An indirect water heater has the advantage of not requiring venting. The boiler you have with a 40 gallon indirect will create 100 gallons of hot water in an hour; this should be sufficient, if not, you can get a very large one and store more. Usually the indirect water heater has more insulation than a gas-fired water heater. You can read a design discussion of the concept at: http://www.hydronic.net
in the Lessons section.