My sister has lived in her house about 3 years, inspection was fine and no issues since. Hot water tank started leaking, the installer for the new one says that the inducer with double wall b venting is incorrectly placed, because the HWT and furnace vents tie together, then the inducer is next in line. He says he can't fix it, is only a plumber not HVAC, but the inducer should be before the furnace and HWT vents come together. CO detector across the room has never gone off. Opinions appreciated.