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Balancing short main to long main? small riser to large riser?

ch352 Member Posts: 25

i finally have the opportunity to correct my steam system now, i know the line you can't over vent a main, however i am wondering if i will need to "restrict" the main vents, and balance the long and short main's.

main #1- is 60' 4" inches long (32' x 3" and 27' x 2.5")
Main #2 - is 20' 8" x 2" Pipe

#1 currently has two newly installed Gorton No. 2 and one old - working no.1 , i'm planning to add another vent
#2 has no vents, leads to a riser - no vent, and than two rads which are the only vents for this line

rads on main 2 start getting hot before any risers on main 1 begin getting hot.

i am working on correcting the riser vents - and than will work on rads

few Questions
1. I've seen very little talk of multiple riser vents , is it common practice to have a no.2 vent or even multiple vents on a single riser ? or am i missing something ?

2. don't i need to "restrict" the vent capacity of the smaller (and closer) risers to have all risers heat up the same time? or is it only the rads that need to be balanced, and the main's and riser's balance themselves out once allowed for maximum venting? (we have a narrow riser 15ft from the boiler while there's a larger riser 65 ft. from the boiler. and the same idea for balancing main 2 against main 1)

3. is the proper and best way to size a main vent by CFM ?
(i saw a Gorton recommendation of 1x No.2 vent for every 20' of 2" Pipe - that's much more than CFM would call for)

Thank you !


  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    I know that many people talk about the cubic volume of pipes, and timing, in relation to main vent selection, but really what matters is the venting resistance of the mains.
    If the main vents are too restrictive, steam will begin to travel up any riser with a large radiator vent before all of the main has filled, and therefore unbalance the system. Think of a soaker hose, with one hole made larger than the others. That area will get more water quicker than the rest of the line.
    One pipe systems are vented in parallel, whereas many two pipe systems are vented in series, through the crossover vents, and are the more self balancing “Cadillacs” of steam heating.
    The most scientific way to approach this is to put slow radiator vents on, such as Hoffman 40’s, (or Maid-O-Mists, with orifices), and large capacity main vents downstairs. This will balance the aggregate resistance of the radiator venting with that of the mains. This should result in the air escaping during the initial venting phase, at less than 2 ounces on the essential low pressure gauge, (less pressure is even more desirable).
    In your case, putting a big mouth vent on each leg would be a good starting point, since the cost of the big mouth is the same as the Gorton 2.
    After the radiators are close to balance, the orifices can be changed out for individual laggards upstairs, most likely on upper floors, if need be.
    I realize this can be a confusing subject to many amateur, as well as experienced steamies, and I hope I have been able to explain it adequately.—NBC