Some info from consumer reports assessing installation, performance and payback
They ballpark savings at around 70-80 dollars a year on energy, significant up front costs on the order of several hundreds of dollars in both the heater and electrical, ventilation upgrades, plus note that heat exchanger maintenance is recommended every 1-2 years which negates the energy savings.
http://www.consumerreports.org/cro/a...heaters-ov.htm