Without knowing the water heater type vent type location and type of termination no one can give you a more definitive response.
Can you vent a hot water heater out the wall.
Yes you can vent a gas fired water heater out of the wall if it is either a power vent unit.
With a direct vent system air for combustion is drawn from a vent pipe that runs through an exterior wall or the roof.
Single wall vent pipe cannot pass through any interior wall.
Water heaters and vents come in multiple forms including induced draft direct venting and conventional draft.
To make venting easiest install your tankless water heater on an outside wall.
The only other option is an electric water heater which requires no vent.
Instead both systems typically expel exhaust through a pipe that runs outside through an exterior wall.
All solid liquid or gas fueled water heater whether they are tank type or tankless must vent through the roof unless they are direct vent.
This one vents out the side wall and it is perfectly fine.
They do make two models of power vent units one is called a direct vent power vent as bob harper pointed out it draws its combustion air from outside unlike a standard power vent unit which burns its combustion air from the dwelling.
Direct vent water heaters.
Two specialized types of water heaters direct vent and mechanical vent also called power vent are exempted from this code requirement.
Single wall natural draft gas water heater vent pipe cannot be used in unheated areas like attic or garage even if only partially penetrating that space because such vent pipe installation will cause excessive condensation on vent pipe walls and compromise proper drafting double wall pipe b vent type is required.
Exceptions may be made for non combustible walls such as solid masonry.
Single wall vent pipe cannot be used for water heaters located in attics and may not pass through attic spaces crawl spaces or any confined or inaccessible location.
However it is very important to check the pvc requirements before installing one.
It can be installed both vertically and horizontally.
Exhaust gases are vented outdoors through a separate vent duct or through a separate chamber of the same pipe this requires a double wall vent duct.
Photos go a long way.
The difference is that a power vent water heater pulls in combustion air from the space.
If the unit is installed on a basement wall the vent pipe can be run up just a few feet and then vented out the side with the use of a 90 degree elbow.