Main vents according to pipe size

papa12603

I have 40 feet of 3 inch main pipe without any vent at all. It branches out to 32 feet of 2" pipe with only one rusty gordon # 2 vent. I already ordered new vents online and waiting for them. From reading answers to other post I understand that one vent for every 20 feet of main. My questions are the following: If a main pipe is 3" instead of 2" does it need a bigger size vent? If it is a smaller size (diameter) do you still put the same size vent?
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JUGHNE

That is the rule of thumb that for every 20' of 2" line. It is all based on the internal volume of the steam main. Just an off the cuff guess would be that you could need up to 5 Gorton #2.

Opinions vary on that, but you can not have too many.
A limiting factor would be what size of tapping you have at the end of the main.

papa12603

Jughne, excuse my ignorance! What do you mean by size of tapping and the end of the main? Also, to install the additional vents do I need to re pipe the sections or can I tap into pipes by drilling?

KC_Jones

He is talking about the size of the hole you have for the vent to screw into. Depending on what size you have available can dictate how much venting. To install multiple vents you add fittings and make an "antler" for all the vents to connect too. You can do a search for vent antler on this site and you should be able to find a bunch of posts with pictures. If you tell us what size the threaded connection is on your main we can tell you how many vents it can handle. I would agree with Jughne you are looking at, at least 5 #2 vents, that's a lot of main to vent. I would say 3 maybe 4 on the 40' section and 2 on the 32' section.

BobC

The volume of air in a 3" pipe is over twice the air in the same length of 2" pipe so 40 ft of 3" main contains enough air for 4ea Gorton #2's.

A 1/2" tapping can handle 2 gorton #2 while a 3/4" tapping will handle 4 gorton #2's.

Bob

nicholas bonham-carter

Remember that (in a one-pipe system), the amount of air escaping per minute from the mains is a function of the resistance of the main vents, (back-pressure of venting). This resistance must be lower than the back-pressure of venting of the radiators, so that the mains will fill with steam completely before steam begins to rise up to each of the radiators simultaneously.
A low-pressure 0-3 psi gauge will show you these figures in ounces.
That is why we say," vent your mains fast, and the radiators slowly".--NBC

papa12603

Thanks KC Jones!! The pipe where the current vent is located is 2 inch in diameter. The tap size is 3/4 inch. Should I put my antler there or in the 3" (3 1/2) diameter.