Window air conditioners can be lifesavers during the hot summer months, especially in older homes or apartments without central air. HomeServe broke down the best ways to run your window air conditioner efficiently. If your window unit is too small, it’ll run constantly and still leave you sweating. Use ‘Eco’ ModeMost newer window air conditioners come with an “eco” or “energy-saver” mode. This simple trick can reduce indoor heat gain by as much as 33%, giving your window unit a much easier job.