Hello my plumbing consists of a tankless water heater for 2 zones hydronic, radiant floor heat,and drinking water.I need a heat exchanger for my domestic but I don't know which is more efficient a flat plate or an indirect fired storage tank.there are 3.5 baths,4 people.thanks for the help

Sorry Cliff but I'm confuse. A tankless water heater heats domestic potable hot water. Are you using a tankless water heater for something else or is it actaully a boiler?

Yes I'm using it as a my boiler

So are you looping the hot water back to the tankless heater once its made its run through your heating zones? Tankless water heaters are vert efficient compared to a normal hot water heater because they do not heat the water to 120 degrees all day long even if you are not using water. I don't have the numbers to crunch but I do not believe they are going to be super efficient for what you have in mind.