First of all, in my own home, I've had the largest Bradford White Combi water heater and had good service.
My daughter's house is only 600 sq ft to heat and she lives alone, so not much hot water simultaneous demand. In looking at replacing her system, the smallest Bradford White Combi2 Residential Atmospheric Vent Energy Saver Gas Water Heater System C-DW2-504T10FBN is 45 gallon and 22" diameter. Is there anything smaller from another manufacturer you could recommend?
The other option is to convert her domestic hot water to tankless. I keep hearing how much maintenance is required and that they have relatively short service life. If you were going tankless in a one person home, which would you choose?