2 main vents in 2 different locations on single pipe main

Marc_J

Single pipe steam system, first main vent was located approximately halfway around the main loop (after ~20' of pipe and 5 radiators) and another at the end of the main, just after the last radiator (another ~20' of pipe and 6 radiators) and a foot before the drop down to the wet return. I've since changed this arrangement, plugged the first hole and built an antler of (3) Hoffman #4 vents at the end location. But still wondering why the previous arrangement had been installed. Any thoughts?

nicholas bonham-carter

The vents belong at the end of the dry return before the drop, otherwise the middle vent will close early, and you lose part of your main venting capacity, before steam has reached the end.
I don’t think your vents are nearly large enough. 2 big mouths would be a good start.
The aim is to allow the air to escape at a back pressure of 2 ounces for maximum speed and efficiency, (not using extra fuel to force the air out through constipated little vents).—NBC

JUGHNE

How large of a tap/fitting is at the end of the main?
The placement of the center vent may have been just the convenience of an available tap point.

I have used opportune ports as such until piping was revised to add a proper antler at the end of the returns. It is better than nothing.

Marc_J

Thanks NBC. 2 Big Mouths on the way. Seems that it wouldn't hurt to put the Hoffman's I already have at that midpoint tap as well after the Big Mouths are in.... Or would that create unforeseen issues?
JUGHNE - 3/4" fitting at both the middle port and the end port as well.

gfrbrookline

NBC, what if you have 2 mains and 4 dry returns because the mains are stepped in the middle of the building? Would you vent all of the dry returns proportionally so all of the radiators receive steam at the same time.

mikeg2015

I added a couple mid points taps on my system and it seems to work well. Boiler is grossly oversized so it helped to keep it balanced as the header warms up. Should act to reduce pressure drop trying to push the air all the way to the end on a longer header. But as mentioned it can impact balancing as once the vents close, you loose that venting capacity. However, as the header heats up you need less and less venting since some steam is consumed.