Indirect radiator box design/construction

mr_pita

I am seeking design and/or construction advice for replacing the radiator boxes/"ductwork" in my basement. I have a 1906-1912 vintage home in Philadelphia with mostly indirect steam radiators (2 pipe with vents, no steam traps) mounted in basement cellar ceiling. The only radiators above the basement are in the bathrooms and laundry area. I'm 48, grew up here and house has been in my family for 10-15 years longer so I have a pretty good handle on its history, etc. otherwise.

The boxes/"ductwork" surrounding the radiators have mostly either fallen into disrepair (2 are missing/completely collapsed), or have been replaced over the years by well meaning Dad and older brothers using whatever materials were on hand, and probably less than careful design consideration. The 'original' or oldest boxes I have/had, had a sheet metal lining surrounding the top and sides of the radiators, completely open on the bottom and then surrounded by wood; horizontal boards (either tongue & groove or ship-lap) fixed to vertical battens at the corners. The replaced ones are a variety of construction, plywood to particleboard to foam board and mostly just the four sides; mostly no sheet metal lining and no obvious lining at the ceiling. Where the ceiling is lined, original or replaced, there is opening for the duct above.

I have sought and not found more details on how these should be made. The closest I came was finding the following passage at https://heatinghelp.com/dead-men-tales/hidden-radiators/

"Within the duct, the indirect radiator has to be about 10 inches below the top, and eight inches above the bottom. The radiators have to be tight against both sides of the duct. These dimensions are crucial to the proper flow of air across any indirect radiator."

Among my many questions...
1. From the quote above, my radiators are roughly 10 inches from the ceiling (I have not measured since finding that). Check. How tight is "tight"? - In contact, no more than an inch (or x inches) of air space to the walls of the box, etc? (It also says "both" sides, I'm assuming "tight" on all 4 sides?)
2. "... 8 inches above the bottom." The bottoms of my boxes are completely open, however I have seen others' are enclosed and reduced down to a smaller duct size, as seen in comments/photos here:https://forum.heatinghelp.com/discussion/comment/1636223 There are no cold-air inlets to the outside, and it has always been this way (open, taking in basement air) for as long as I can recall. Which is right? Either way, I'll be sure to extend the larger box 8" down. Is there any advantage to extend them further - should the end be 8" lower or at least 8"? (Thinking extra length might help induce more of a "chimney" effect, but I'm a fireman, not an HVAC guy)
3. If I should be enclosing bottoms and adding a longer/smaller "duct", what size do I make it? I presume it'd function best centered in the box; would it matter if it were closer to exterior wall? (Just looking for practicality of storage underneath, etc.)
4. Instead of sheet metal to line the boxes, would there be any disadvantage or advantage to using foil-faced polyisocyanurate foam board as a lining?
5. My plans currently are to build plywood boxes lined with foil-faced foam, fit "tight" to the radiators leaving about 1/2-1" space or less to foil-facing. Cutouts as appropriate for piping/vents to exit the boxes. Any issues or suggested improvements?

These have been a nagging issue for my family for years; I asked here about them 11 years ago and didn't know then what I know now. Now house is mine and I'm more focused, LOL. My basement this winter has been as high as 90* after replacing boiler earlier this fall so I know there is just a TON of wasted heat from not having these boxes fully in place. (2 of 6 are completely missing) My contractor who replaced the boiler has many years with steam experience, but did not have much input beyond "eh, just build a box". I'm fairly accomplished/experienced as carpenter/DIYer so looking to build myself to further save money. I know I may be overthinking things and if I am, tell me!

If you have continued to read all the way down here, my apologies for the length and great thanks in advance for any/all help!

hot_rod

You could always prototype some options out or cardboard to see how they work

The heat transfers off a sectional cast iron radiator by radiation, think of sunshine, and also convection, air currents moving across the passageways.

Depending on your goal? Reduce head, nicer look?

Now if these are thin fin type heat emitters, those are mostly convection, so cool air needs to enter the box at the bottom and have top opening for the warmed connective currents to leave. The entire front could be covered to aid convection through the fins

Movable dampers on any openings would allow you tho adjust output somewhat

clammy

Usually there enclosed in sheet metal ,I would stick to using it over any other material . Is there a grill directly over the radiator ? Most of the time on ones which I have seen are just enclosed in a metal box w either a floor register directly over the radiator and usually to one end w a air inlet duct cut into the opposite side . Some would have sheet metal boxes and duct work going to either remote floor or wall registers . They would also have a ducted return to some place in the same room , to get it all circulating there’s was usually a outside air inlet in the bottom or side of the radiator enclosure ,without which circulation stunk and very little heat out put .Most in older homes went outside through window well and such ,most had manual dampers on them ,later in time motorized dampers where they would open and close in response to steam supply temps . In lot of cases I have ran into the outside air inlets where boarded up and holes just cut into the duct work in the basement , I can’t say what the result where but when the stock market die down in days of old it was some thing that went and the rising fuel cost of the 70 s sealed the rest of them up .Depending on the size of the supply register I would only think you would need a inlet opening about 1/3 to half it’s area .If the basement is that cold possibly a rectangular duct could be dropped down a ft or so from the floor would help create some convection . Peace and good luck clammy