You have several choices: power vent, and closed combustion. Either of those could be vented out the side wall without having to go up. The location of the exhaust has some restrictions as does the length of the piping and how many turns it has. The power vent usually uses metal duct, the closed combustion often uses pvc (if it also is a condensing unit). Any combustion device needs combustion air, so a closed combustion (where it draws air directly from outside for combustion) ends up being more efficient. A traditional gas appliance uses interior air, which creates a pressure difference, which draws outside air in through cracks and openings all over the house. This also means you are losing your conditioned air. When you close the combustion loop with outside air, you have less air being drawn into the house that you then have to heat or cool. these types of WH are more expensive than a traditional one, but you may save some on flue costs. The power vent version is noisier, since it tends to have a bigger fan to blow the exhaust gasses out a 'non-traditional' flue (i.e. one that can rise naturally by convection).