This is very common in boiler country. It is called an indirect water heater. There are many manuf of these tanks. I would not increase the size of the boiler for this. You pay a penalty in oversizing the boiler and you can achieve the same thing with a "domestic priority control". Essentially, when there is a call for hot water the heating zones close and the boiler capacity is converted to dhw recovery/output. I should ask, how big is the house, heat loss calc. Number of baths and any high flow fixtures, etc.