Dunham valve rebuild?

I think that is a check valve on the vent trap.

If there is enough differential between the supply and return it will push the air through the water although that won't happen until the boiler heats enough to build pressure so this radiator likely won't start heating until late in the cycle and if the system is building that much pressure the orifices might not limit the steam to what the radiator can condense(although that is only an inch or 2 wc.). I am pretty sure the controls on the boiler are pressuretrols that are not precise enough to limit the pressure to vapor levels.

All that being said, you can probably tweak that pipe to get it all heading down hill.

I believe your friend's system is a blend of the equipment found on page 12 of the document (page 14 of the PDF) and page 61 of the document (page 62 of the PDF) mentioned above and here.

https://heatinghelp.com/assets/documents/dunham.pdf

Yes traps should be checked for proper functionality an incorrect pipe pitch should be corrected.

Upon inspection you may find this is the only vent or vent pipe in the system.

Does this gauge work ? Notice the vacuum part of the gauge.

I bet when this system was working correctly (and was air tight) it would fall into a vacuum between heating cycles so the need for significant venting to get the air out with every cycle was not needed. Since between heating cycles the majority of the air is not let back into the system, hence the vacuum generated.

Does your system still have the Dunham Air Eliminator with Air Check and the Dunham Return Trap ?

Looks like a check valve to me, see the arrow. Hopefully the crack pressure is less than 1 inch of Water column. To purge the air, if fuel is burned to build pressure to overcome too high a crack pressure of an inappropriately high crack pressure check valve, that is a waste.

The Dunham Air Eliminator with Air Check is the functional equivalent of a Vacuum Vent, which lets air out but not back in with a very low vent crack pressure. The air check is on top, however that does not look original. Your Gorton vents (on your system) let the air back in, only to be pushed back out with the next heating cycle, burning fuel to do it.

I'd replace the gauge with the same type to see if there is any vacuum, if you want to add a low pressure gauge in addition that is OK.

Without, disassembly cleaning / rebuilding, etc. system performance and system pressure / vacuum. I would expect to see a vacuum, but if you don't have a working vacuum gauge or a Manometer you can't monitor that characteristic.

I'll have to look through that document (I'm the curious type) to see if there is any typical pressures stated. Back in the day a gasoline engine has about 18-20 inches manifold vacuum at idle, probably still close to that.

This type of valve takes very little pressure to vent. No spring just gravity pulling down on the weigh of that little disk. I'll see if I can find any info on that check valve in the system.

Looks like the system may be designed to run at 10 Oz. or less.

With the design of these systems steam probably never gets to the Air eliminator.

If that vacuum vent on the air eliminator assembly is working — or can be made to work — you don't need and don't want any other vents anywhere else on the system. The system will work just as well — if not better — without them.

With one caution: the crossover traps must be working. Easy enough to check: when the system fires up, the steam main and inlet to the crossover trap should get nice and steam hot, but the outlet should be cooler (it won't be cold — could be as much as 200 to 205 — but distinctly cooler).

If it isn't working, there's no real harm to putting a Gorton #2 on there instead. Unless your system is truly very large, that will be quite adequate. The only downside to that is that the system won't hold a vacuum — which is really no big deal anyway.

The desired maximum pressure for that system is 8 OUNCES per square inch, gauge. You will need to have a 0 to 16 ounce vapourstat to manage that. And no, a greater pressure is not useful. In fact, it will slow or stop the system from running! Not that it will break anything, at least within reason, but you won't get much heat, either, from the fuel you burn…

I found two names associated with a very similar check valve Swagelok and Nupro and the one spec page does mention gravity, but sadly they don't mention the crack pressure, just what I believe is maximum working pressure. I'd see what the system does, pressure / vacuum and heating performance in general.

Personally I like vacuum, (wish I had a 2-pipe system or all vacuum vents) but it can have issues like hard to find leaks. I think the benefits are less air in the system, minimizing corrosion, less air to push out each cycle, so faster heating, slightly lower boiling point.

Are the benefits worth the maintenance expense ? I have no data on that.

As @mattmia2 stated many of the traps may be good, if never actually used no cyclic expansion stress. If they did not corrode and leak out they may work. If it were me I'd test them, but that takes time, and 'New' does make some folks happy.

No heat could be trapped condensate or trapped air. New traps won't hurt other than the cost. Testing the old ones may provide spares.

Maybe 1 inch, 2 at the most. 2 inches of water column is not much pressure (1.16 Oz, 0.072 PSIG). The air would push through if the differential pressure was there. Really the air just needs to push through at the arrow

Still should be corrected.

In this system (the friend's system) I don't think there is any thermal venting closure.

Is there a union between the trap and the radiator ?

If not, I'm having second thoughts. I know I said " Still should be corrected. " and it should…..

However, if you put the big wrench on it to turn it 15 degrees and something breaks you are probably in for a lot more work. Work that may require some serious finesse.

Maybe when originally assembled it got to a point where it got real tight so they stopped, and since it worked they left well enough alone. You may already realize that.