An indirect water heater uses a boiler to heat a fluid that’s circulated through a heat exchanger in the storage tank. An indirect water heater, if used with a high-efficiency boiler and well-insulated tank, can be the least expensive means of providing hot water, particularly if the heat source boiler is set to “cold start.” Indirect water heaters are a more efficient choice for most homes, even though they require a storage tank and can be and excellent and efficient hot water solution for your home or business!
Efficient, but not as efficient as on demand
Fast hot water recovery time
If you already have a boiler this makes the most sense
Low maintenance (less mechanical devices for something to go wrong)
Initial cost is higher like the on demand