You don't. You simply need to add additional venting area if it's in a smaller space. Check the manuals for them, not the poorly written articles. ;)

Many places (and just about everywhere without a basement or attached garage) have water heaters closed up in tight spaces. A heat pump heater won't work there, but if you add some vents to the door so it can exchange air with the rest of the house, they're just fine. Or, depending on where you live, you might consider venting it to the attic - if you have an attic that runs hot, "extracting heat directly from it into the hot water and then cooling the attic" is a good use of time.