Looking to replace water heater with a new indirect model, either superstor or triangle tube and trying to make the best decision on sizing the unit for my home. Currently have a oil fired peerless boiler with output of 119,000 btu to feed the unit and have a home of 5 people that need to shower, wash dishes and perform laundry during the day. When performing the calcs I cannot seem to determine which would be more efficient, a smaller unit such as a 30 gallon which falls within the btu input requirements for the boiler but is on the verge of the first hour rating (which I estimate requiring 180 to 200 gallons) or increasing to a 45 - 50 gallon unit which exceeds the first hour rating requirement but is oversized for the boiler. Would the purchase of a larger unit be a waste of money or would it be better to have a larger unit for the water storage with the understanding that the boiler would need to run more to keep up with the unit.

Any input would be great appreciated