With a steam line, when you shut that valve, you could have it down in maybe half an hour, and it would be cool enough for welders to work on it within two to three hours. With a hot-water system that size, depending on where the valving is, it could take 24 hours to drain a small section of pipe, because you can only drain it so fast. We're talking about pipes that are almost as tall as this room—they're huge—and they're not in that tunnel yet. They have to build the tunnels for these pipes. These tunnels were never designed for a system with such a low enthalpy of temperature in there. If they're going to low-temperature hot water, they're going to have to rip a lot of concrete down to try to get them to fit in there—that's for sure.
On February 4th, 2019. See this statement in context.