Too small a boiler
Can a boiler which just meets your heat loss be too small to use an indirect water heater? Indirect WH seem to rely on significant amounts of boiler to do the job. If you are at design outside temperature, do you have to make a choice between hot showers or warm house. Is there some point were a standalone water heater is necessary and what is the effect on overall energy use. Oil is my fuel of choice if that matters, no gas.