steam vs vapor system

Depends. Some vapor systems may not have traps on the rads, for example.

AFAIK, all vapor systems are 2 pipe.

They are all designed to operate on less than 1psi, often half that.

Or another way to look at it. A vapour system is a steam system, with a difference.

First, they are always two pipe systems, often with wet returns as well.

Second, they are designed to work at very low differential pressure between the steam mains and the dry returns — typically 8 OUNCES per square inch as a maximum.

Third, from the initial construction standpoint, as a result they don't need as much headroom from the boiler water line to dry returns — 20 inches is ample.

Fourth, as noted above, they almost always used metering valves adjusted to control the maximum amount of steam going into a radiator to what the radiator can condense. These are defeated by excess pressure differential

As a result of that, the outlet arrangements can vary widely. Some — Hoffman Equipped — always used traps. Others used various patented devices which work well at low pressure, but not at all at higher pressure differentials. Some early ones had no output device at all — just an elbow going to the dry return (but don't be fooled — some of those elbows had traps or other devices built in).

Next, many — though not all — had various patented devices to prevent that pressure differential from becoming excessive in the event the boiler got too enthusiastic. These vary from simple (Hoffman Differential Loop) to remarkably complex.

Sixth (I think) as a result of the wizardry, they almost always used crossover traps from the mains to the dry returns to vent the mains, and all venting was concentrated at the boiler where the dry returns join together.

Now having said all that… there are a number of present day advantages. Among them are that traps and vents last pretty much forever, except for the crossovers, since neither are usually called on to do anything. The crossovers are, but since the differential pressures are so low they tend to last forever, too (to give you the idea, Cedric's crossovers were installed in 1930 and are still just fine, thank you. 95 years young)..

There are a few disadvantages as well. The most critical and most common is that maximum pressure differential. They requite a vapourstat or other controller capable of managing the boiler pressure to below that 8 ounces or so. A corollary is that they misbehave rather badly if someone decides to crank up the pressure to more conventional pressures. Another is that they do not like to be kludged. Vents added where they don't belong. Traps removed or changed where they aren't needed. The differential pressure control devices removed because people don't understand them. That sort of thing. Another is that if someone does make them run on a higher than designed pressure, they can get absurdly out of balance, since the metering valves are defeated.

Like "bodged", only more so.

Vapor.

Don’t forget @Jamie Hall , Gorton made a 1-pipe vapor system (which I recreated in my home).

Any difference in pipe sizing between vapor and higher pressure 2 pipe?

And mine runs at 0.1445oz to 0.578oz most of the time.

8 ounces is annoying with vents on radiators.

Actually, my partner @Gordo does those videos. I'm rarely in them, we don't want the camera to break.

What is "steam vapor" as opposed to "steam"?

yes but be careful, sometimes you have to bump it up a little for it to reliably cutin, make sure you are really sure it is reliably cutting in at the low setting before you leave it unattended for a while.

I give up. Again.

You might be interested, @Mad Dog_2 , that it was Noel who came by one long afternoon many years ago and helped me learn how the Hoffman Equipped System in my main house worked — now powered by Cedric — and how such systems (I guess we can't use the term "vapor" since it now applies to anything running under 1 psig… so what can we call them?) (I'm open to suggestions) were designed and function.

Be nice to have specific differences, rather than "runs under 1 psig".

My MIL's one pipe system does that.

I would say that vapor systems control the steam by controlling pressure and using orifices and other piping configurations, they use very few if any mechanical devices to control the system and ordinary steam systems use mostly mechanical devices like steam traps or air vents. Vapor systems still sometimes have a mechanical device like some sort of pressure regulator at the boiler to equalize the supply and the return and crossover traps and main or return vents but they tend to be more centralized and fewer rather than distributed throughout the system.

So i would say they rely on the low pressure for proper operation rather than just being capable of operating on low pressure.

Differential pressures, got that.

More looking at visible mechanics to identify them. Some, like loop seals are pretty obvious. Others less so. Like running a vapor system at low pressure means rad traps don't need to close and thus last indefinitely etc.

Aside from things like vent traps and the like, who's to know?

Exactly what I was going to say!

Right. But as @mattmia2 says, make sure it cuts in reliably. As long as the Return Trap works, you should be fine. But I'd replace it with a Vaporstat when it's time for a new one.

find an nos honeywell with a mercury switch?

keep the pressuretrol as a secondary safety set to maybe 2 or 3 psig