Dry returns run above the water line and carry condensate, air and sometimes steam; once they drop below it, they become wet returns. Wet returns carry no air or steam, so they don't hammer, but they collect sludge and rust and slowly choke, and because they often run along or under the basement floor they rot out. Flushing the wet returns is a standard part of reviving an old steam system.
Wet return is so clogged the water's backing up into the dry return, cut it out and replace it with new pipe.
Before replacing a wet return, flush it with a hose from the end; sometimes the clog is just sludge at one fitting.
Related terms
Hartford loop
A residential steam boiler piping arrangement that stops a leaking return from draining the boiler below a safe level.
Condensate return
The piping and equipment that bring condensed steam back to the boiler room for reuse as feedwater.
Dead men
Steam heating slang for the long-gone engineers and fitters who designed and piped the steam systems that remain in service.
Main vent
A large-capacity air vent at the end of a steam main that lets air out fast so steam can fill the main.