Best Of

I'm going out on a limb a bit, and maybe you've already taken care of this -- but the Dunham residential systems were and remain exceedingly sensitive to excess steam pressure. In theory the air elinator trap float should close the air vent -- which, incidentally, must be the ONLY air vent on the system -- when the pressure in the boiler gets too high to allow condensate to flow back to the boiler. This will allow pressure from the boiler to build in the dry return, and hold the water level where it belongs. If that float doesn't close properly -- or if it does, but there is another vent ANYWHERE on the system, ... repeat: ANYWHERE -- even on a radiator! ... that won't happen, and the boiler water will back out into and flood the dry return and, very likely, the returning steam main.

This sounds to me very like what you are experiencing.

There are three things to do. First, the boiler must be controlled by a vapourstat, not a pressuretrol, and set to cut out at not more than 10 ounces. This must be verified by an accurate low pressure gauge. Second, go out in the system and, if you find a vent anywhere, remove and plug it. Third, ensure that the float in the air eliminator is free and that the vent opening on the eliminator is really closed when the float is up

Come to think of it, there's a fourth: if you find a steam main somewhere out in the wilderness which terminates, there must be a crossover trap there to the adjacent dry return (and a drip from both the man and dry return to a wet return at floor level) -- and that crossover trap, and the crossover shown in the diagrams near the boiler (which I think you said was missing?), must be operating properly.

If the Dunham air check is in place on the air eliminator, then the system may be able to produce a vacuum of considerable depth -- in which case the check valve on the return must really work.