Odd failure (long)

ratio

I had a no-heat at the li'l school building I take care of. Two classrooms blowing cold air. They happened to be two of the four rooms heated by Nesbitt cabinet heaters, fed from a steam->water HX. Frozen, of course: I posted pics in another thread. These were the two on the first floor, there're two more on the second floor. One of those was frozen as well, but not burst. The fourth was heating just fine. Now, I'm back in the boiler room. I knew we had lost water, the signs were all over the floor in the first room I looked at, and sure enough, 0 PSI on the gauge. Wait—one unit on the second floor was hot, remember? Sure enough, one of the second floor units was operating like enough to normal that I couldn't see anything wrong with it. Anyway, I refilled the system, started the pump, & sat back to watch it run. The return was scorching hot. What? I don't even... The pump (Armstrong H-54) has the huevos to move the water around even with a little air, I can hear it circulating. But the return is HOT, and the furthest coil was heating. With no pressure in the system. I wander around and notice that there's no air pressure either, turns out the compressor motor's huffed as well. So lot of hot classrooms for a few days, but I realized that the HX was getting steam the whole time as well.

Now, what I think was happening is that the HX, being fed via steam at 8-24 oz/in, was actually boiling the water and it moseyed out the return line to the coldest place it could actually move to, the one unfrozen heater on the second floor, heating the piping on it's journey. Plausible? Can you think of another explanation?

Not done yet...

On my walkthrough yesterday, I fount the top two heating but the bottom two not.

Wha‽ Again?

Nope, the pipes leading to the diverter valve are cool, very cool, but not freezing. But the second floor is heating. In the boiler room I find that the H-54 doesn't sound completely healthy. The return pipe is about 110°, the supply is scorching hot. We're not moving enough water. Turns out the impeller had broken off the shaft & wasn't really pumping any more. We were circulating mostly via gravity, and all that hot water just didn't want to do down to the first floor units, since the piping ran down the ceiling of the first floor hallway. What a day!

I think the impeller fault is the original problem, everything else stemmed from that except for the compressor motor burnout. Not sure how that fits into the puzzle other than a happy coincidence.

JUGHNE

Do you have outside fresh air inlet/mixing dampers?

I have a pretty well unused school with HW Nesbitt cabinet heaters. It has outside air inlet controlled all by pneumatics.
I disabled the air dampers, plugged the inlets with foam insulation and locked the OA dampers shut. (plenty of infiltration thru windows....even when new in 1961).

It was fail safe as if no air pressure then everything would overheat.
But IIRC the outside dampers would close and fans would shut off with the higher night pressure??
Perhaps you have dampers stuck open without air pressure?? Leading to freezing.

ratio

The OA dampers have been disconnected for a goodly number of years. I think spray foam might help seal them up better.