Can't Visualize Flow & Venting of a One-Pipe Steam Radiator

Water is most dense; then air; then steam. So steam condenses at top and condensate washes air down into water which goes down the waterspout. Sometimes vent is installed at top of radiator so that liquid drops in steam push the air right out and tweet a whistle as well. Now you have me confused.

https://forum.heatinghelp.com/discussion/178998/time-lapse-infrared-video-of-steam-moving-through-a-radiator-in-a-counter-intuitive-way

leonz said:

To understand fully it you should wander over to www.castrads.com.. The first 2 paragraphs on the first page will explain it for you.

The diagram they provide shows a vent halfway up one of their radiators and shows in the side view that there is a vacuum breaker on the same side in the top plug which allows the radiator to work more efficiently allowing cool air to enter and sealing the radiator when steam enters it to hold the heat in
longer.

Credit for the following goes to campbell-sevey inc.

The vacuum breaker installed in a radiator helps your heating system in in four ways.

1. improving condensate drainage flow back to the boiler
2. reduces the possibility of water hammer
3. minimizes temperature swings and uneven temperature
4. minimizes steam loss

A vacuum breaker in a one-pipe steam radiator in addition to the vent? Seriously, how many people have that? And why wouldn't a well-working vent accomplish the same thing?

@PGB1 I am going to take a stab at the explanation to understand the answer to this query

I like to think of the radiator as a bucket of air. When you fill the radiator with water, the air gets pushed out. if both the top and the bottom of the sections are push-nippled together then it does not matter how the water gets in there. Water will flow from one section to the other section thru the bottom nipples and the air will flow thru the top nipples until the water completely fills the radiator. Once all the air is out, you can close the vent. There may be some very small pockets of air at the top of each radiator above the height of the push nipple but that is miniscule compared to the amount of water. You can see that in your mind's eye right?

Now think about what would happen if there were only top nipples on that hot water radiator. As the water entered the left (or right) section and filled that section, the air would leave that section by way of the top of the section. Water going in and air leaving at the same time as that section filled with water. the air would get pushed out by the weight of the heavier water. Once that first section is full of water, the additional water will fill the next section, heavier water will push the air from the second section. This happens over and over again in each section until the radiator is filled with water. Eventually the radiator will be filled with water and the last section will be filled with water as high as the vent opening will allow air to leave the radiator. You can picture that now, right? So there will be a small amount of air left in the last section and that will float to the top after the water stops pushing the air out and water flow stops


That is how steam works in any container. Just upside down. Since the steam is lighter it will go to the top and the air will find its way out the bottom of the radiator that has bottom only connections. The steam will then pour over to the next section displacing the heavier air to the vent at the end section. It will then pour over to the next then the next until the radiator is full of steam and the steam reaches the vent to close off the air vent. After the steam flow stops all the air finds its way to the bottom of the radiator. During this settling of the air to the bottom, the air has brushed up against the hot steam and hot cast iron, and that air is now very hot. Think of steam as upside down slow motion hot water because it is a fluid just like any other liquid or gas, the heavier stuff will move to the bottom and the lighter stuff will move to the top and once the pressure is equalized it will find its own level.


As the steam looses its latent heat, the steam condenses into water. The water is heavier than both the steam and the air so it finds its way to toe bottom. It also takes up 1700 times less space in the radiator. This leaves more room for fresh steam to enter the radiator. And as long as there is pressure in the boiler that steam is moving into the radiator to replace the condensed steam that is leaving by becoming water, and that water is leaving the radiator, just like the air left the example top only connection radiator above.

If it's a thermostatic valve, yes -- but that's the wrong device. It should be a thermostatic vent. There is a difference. A properly functioning vent -- thermostatic or otherwise -- will not hold a vacuum, and is not intended to.

If the system pressure gets excessive (over 3 psi) the vent mat jam shut. If you are lucky, a sharp rap will open it. If you aren't, you just bought a new vent.

@PGB1 :Time-lapse infrared view of a single-pipe steam radiator heating up
https://youtube.com/shorts/rj7Y9RVyXGg

Gordo said:

@PGB1 :Time-lapse infrared view of a single-pipe steam radiator heating up
https://youtube.com/shorts/rj7Y9RVyXGg

That is exactly how my steam only radiators fill. Monitoring mine simply by feel progression is bottom to top one section at a time. However, I have mostly two column radiators and there is a circulation in the two radiators where the steam comes in from the bottom but tends to go up one tube before the other per section. Intuitively I would have expected it to be consistent one way or the other (left tube before right tube or back before front or vice versa) but find that it is not. Seems to vary from section to section and from radiator to radiator but always one tube heats slightly before the other in the same section.