to vents) 28.5' of 11/2" pipe=tot pipe CFA of .83, rads have 3.34 CFA.
Back: SM, 84sf of att rad,; 28' of 2" and return main 26.5' of 11/2" pipe=tot CFA .99, rads have 2.12 CFA.
Considering above, what would be proper main venting? Currently has MOM1 and G2 on front and 2 MOM1's on back.

nicholas bonham-carter

Try an additional Big Mouth on each main, for a start.
Remember you are trying to balance the resistance of escaping air, (back pressure), with the aggregate back pressure of all the rads on each main. That way steam will take the easy way, and fill all the main pipes first, and then move up into all the radiators,
This is why we suggest venting the mains fast, and the radiators more slowly, so steam arrives simultaneously.
On your good low pressure gauge, you should see the starting air removal take place at less than 2 ounces pressure.—NBC

Fred

What size tapping is on each main for the vent(s)? You can keep adding vents but there is a limit on the capacity of the tapping as well and exceeding that is just wasting money.

Fizz

Fred,tappings are 1/2". NBC, how big are big mouths? Don't know if have enough room.

ethicalpaul

Big mouths have 3/4" threads and they need maybe 5"-6" of vertical space above the pipe

Fizz

Thanks Paul!

Steamhead

From what you posted, it looks to me like you already have the vents you need.

Rear main has 0.99 CF, and a Gorton #2 is good for 1.10 CFM at one ounce pressure. Front main has 0.83 CF, and a single MOM #1 is good for 0.333 CFM, so the three you have would equal 0.999 CFM. I'd try installing the vents as indicated here and see how well it works before buying anything else.

Fizz

Will do; thanks Steamhead!