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!

Pros:
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)

Cons:
Initial cost is higher like the on demand