For prewar buildings looking to reduce their carbon footprint, it might well be worth evaluating how much fossil fuel is being used to heat your lobby. That’s exactly what the board at 1120 Fifth Ave., a landmarked 45-unit co-op in Carnegie Hill, did, which turned out to be the first step in transforming the 1,800-square-foot lobby from a gas-guzzling space to one that’s fully electrified.   Read the article…………………………….