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If this is indeed a steam system water does not travel up into upper floors, steam yes water (condensate) no. It is possible for water to come out of steam vents if the system is flooded. What I suspect is happening is that you have a clogged return most likely at the boiler and condensate is not finding its way back to the boiler and is backing up into the system.
Pressure relief valve is on the boiler not a pipe on an upper floor. I wouldn't waste any time looking for pipes on upper floors. You need to start with pulling the return manifold apart at the boiler. If it was piped right there should be a nipple and cap off the end of the manifold that you can remove to clean/flush it out. I don't know what you mean by a double looped pipe (Hartford loop?) but that's on the supply. You need to check the returns which are connected to the bottom of the boiler not the top.
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