We are planning to use tankless hot water in our new construction. Our water heater will be near an outside wall so the vent run will be fairly short. Is it worth the extra efficiency to get the condensing heater? What are the pros and cons for our situation?

My second question is sizing the unit. Our house will have 4 bedrooms and 3.5 bathrooms. While there are only two of us there most of the time, I want to be able to take a shower and do the laundry or run the dishwasher at the same time, and I want to be sure two people can shower at the same time when we have company. We live in a cold climate. I've played with some of the online calculators, but they seem unrealistic. I have showered at my brother's with his undersized unit and I definitely want to make sure we don't undersize.