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Exposed duct doubles as radiant heat source?

My energy audit client's condo building was converted in 1995 from a factory, keeping brick walls uninsulated (aesthetics), and with only standard double pane windows (budget). Not the current standard for a Chicago climate & clients feel chilly regardless what the thermostat says! There's an 80afue furnace and ND DHW tank. There's exposed spiral duct in this "lofty" condo, aligned (you guessed it!) with exterior walls/windows. Nothing prevents installing 95% heating equipment.
Question: how about securing PEX (or other) tubing Zig-Zag fashion to the outside of spiral duct, oriented toward people, circulating water from a tankless boiler? Paint the spiral duct a nice matt radiating color and make it function as a radiator. How to bond any tubing to transfer heat into the duct? Any history of this among our august and imaginative Wally's?