I don't know if the oil-based Trianco boiler is the same general design of the gas-fired one, but my experience with the thing is that it is not particularly reliable, and I'm going to replace mine soon (I keep saying that! -one of these days it will happen). In mine, the sensor that keeps the reservoir hot is slow to respond which allows the heat transfer to be lacking until it catches up. You could add a zone, use an indirect storage tank, and get similar (probably better) results than you had with your old hot water tank. Also, as I understand it Trianco is out of business, getting parts is possible, but not very timely. My local contractors won't work on them anymore, so I have to do it myself until I replace it. My unprofessional opinion.