wetdryguy odd question but looking for even rough percentages

mastercraft

How would I go about estimating how much fuel oil usage a domestic internal tankless coil uses vs. the heating zones?

STEVEusaPA

Some more details please...
Easiest would be fill up oil tank in Spring, when done with heating. Fill up tank in Fall, before you need heat. Note gallons used, divide by number of days between fills.
If this is a house you're considering buying, maybe their oil company can provide historical data.
It's all an estimated guess, because in the winter, the boiler maybe be running and hot while heating, using no more oil for domestic use.
Keep in mind, an indirect would provide nearly endless hot water, use less oil, and be more efficient than a tankless.

mastercraft

thank you