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Steam vents

You are touching on something that harks back to about 80 years ago...

When steam condenses, that means that it has given up it's useful (latent) heat. If you do not let the air back in, you will create a vacuum in that radiator.

Because steam goes from high pressure to low pressure, always, and because a vacuum is the lowest of low pressures, it would otherwise fill with steam.

<i>("Nature abhors a vacuum", my dad always said. Tossing that back at him as a personal statement did not get me out of pushing the Hoover around the house mind you.)</i>

This vacuum and immediate refill with steam may cause the room to overheat; the steam "just got done", remember. That vacuum also can, to a lesser extent, hold back some of the returning condensate, the "finger on the straw" concept.

Now, back in the 1920's or 30's, there were in fact vacuum vent valves that purposely did hold a vacuum. For coal-fired boilers in particular, this was a great way to prolong steam production, more gently, when the coal fire was dying. (There are far better explanations, but this is the gist of it.) It was a perfect match.

With the advent of on-off control (oil then gas), vacuum would be created without regard for air removal elsewhere. Pandemonium in there, I tell you. :)


  • Robert O'Brien
    Robert O'Brien Member Posts: 3,542
    Why let the air back in?

    Okay, I have a question about steam radiator vents. My understanding is that they are for evacuating air from the system so that steam can circulate throughout. The air escapes through small holes and there is a valve inside that will close when it senses the air has escaped, sensing either by temperature or by a pressure-sensitive float.

    With most or all valves open, pressure in the system will not build. As valves begin closing, the pressure will mount. When all valves are closed, pressure will rise rapidly and a boiler pressure control will soon cut the flame, because the pressure rise implies that all vents are closed, which in turn implies that all radiators are hot enough and that additional flame will not make them any hotter.

    Once the flame is out, the system will begin cooling and pressure dropping. At a certain point, vents will open again and air will begin rushing back into the system's holes. This will need to be evacuated again in the next flame cycle, which will begin when the boiler pressure control his reached its cut-in minimum.

    What I don't understand is why you would want air to rush back in, or at least why you would want it to rush in as fast as it came out. Wouldn't it make more sense to have the vents admit air much more slowly on the way in than on the way out? That way, the off cycle will be longer and the system should waste less fuel.
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