Having a tankless coil in an oil fired boiler is like leaving your car in the driveway idling because you might go for a ride. An indirect is really the way to go.
Ooooo, that's a good visual. Let's build a top 10.
Like leaving your oven on, in the summer, because you might bake cookies...
Which makes more sense to you? CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10% ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%