Depends on where you are, America's a big country from frozen tundra to baked deserts and all points in between so heating systems are varied too. Local codes determine fluing and concentric is available in some areas. Most are parallel flued many are sited outdoors. You'll find Worcester (as Bosch) Rinnai, Navian Takagi (I think I may still be a trained installer for them though never did), Noritz and ourselves (badged differently) over there and I saw Vaillant in CA but concentrating on solar unsurprisingly. In crowded areas with apartments combis are used. Tankless up to 300,000 btu's but not off 15mm either. Cheap energy means the boilers are sometimes dedicated to snow clearing the drive with underground heating matrixes, that surprised me somewhat.