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Master venting for a tall two-pipe system—or master trapping instead?

I’m looking at a variety of efficiency improvements for my tall 2-pipe steam heating system. NYC Retrofit Accelerator has some recommendations, and even a case study, suggesting master venting for 2-pipe.

Many years ago in a different building, I used master venting with great success on a 1-pipe system. And I can certainly see how venting the supply riser tops could help get the air out of the system and establish full steam distribution more quickly. But I also thought: air vents on a 2-pipe system? Seriously?

So my engineer brain got to thinking, and I thought “what if, instead of a master vent, I bypassed the top-floor radiator on each riser with a length of pipe, independently thermostatically trapped into the return?” And I checked the Lost Art’s venting chapter, which quotes Gerry as saying “Before a radiator trap is a radiator trap, it is an air vent.” Certainly it seems like some of my favorite traps, like the Barnes & Jones 122/134 out-flow some of my favorite vents, like the Gorton #2.

Part of this exercise is a way to guarantee that the Heat-Timer’s system sensor, up on a high floor, sees steam even if a resident (or TRV) has turned off the last radiator on the riser. But if there’s a benefit to be had from master trapping or even master venting all of my riser tops, I might just go ahead and do this to all of them.

What do you all think? Master-venting on a 2-pipe system, for or against? And has anyone master-trapped riser tops like this before?



  • Jamie Hall
    Jamie Hall Member Posts: 23,273
    You had me scared. Have you ever heard of crossover traps? That is what you are proposing. The vast majority of vapour steam systems -- two pipe steam systems running at very low pressures -- use them. What they are are ordinary steam traps, but arranged so that they don't have to handle condensate (on horizontal lines what is done is a line is run vertically up, then elbow over to the inlet to the trap, then from the outlet of the trap straight done) and they discharge air into a dry return.

    They work perfectly well on any two pipe system, vapour or not.

    There is absolutely no reason why such an arrangement couldn't be placed at the top of each riser -- in fact, on a tall building, I'd recommend it.

    Now this does mean that your dry returns have to be adequately vented -- but you have to do that on a two pipe system anyway.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England