Best Of

I admit to getting somewhat bemused by the discussions of how to vent steam heating systems which turn up from time to time -- often involving detailed calculations of pipe size and volume and the like, and often using (usually somewhat erratically) @gerry gill 's invaluable research and measurements on vents.

The first thing to recognize is that there really are two aspects to venting. The first is to ensure that the steam mains -- including any associated longer risers -- are filled with steam as evenly and as quickly as possible. The question here is usually posed as something along the lines of "how big a vent do I need". Wrong question. The real question is, "how small a vent can I get away with". The velocity (not really the right word but it will do) at which a steam main will fill with steam and begin to deliver steam to the radiation is governed by three things: how long it takes the main itself to warm up and stop condensing the steam, what the boiler power is to generate steam to fill the main, and how fast air can get out of it. While it may be entertaining to calculate the volume of air in the main and what size vent will allow that air to escape at a certain rate and pressure, that ignores the effect of the other two aspects -- which are at least as important. It may make economic sense to skimp on main vent size (big ones don't come cheap) but, on the other hand, it may also be worth remembering that in the bad old days some two pipe systems vented through open pipes to the chimney or even the basement -- and many, but not all, two pipe systems vented into the dry returns with crossover traps, which have a huge airflow capacity (the Barnes & Jones Big Mouth is just a standard trap minus the float, after all).

So for mains... one can say that in general bigger is better.

Risers and runouts are a slightly more complicated situation, and here the consideration is different for one pipe systems and two pipe systems. First, in two pipe systems, the riser or runout will likely be adequately vented by the radiation, unless it is very long and the radiation at the end is relatively small. On one pipe systems, however, a very good case can be made for providing "main" vents on the risers or runouts, unless they are very short. Why? Because in those systems, the vents on the radiators serve a very different function than the traps or other devices on two pipe systems.

On two pipe systems, the heat output of the radiator is controlled by two factors: the size of the radiator and the rate at which steam can enter the radiator (there are a few systems where the steam entry is controlled by limiting the air exit with an orifice at the outlet -- but these behave like one pipe radiators, not two pipe radiators). Venting is not a consideration here, save only that the dry returns are adequately vented (and, like mains, bigger is better here). Rather, the power output may be controlled for the heating demand of the space by adjusting the inlet valve or, if so fitted, by changing the inlet orifice, provided only that the radiator is big enough to begin with.

One pipe is another matter entirely. For a one pipe system once the radiator is filled with steam and the vent is closed, there is no further control of power output possible; the radiator will put out its full rated power until the boiler stops supplying steam (which, incidentally, is why -- besides controlling pressure -- it is essential that a boiler on a one pipe steam system is cycled off at reasonably regular intervals; to regain control of heat in various spaces). What the radiator vent does here is control how long it takes for the radiator to fill with steam from the time that steam reaches it, and thus how long it takes to ramp up to full power -- and therefore the total energy output, averaged over the boiler cycle length, it will produce. Now you can slow this down with a smaller vent, but once the vent size is enlarged to be more or less equivalent to an open pipe of the vent attachment, you can't speed it up. It can be seen, though, that the more evenly steam reaches all the radiators the better one's control can be -- which is an argument as noted above for providing main type venting on longer or more heavily loaded risers and runouts.

Miscellaneous rambling. For what they're worth.