Supplying steam to one radiator from 2 zones

Never tried it. But… let's think about this for a minute.

You have two boilers. You want either one to be able to feed steam to a single radiator, but not both at once.

System is two pipe, so the radiator is vented through dry returns. Presumably there are also two separate dry returns, one for each boiler circuit? And presumably those dry returns are vented and have drips to the respective wet returns?

OK. If that's the picture, the best way I can think of doing this — and I'm sure @Steamhead will have a better idea! — is a total of four powered, full port ball valves. Two on what we will call system A — one on the runout from an A main to the radiator, the other on the return from the radiator to the A dry return. The other two, correspondingly, connected to the B main and dry return.

All four valves are normally closed. If you want system A to power the radiator, the two A valves are commanded open with a nice relay controlled by something, and that same relay opens the circuit to the B valves so they can't open. If you want B running, its valves are opened by a different relay which also opens the circuits to the A valves.

I'd love to use a swing check — so simple — but I doubt very much that it would open reliably at the pressures you like to run. And that doesn't solve the problem of directing the condensate back to where it belongs…

Now if both of your zones are running off the same boiler, then you only need two of those powered ball valves — one for each runout — since the dry returns and wet returns are common at the boiler anyway. And neither should ever have steam in them!

I always make things more complicated that they really are! If that's so, then all you need are the two ball valves, one from each main to the runout to the radiator.

I believe the motorized valve idea is better than check valve idea.

I had one church where I replaced a steam boiler that had 3 zones. Convoluted history that I can only speculate on but it was in Cape May. The boiler was one pipe steam, in a basement below sea level and every once in a while I'm sure the old coal boiler would be under water. No big problem when that boiler was hand fired. Just pump out the basement after the storm and shovel in more coal.

When it was converted to oil in the 1940s or 50s there was a boiler room built in the parking lot and the old boiler was moved to the boiler room above sea level. At that time three zone valves were added, in the basement ceiling and a condensate return pump on the basement floor. That pump motor is the only part that needed replacing after a basement floor event. The zone valves that I removed were two old Honeywell 3" threaded valves and one Honeywell 1-1/2" threaded valve.

I didn't want to depend on those old valves as there are no replacement parts available any longer so I welded flanges on all three pipes at the valve locations and replaced the valves with something that can be easily purchased and replaced without tearing out a bunch of pipes until you reached an appropriate union.  Since there were 4 rooms that needed to be heated separately and at times when the other zones were not used I came up with the idea of adding a baseboard hot water zone using a tankless coil in the replacement boiler.  

Zone 1 was a 5000 sq ft sanctuary where all the Sunday services were celebrated.

Zone 2 was a 4000 sq ft auditorium on the second floor with an additional 4000 sq ft first floor that had several classrooms and activity rooms. 

Zone 3 was the 1-½ zone CO2 that heated only 4 large radiators in a smaller chapel that might be used for small weddings or funerals where there were not a lot of guests expected to attend. 

 The pastor’s office, the secretaries office and the mens and ladies bathroom radiators were all on the largest zone for the two floor 8000 sq foot auditorium/classroom zone.  This meant that when someone wanted to use those rooms, the BIG zone needed to be turned up to 70° 3 or 4 hours in advance. The secretary opted for an electric space heater when she worked on a Tuesdayor Wednesday afternoon

I added the hydronic baseboard zone with a dedicated thermostat that did not need steam for heat so the boiler only went to 170° when those 4 small rooms were turned on.  BIG savings on fuel usage with that idea.

The reason I mentioned the zone system is that it is often recommended that you stay away from zoning steam boilers.  This system seemed to work with the old boiler so I just went with it with a prayer that I was not getting into a nightmare by replacing the old boiler and zone valves with something that was very similar.  Of course after calculating the standing radiator square feet of equivalent radiation, I did reduce the size by one boiler section, compared to replacing the old boiler size for size.  I also asked some people smarter than me to guide me in this project.  And that is why I believe that motorized valves will offer you a better chance of success over the check valve approach.

I hope you liked this story. If you did, Please tell your friends. If you didn't, keep your mouth shut. 🤣

Why just do the thing you were trying to accomplish to begin with…