Watch: the walkthrough
Sound's on — this is me walking the basement in real time, working out what I was looking at. Music's just underneath; the voice is the real thing.
What I walked into
The job was a dead commercial kitchen — hoods, makeup air, the usual. But the basement stopped me. Empty stone-and-brick rooms, big insulated ductwork running everywhere, riveted tanks, copper mains gone green with age, and these big white boxes hung up above the ducts. You can hear me on the video trying to place it: "these were either some kind of fans or coils originally — I don't know."
Turns out the instinct was right. What I was looking at is one of the cleverest heating setups of the era, and it was built for serious money.
The whole basement was the plenum
Here's the part that got me, and I said it out loud on camera: "essentially this entire basement was the supply plenum." This wasn't a furnace with a few ducts hanging off it. The entire basement was sealed and pressurized as one giant air chamber.
The white boxes above the ducts are radiators — heating coils — hung right inside the air stream. Blower motors pushed air across those hot radiators, the air warmed up, and it rose through decorative grilles in the floors and walls into the rooms above. Heating people call it indirect heating, or plenum heating.
No radiators in the parlors. No radiators in the bedrooms. Just warm air rising up through the floors, like magic — if you had the money.
And that's the whole point. This was the rich man's system. Wealthy owners didn't want ugly radiators showing in the fancy rooms — those only went in the servant areas. So they buried the heat down in the basement and let the structure carry it up invisibly. The same heating-history folks have documented this exact setup in places like the Duke Mansion and The Breakers in Newport.
Built for one of the richest men in America
Rainbow Hill was completed in 1917, connected to Edward T. Stotesbury — a senior partner at J.P. Morgan and Drexel, and one of the richest men in America at the time. At his peak his fortune was estimated at roughly $100 million — well over a billion in today's dollars. He was building his legendary 147-room "American Versailles," Whitemarsh Hall, in those very same years. General Douglas MacArthur later lived at Rainbow Hill and renamed it after his WWI 42nd "Rainbow" Division.
That's what it took to have hidden, whole-house climate distribution in 1917. While most of the country was heating with stoves and coal, a $100-million fortune bought a building engineered as one big air machine.
The opening room — an icehouse?
The room I open the video in — tall paneled doors, old riveted tanks — I think it might have been the estate's cold storage. An icehouse. Gilded Age mansions routinely had dedicated basement ice rooms: thick walls and stone giving you natural thermal mass, a walk-in icebox before mechanical refrigeration was a thing.
I want to be straight, though: I can't confirm that one. There's no record I found that names an icehouse at Rainbow Hill specifically. So I'm raising it as a question, not stating it as fact. If you know this house, I'd love to hear it.
Why it still gets me
The coolest thing for a guy who does this for a living: they still use that original 1917 ductwork to make the modern system function. A new air handler plugs straight into the hundred-year-old air-distribution skeleton. The equipment got swapped out over the decades, but the bones — the ducts, the plenum, the whole layout — were built so well they outlived the machinery by a century.
That's a hundred-plus years of a design still doing real work. We don't always build like that anymore. Worth stopping to look at when you find it.
Sources & honest notes: Indirect/plenum heating from Dan Holohan's "Indirect Heating Systems" and Britannica; documented mansion examples from HeatingHelp; Stotesbury wealth from Wikipedia and American Aristocracy. I've kept this to what the footage and the records actually support: it's a heating plenum, not air conditioning; "one of the richest," not "the richest"; and the icehouse stays an open question.
About the author: Gregory Frazier is a heavy commercial HVAC estimator working Maryland, Pennsylvania, Delaware, Virginia and Florida for 25+ years. Read his story →