The heating hot water loop, which is the loop that supplies hot water to the HVAC system for heating the building, is mainly heated by a hydronic boiler. In this setup, water flows directly through the boiler, where it’s heated before being sent out to air handlers or other HVAC units.
In some cases, the hot water loop can also be heated by steam from a steam boiler using a heat exchanger. In that setup, the steam doesn’t touch the water—it simply transfers its heat through the heat exchanger, warming the water indirectly. Both methods help ensure the building stays warm and comfortable, especially in colder months.