Heating Help

Heat exchanger

Good evening. What(if any)effect does using a water to water heat exchanger to add a hot water loop to a steam boiler have on the sizing of the boiler? I know that when adding a hot water loop directly off the boiler that you don't increase the size off the boiler(provided that the btu output off the zone is within the pickup factor rating). I was wondering if the same rules would apply when using a water to water heat exchanger? Thanks in advance.