Whether you are faced with replacing an old, large hot water boiler, or when designing a new hydronic heating system, you must decide whether to install one large hot water boiler or multiple smaller boilers. Although most systems are designed around one large boiler, when you think about how most heating systems operate, multiple smaller boilers can make a lot of sense.
The first thing you want to do is perform a heat loss calculation. By establishing the true load on the building, the replacement boiler or boilers will not be oversized. However, the conditions we use to establish the heat loss of the building are assumed to be at design conditions, which means the coldest day of the year. If you want this capability, then you need to size the boiler or boilers for this maximum load. But, there are other considerations.