Vent placement in home 2-pipe steam heat system

@pacoit , here is a discussion of steam radiator inlet orifices and the first systems they were used on:

https://www.heatinghelp.com/systems-help-center/european-heating-systems-circa-1907/

I'm wondering where the pipes go after they drop below those old vents. There are elbows that go into what looks like asbestos. Ideally they should drop from that point down into the wet return before connecting to each other or anything else.

Those vents may be some variation of the KMC vacuum vent that it sounds like was just a regulator that let air out under very little pressure but would only let air in when the system was under some amount of vacuum regulated by the weight of a column of mercury. If you disturb those I would be very careful to keep them upright.

Each radiator drops individually in to the wet return? Is there still a vent pipe connected to that?

is there a vent at each drop from each radiator?

I think we've discovered a new (to us) variation of Vapor. More pics please!

you just need vents on the mains now. you can use the radiator valve to balance the system. you don't need orifice plates, the vent and the water in the wet return will keep the steam from going someplace it shouldn't.

i'd check the vent on radiator that doesn't heat. all good vents on the radiator drops and big vents on the mains should make it heat quickly(though your system isn't super slow now)

Looks like the radiator in the diagram was installed in the basement- maybe an indirect rad that fed ducts going to main living areas?

I don't know what is happening where there is a tee and the pipe goes up and down and off the branch of the tee. Might not just be a tee in there.

the trim was probably similar. the indirect radiators would be sized to heat the room. they were to heat fancy rooms without you seeing the radiator in higher end houses. frequently they were ducted to use 100% outside air from a vent outside somewhere.

Somewhere on this site is an article that @DanHolohan wrote about how after the 1918 flu pandemic they built houses to be heated with the windows open or otherwise pull in lots of outside air because they thought that prevented illness. The duct that was around the radiator under the floor that pulled air through the radiator and out that grill in the floor or wall instead of starting with a return vent in that same room was usually connected to a duct that went outside so it would pull in and heat fresh air.

which is also probably why they converted it to standing radiators in the room in the 70's when fuel got expensive.

If it was designed to have a vacuum created by some sort of vacuum pump, probably some sort of eductor rather than a mechanical pump, the mains probably aren't designed for full capacity under atmospheric pressure. fortunately you probably don't need the full capacity for a lot of reasons.

With regard to vacuum. Remember that steam systems work entirely on the basis of relatively small pressure differences between the boiler, the mains, the radiators, and the returns.

There are no pumps!

Now if the system is sealed, it really doesn't care what the absolute pressures are, except that the absolute pressure controls the temperature at which the water boils in the boiler and condenses in the radiators. Higher pressure (absolute!!!), higher temperature. Lower pressure, lower temperature.

For practical reasons, we often use a shorthand reference: the atmosphere. Any pressure in the system in excess of the local atmosphere we call "pressure" — and any pressure less than the atmosphere we call "vacuum". When we are talking about pressures with the atmosphere as the reference, an engineer will use the term gauge pressure — and this is the pressure which we see on a pressure gauge on a boiler. If the engineer is concerned about the way the system operates, though, he or she will use the term absolute pressure, which is referenced to what the pressure would be in a vessel completely empty or air (or steam, refrigerant, or whatever)

All this can take a while to wrap one's head around, but it really is helpful in understanding how the system really operates.

Since we also play with liquids — condensate — we also need to take into account the change in pressure in the liquid with vertical depth. This will be the pressure at the free surface of the liquid (such as the water standing in a drip to a wet return — or the boiler) plus an amount related to the depth of the liquid and its density. In the case of water, this increase is approximately 0.43 psi per foot of depth.

Now let's look — finally! — at your question about the effect of a vacuum on the height of the water column in a drip to a wet return. And let us suppose that that wet return is directly connected to the boiler water. Remember that pressure in a liquid is related only to depth, not horizontal distance. So… if that drip is connected at the upper end to a steam main, which in turn is connected the boiler, the water in the drip will stand at exactly the height of the water in the boiler (well, there is a complication, but it's minor). Applying a vacuum to the steam main and hence the boiler makes no difference at all.

However, suppose that the boiler and steam main is operated under a "vacuum" — remember, that's a pressure less than atmospheric — but the upper end of the drip is attached to a dry return which is at atmospheric pressure. To make the pressures balanced, the water in the drip will have to fall, so that the pressure at some depth in the water is equal everywhere. Conversely, if the pressure in the pipe at the upper end of the drip is a vacuum, the water will have to rise in the drip to compensate (this, by the way, is why water rises in a straw…).

Now it is very important to distinguish between steam systems in which the entire system is sealed from the atmosphere, and which can, therefore, operate at an absolute pressure less than atmospheric — a "vacuum" — and steam systems which use a pump or eductor or some other method to evacuate air from the system on startup. They are both sometimes referred to as vacuum systems, but the principle of operation and the effect on operation is quite different, and the "vacuum" is there for different reasons.

One aspect of this affects two pipe steam systems. These, as noted, operated correctly on rather small pressure differences. Absolute pressure in the system affects the temperature at which they operate.. I have sometimes seen it suggested that the dry return on such systems, which normally never contains steam and operates at atmospheric pressure, but subjected to a vacuum — a pressure lower than atmospheric. That can be done, if the system is or can be sealed against a vacuum — but what happens to the pressure differential which drives the system? It is, effectively, increased by the vacuum — which will raise havoc with the balance of the system and the water will stand in various drips. In fact, one can get precisely the same effect by raising the gauge pressure on the steam main/boiler side!

the temp needs to stay hot enough that the water in the flu gases doesn't condense in the boiler. you can do what you describe with hot water systems and mod con boilers but it isn't really practical in a steam system beyond a marginal reduction in operating temp. most of the energy goes to the phase change, the energy in changing the temp a little is small.