January. Teaneck, maybe Paramus. The thermostat’s set to 68, the boiler’s doing its thing down in the basement (you can hear it), and the living room’s toasty. Then there’s the back bedroom over the garage. Walk in there and it’s like somebody cracked a window. Touch the radiator. Cool on top, kind of lukewarm underneath. Annoying, sure. But that cool-on-top detail is a clue, and a pretty good one. Cold up top? Air’s trapped in…