A condensing WH is very similar to a boiler. On those, the fan is often pressurizing the air/fuel mixture rather than trying to vent the exhaust, so it is sort of like a turbo charger on a car...it's function is significantly muffled. A good boiler is virtually silent a few feet from it. You didn't say how you heat the house, or I missed it. If you have a boiler, then an indirect WH is the way to go, plumbed as a zone to the boiler. Obviously, doesn't work with a furnace, though. I've lived in several houses with (older) tankless systems, and know how they worked...didn't like them, so maybe I'm biased...I'm sure the new ones are a lot better, but they still have their problems, especially when living as far north as you do where incoming winter water temperatures are near freezing at times. Heating the water as you use it those extra 10-20 degrees or more means significant size grades, or multiple units over what those living much further south require. Then, you may need to upgrade your gas service, and don't think about a big electrical one where you live. Given that you have a rental unit, and you can't control their use, you are probably best off with a bigger tank to minimize complaints. Two modest users at the same time is probably all you could expect out of 1/2" supply lines to the house. If you run the WH at a higher temperature and then use a tempering valve on the outlet (required by code where I live), that can make the WH tank 'look' larger since it's mixing cold to bring the temp down, at least when the tank is still hot. By going to any closed combustion, where you're drawing combustion air from outside rather than inside, you'll notice an increase in comfort and economy in the home.