Harold Kestenholz
The above answered your question. IMO the proper way to make hot water while putting a tankless coil out of service is to use an external water heating storage tank, either one with a heat exchanger built as part of the tank or through use of a heat exchanger.
The boiler already has a burner and flue. Using the boiler to make hot water makes sense because only one burner has to be serviced and no changes to the flue are necessary.
A 3-gallon-per-minute tankless-coil boiler with a 120,000 btuh burner can make 180 gallons of hot water in one hour. Nothing else comes close to that, including a residential external boiler-fired water heater. But a 60 gallon boiler-fired water heater can make 100 gallons of hot water in the first hour, and that is a good amount of hot water.
A separate oil-fired water heater has a smaller input. it makes a good amount of hot water, but it requires another burner to service and flue modification.
