Is there any smell when the Navien tankless waterheater is running?
Does the Navien NR-240A generate any odours when it runs?
Inside your home, you won't notice any odours as the unit runs or is idle. This is because the air for the system is drawn directly from the outside, and the exhaust gases are vented directly outside. If you feel you are smelling natural gas or exhaust, the first thing to do is to check that the intake and vent pipes are sealed correctly. As well, ensure that the vent is located far enough away (usually 3 feet or more) from any window that opens.
Another thing to consider is where your unit vents outside. If you have it vent to a sitting area on an outdoor patio, you will notice the smell of natural gas quite frequently when outside in this area. Because the NR-240A doesn't have a pilot light, it must ignite gas when hot water is required. A small amount of gas will be in the exhaust until the gas is ignited, thus generating the natural gas smell at the vent.