I just moved into a 10 year old home that has a Trianco tankless 3 zone oil burner system (Beckett Burner). When I go to take a shower, the first minute produces more than adequate hot water temperature. After that, the temperature slowly goes to luke warm, even with the shower valve turned all the way to hot. At some point, it does become hot again but only for a short time. I am the only one using water when this happens.
This doesn't appear to be a mixing valve setting because the initial temperature is fine. My previous home had a gas burner with a 50 Gal hot water tank, so this is a new experience for me. As I mentioned, there are 3 zones. 2 of the zones are for baseboard radiators and the third is for a radiant floor system in our family room. Any suggestions on what may be causing this?
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.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013