I have a 3 family house that has 2 very old oil fired boiers that need to be replaced. I am having gas run into the house and am debating whether to use the new type of tankless condensing units that also provide hot water. The brand that was recommended was Navien. I live in Mass. and would either replace both boilers with 2 units or maybe get 3 units and have each apartment pay their own heat as opposed to me paying for the heat for 2 apartments. I don't know if it would be worth it moneywise. Are these tankless heaters as good and reliable as I was told? What are the down sides?