Pro- Size is the online water heater sizing software from A. O. Smith, who have built a global reputation for water heating innovation and efficiency. The first involves determining what size and heating capacity you need. There are different things to look for when comparing a storage tank water heater with a. Learn how to calculate what size water heater you need. Tank capacity or continuous flow rate are compared to peak usage rate. Get it right & save money. Pro-Size is intended to assist in selecting water heaters or boilers that best meet the specific job requirements. However, the information may not be error free and may not be applicable in slot book of ra 3 circumstances. The result is that many people end up buying a larger system than they actually need. Choosing Hot Water Heaters for Cold Climates There is less hot water ouput when the incoming water supply gets colder. Always check the energy efficiency rating and install the system with the highest star rating you can find and afford. Write this down and move on to the next hot water outlet.