HVAC Decisions for old house

That was a beautiful heating system, if not wildly efficient. The flues to near floor level in the basement are unusual, but not unique. The recirculation is from the upper floors down eventually to the basement, then back up through the flues and the radiators to the main floor.

Now… what to do next. In the Philadelphia area, both mini-splits and ducted heat pumps are a reasonable way to heat a house, so they can be included in the thinking.

My own preference would be a new steam boiler and work to get the original steam working as was intended — but keep in mind that my bias is towards restoration, not renovation. A good installer could figure out how to manage placing the boiler and managing the breaching so it wasn't a problem.

My second choice would be to evaluate —thoroughly — the actual design day heating requirement of the house, room by room, and go with the heat pumps. The existing mini-splits may or may not be adequate; you can judge that, since you are there. The third floor could, as you suggest, be ducted. If you were to do that, I would close off those underfloor radiators, though I might not remove them.

The one option I wouldn't even think about is converting to hot water heat. That is a switch which rarely works well.

Inefficiencies happen in the boiler getting the heat from the fuel into the steam, not in the steam getting the heat into the house. As long as those indirects are recirculating house air, I don't think they'd be that bad.

The inefficient systems used outside air as the intake. "One and done".

So no, not "wildly inefficient".

I have to admit I really don't know where to look for more information on those basement flues. I can tell you where I've seen them before: some old Vermont schools used them, and they did work and did work well. The "inefficiency" comes from using the basement as a sort of a plenum, if you will, for the return air — which means the basement is warmer and drier than it might otherwise be. This is not necessarily a bad thing! And you get the benefit of good air circulation.

Estimating the EDR of the old radiation may be a bit tricky, but remember this: it is an estimate, not a hard number, and there will be no harm if you are off by even ten or 20 percent.

It might be worth the expense of getting @Steamhead to come and look at the system (he's in Baltimore) and you might also be able to persuade @STEVEusaPA to come by. I think he's not that far from you.

I'm glad you are sort of thinking in terms of restoration. It can be daunting at first — you often will not be able to go down to the Big Box and get a bit of something you need — but it needn't be that much more expensive than just remodelling. Perhaps the biggest thing is actually in your favour: take it slow!

I am sure that @Jamie Hall spoke correctly when saying the system was "Efficient". referring back to the days of district steam in Philadelphia. 🦅Go Birds 🦅!

Getting a steam boiler that works properly with a proper exhaust vent should be no problem for a good Steam Pro. All the Phila Steam guys I know are retired or dead by now. But you can find a good one. The Crown boiler folks are still located in Philadelphia and they will know who is a good steam guy in the city. It is called Velocity Boiler Works now. You can drop my name Edward Young from South Jersey and you may get a little better response. I Installed the boiler in Yale's previous home in Margate NJ.

The Crown boiler representatives are not supposed to recommend anyone company over another, but if you ask them if there is a particular steam person that might be available to consult before making another decision on weather or no to use a replacement crown boiler. Like someone as good as "Eddie Young". Have your model and serial number from the old crown boiler ready for them. You may get a referral but don't tell anyone that the factory recommended someone over another. That is not good for business and could cause some problems

That appears to be a 1 pipe steam system. To convert to water would require an almost complete and total re-pipe of the $y$tem, yes I used dollar signs on purpose. That said, pressure won't be an issue for pipes because you won't be able to use any of them, or at least, very few. This could be quite intrusive and most likely would involve opening up walls and such. Personally I wouldn't keep that option on the table. Yes you may gain some efficiency, but in no way shape or form will the conversion costs ever get paid back in fuel savings, in your life, or maybe even your kids life. Also a conversion assumes all radiation can be converted. Original steam radiators cannot be converted because of no top connections, so it depends what you have.

For me, I'd get a new boiler, move it to a better spot, hook it up, get the system dialed in and bask in all it's warm glory. BUT and it's a big one, you need to have someone that knows steam to do this. I do my own work, including replacing my boiler. I find steam simple and relatively easy to understand, that doesn't seem to be common in the industry. I'd say if everyone you are calling is suggesting changes or conversions, you probably haven't found a good steam person yet.

As far as the heat pump mini split idea, they are efficient to a point, but it's really about personal preference. I don't care for the barely warm air from a heat pump, but that is my personal preference, you may be fine with it. I can't deny the efficiency of them, it's real.

As far as why the old boiler died so quickly, it's almost certainly due to leaks in the system. Excess makeup water kills these boilers, so that would be another thing on your list to address, finding and fixing any leaks there may be. Wet returns are a common culprit, after that look for steam leaks and address them. It's typically bad valve packing (easy fix) or bad vents, another easy fix.

That would require adding fans to the indirects as you would lose the chimney effect which produces the air convection through the radiators.

With all due respect, I think gravity circulation would work. The radiators are probably oversized for the existing load, and the heat pumps exist to supplement them.

Uncomfortable cold floors over basement are caused by air leaks. Much simpler to fix the leaks than throw more heat at it. For example in my uninsulated stone rubble foundation, I replaced the old leaky windows and air sealed the rim joist and even when in the teens here, the basement is around 62F.

As for the heat, in any place that needs a lot of cooling, you are better of on figuring out a ducted solution. Since you already have the SM48NAMHZ, you can replace the main floor wallmounts with a ducted unit (around SVZ-KP30NA) in the basement and install a 2nd ducted unit (PEAD-A18AA) on the 3rd floor as you suggest. That should be able to carry a 2800sqft house in zone 5 with some new windows and some air sealing improvements. You can also esaly add a couple of takeoffs to add some extra heat to the basement, a warm basement means much more comfortable main floor.

EDIT. An option if you can't get your load low enough, is to install a regular furnace in the basement with a PAA-A24AA1 coil connected to the SM48. This will get you much more heat plus still allow you to use the existing heat pump for AC.