They both operate on the same principle--conduct the heat of the water through the cast iron into the air. You're trading aesthetics off. One issue is that you need a fairly long run of baseboard to give the equivalent heating "power" (i.e., amount of heat given off by a unit) to equal one of those decent sized old radiators. A heating guy can do the calcs, but when we did a renovation we mostly reused the old radiators in part because we needed a lot of wall if we wanted to switch to baseboard.

It can be done, but it really should be done by someone who knows exactly what they're doing and who has experience in working with old heating systems. Probably the easiest thing to do would be to remove the radiators, pull out the floor boards, put new over-length stubs on the pipes, and then relay the floor. When you have the final floor in place, you can have the plumber or heating person back in to trim the stubs to the proper length and reattach the radiators.

Hi. I am assuming that you have a boiler, not just a domestic water heater. I would definitely separate the upstairs from the downstairs. One main reason is the upstairs usually gets more sun and requires less heat than the downstairs. If you have the option, why not zone the upstairs bedrooms separately? It sounds like the baseboard on your added bedroom upstairs was installed at the end of the hydronic zone loop. If the piping was undersized, or the pump not large enough, and the other radiators were on, there may not be much heat left in the piping to support another baseboard or cast iron radiator. One way to check is to turn off the flow to the cast iron radiators (usually via a gate valve on the radiator supply or return piping) and see if the other baseboard gets hot. If it does not, it may be air locked and need to be purged. It it does get hot, consider isolating the room as another zone. The current piping is insufficient to provide the needed btu's to the baseboard. Also, on your boiler you should be running at around 180 degrees if possible (assuming it is not steam). If you had a slab system, you would not want to exceed 140 degrees. You might want to have a service man come and check the temperature split between the supply and return lines at the boiler. You may also have a balancing issue, depending on how the header was piped.

Eh, I'd say that if the rest of your house is cast iron, get more cast iron. You can't go wrong with cast iron for hot water heat - it holds heat for a very long time so your heating is super steady and even. I think you'll be happy with a bigger cast iron radiator in the powder room - we have a huge cast iron radiator in our small main bathroom and it's always gloriously warm in there, even if the heat isn't actively running, thanks to the cast iron radiator holding onto residual heat and, well, radiating it. I looked up Runtal radiators because I'd never heard of them. They are quite pretty, especially if you want a more modern look and to save space. But they aren't cast iron, which means they'll heat up faster but also cool off faster, leaving you with greater temperature fluctuations than you'd likely find with cast iron. Not sure that's really an issue in a powder room though.