If you're tight on space, a tankless water heater is the perfect addition to your boiler room. Since there is no tank of hot water to be stored, tankless water heaters are typically very small. The Takagi T-K3 is only 13" x 20.5". When you compare that to a tank style heater or an indirect water heater, a tankless is truly tiny.
Tankless heaters can be found at www.supplyhouse.com