Main air line vent sizing and operation

midiman143

Hello friends!

I got some great advice from you helpful chaps before and I am hoping you will come through again. I have a picture of my two mail line vents. One works, one doesnt. I obviously need to replace the other as it does not work and for the 20 minutes or less that the heat will cycle on the radiator vents really never stop hissing. I assume this to be because of not enough air leaving the system and it is being forced out the radiator vents. This is when the whole system is warm and I would assume the radiator vents would shut down the flow as they are too hot to touch. They are not stuck open as we have washed and checked each one and they all hiss during operation. So I have the following questions:

1. Are the 2 #35 vent rite vents sized right for my system?
2. Is my assumption about why the radiator vents hiss for so long correct?
3. Any helpful comments are welcomed.

Fred

No, those vents aren't enough for your system. How long is each main and what diameter pipe? That will tell us how much venting you need to put on each Main. Also, what pressure do you have your Pressuretol set at? Have you ever taken the pigtail (looped pipe the Pressuretrol is mounted on) off and cleaned it out? They get clogged and can prevent your Pressuretrol from operating.

midiman143

Thank you for the reply fred. So the 2.5" lines which i am assuming you are calling the mains are about 20' each with about the same distance of 1.5" lines returning to the boiler to form 2 loops. Off of these 2 loops the line spider out to 7 radiators with one line capped off. The system is 3 years old and we have no taken the pigtail off yet, it is set to 0.5-1 psi. I did see the pressure gauge finally move from 0 to about 1-2 psi after having the system run for 1.5 hours straight. ( The house was at 48 degrees and we brought it up to 64)

Thanks again

Fred

You need to replace those vents with Gorton #2's, one on each main. They will vent a lot faster and help get heat to your rads a lot faster. While you don't need the full venting capacity of the Barnes and Jones BigMouth vents, you can use those as well. They are much better built, solid brass and they are actually a little less costly than the Gorton #2. Here is the link:
https://www.amazon.com/Barnes-Jones-Big-Mouth-Vent/dp/B01F26P13C/ref=sr_1_fkmr1_1?ie=UTF8&qid=1481651680&sr=8-1-fkmr1&keywords=Barnes+and+Jones+Bigmouth+vent

midiman143

Thank you for your insight Fred. Can I ask, how did you come up with the correct size? I am under the impression, you more or less come up with the amount of air in your main lines, assume some kind of speed of the steam and can come up with a CFM to size the mains. Am I correct about any of this? I am also aware of the notion that you can not vent enough as well in 95% of applications.

midiman143

I also have a side question for anyone- do the main air vents work like the air vents on the radiators, aka close once they are hot enough and are open otherwise

Fred

Typically you can calculate the cubic feet of air in a pipe (per ft of pipe) by it's diameter and multiply that by the length of pipe. Ideally you have at least a vent pipe that is 1/2" diameter. That will accommodate at least one Bigmouth or two Gorton #2's. You do want to vent the mains as fast as possible but, you don't want to spend money on vents where they are mounted on a vent pipe that is less capacity than the vent itself. There is a publication by Gerry Gill and Steve Pajek, called Balancing Steam Systems using a Venting Capacity Chart that is available in the store on this site for $10.00 and that money is donated to charity. It has tons of info on pipe and vent capacities.

To your second question, yes, the main vents are open until steam hits them and they close. Once the heating cycle ends, and the steam condenses, the vents open.