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how does water get into radiators in a 1-pipe system?
while looking at different architectures of radiant/hydronic/hot water heating, it seems there are 3 main methods: series, 1-pipe, and 2-pipe. series and 2-pipe are easy to understand, but I'm a little bit uncertain about 1-pipe. the diagrams I see show a single loop of pipe, but then the radiators hang off in parallel to the main pipe. it seems like the radiator would be a higher resistance flow path, and most of the wanter would just circle back to the boiler without losing much heat. does it just rely on that slight pressure difference on each end to put water into the radiator, or is the main mechanism that he hotter water will rise up into the radiators, forcing out cooler water? (heat rising, cold sinking). I would assume that either way, it would be a bad architecture for a condensing boiler as the temperature drop seems like it would always be smaller than the other methods that require the return water to have gone through at least 1 radiator for the 2-pipe system and all radiators for the series system.
secondary thought/question: could one tap off of the return from a 2-pipe system, making a small segment of 1-pipe, and run a hydronic floor off of the cooler return water? obviously, you would need two thermostatic valves so that the water going to the floor is cut off if it exceeds the allowable temperature for your flooring, say 85°F/30°C. this seems like a way of extracting even more heat from return water, helping the efficiency of a condensing boiler, while also allowing slightly higher temperatures in the radiators than is presented to the floor loop (due to it passing through a radiator before getting to the floor loop). is this something that is done, or am I just being a mad-scientist with this idea?