Open the door to an apartment at 2 Charlton Street, and the space will look like any of the 3.6 million units in New York.

That is, with one notable exception: The windows are likely to be closed in the winter, a rarity in older New York buildings where the heat is often blasting from radiators. And yet the unit will be a Goldilocks temperature, just right.

The secret to 2 Charlton’s comfort is an easy-to-miss white box tucked into the corner, what New York-based technology startup Kelvin calls “the Cozy.”