With oil prices being what they are, I have been considering going to an electric water heater rather than the tankless coil.
I realize that oil BTUs are cheaper than electric ones, but, with oil getting more expensive, I'm not so sure that the difference is that big.
I would leave the tankless coil in place to heat the water during cold months when the boiler is working anyway. I would add the electric water heater down stream of the coil. It would handle all the heating chores when the boiler was off and serve as a tempering valve, sort of, when the tankless was providing the hot water.
I currently have a tempering valve, but, it seems to not be doing the job currently. I suspect it needs cleaning/replacement. If it does need replacement, I would probably just go ahead and get the electric heater and do away with the tempering valve.
Any suggestions appreciated.