This type of steam system is only used (hopefully) on very small systems. It serves somewhat well when the pipe has to go out thru a crawl space for example. In this system the steam flows out of the boiler and into the system main which continually slants upwards. That of course is the problem as the condensate is draining back down the same pipe against the flow of the steam. Picture in your minds eye a surfer as he paddles against the incoming flow of the tides. He's working hard to get out to sea. But once he is out there he rockets back in as he is going the same direction as the waves. In steam, its much better to move the steam in the same direction as the condensate is moving. This link is a little description of a counterflow steam system from a 1919 heating & ventilating magazine issue. The second paragraph pretty much sums it up in rather unglowing terms.
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