TANKLESS WATER HEATERS
Tankless water heaters also called Instantaneous or Demand Water Heaters, provide hot water only as it is needed. Traditional storage water heaters produce standby energy losses that cost you money. We do not leave our homes heated while vacationing. We only heat our homes when there is a demand for heat. In the same way, a Tankless Water Heater is used only when there is a demand for hot water.
As an inspector I do come across tankless water heaters and I don’t have a stand for or against them. I can tell you that some people have complained to me about the lack of hot water in large homes where multiple plumbing fixtures are being used at the same time the same time. I recommend that you have your home and specific needs properly sized to determine the appropriate unit or units. Energy.gov has a section on Sizing a New Water Heater. I recommend a rental unit, so if anything goes wrong you have a warranty and service.
I found a couple of sites which have some good information. Energy.gov Tankless Water Heaters and This Old House – Tankless Water Heater Buyers Guide. The information should only be used as a guide. I recommend to consult with a local tankless water heater specialist who is appropriately licensed and make sure you ask about warranty and maintenance.