There are always heat losses from an oil-fired boiler when it is not running, and it is necessary to keep it hot if you are using it for water heating. That applies if you are using the tankless coil or an indirect water heater heated by the boiler.
Adding an electric water heater is a lot less expensive than adding an indirect water heater and with $4 to $5 oil the electric unit saves money in the non-heating season.
With the current price of fuel oil it saves money to use electric heating in the non-heating season. I installed a 40 gallon electric water heater connected into my hot water line after it leaves the tankless coil.
There are 3 ways you can plumb it:
1. So the electric is always in the line after the tankless coil, just cut the pipe and connect to the inlet and outlet of the electric water heater. This is the simplest.
2. If you use Tees off the hot water line and put a valve between the tees the electric can be bypassed.
3. You can connect it so either the tankless coil or the electric water heater can be bypassed.
Mine is connected so that the water always passes through the tankless coil, but there are bypass and supply connections so that either can be taken out of service if necessary.
There is enough mixing in the electric water heater so the temperature variations from the tankless coil have completely disappeared.
There is usually a temperature modulating valve on a tankless coil system and that should remain in the system if you are using the tankless.
1. The electric water heater is set at a temperature below the normal output of the tankless coil.
2. During the heating season the tankless coil heats the water going into the electric water heater and no electricity is used, unless you are away and don't use any hot water for a very long time.
3. When the heating season ends, turn off the boiler. There is no requirement to open or close valves. The water still flows through the tankless coil but the electric heater provides the heat.
Installing and electric water heater is much simpler and much less expensive than installing an indirect water heater and additional zone off your boiler, with the priority controls that are usually used with such installations. With the price of oil you will reduce energy costs because you will eliminate heat losses from the boiler during the summer. If you are using air conditioning you will also save on A/C because some of those boiler losses contribute to the heat load in the house.