Do not believe that 5-20 degree difference in water temperature will have much affect on wood usage. Believe what temperature do you keep your house set at, how well insulated is your home, how many square feet are you heating, using a plate heater for hot water, how many people in your family and how much hot water do you use for bathing, dishwasher, number of loads of laundry do you do? etc.
Cool. Was just curious.
I don't believe any of those factors make a difference in my wood consumption. The blower on my stove doesn't kick on when the temperature in my house changes. It only kicks on, when the water temperature in my system drops below 165. My hot water might affect it a little but, I believe it's insignificant. I have one of those in line heat exchange modules right before my water heater.
I notice the biggest difference in fuel efficiency to be, based on outside temperature. The colder it is, the more wood I burn. I also believe the 2nd biggest, or biggest depending on your perspective, is the fact that I also heat my garage, & it is located 300' from my stove. So the buried water lines that go to my house, & heat exchange in my attic, total approx 100' or so (roundtrip) as a guess. Best I calculate, I have approx 650' of water lines to heat my garage (heat exchange hanging from ceiling). I'm convinced the majority of the temp drop of my water comes from the long haul to the garage.
I'm not really complaining as, I absolutely love walking into my garage at 70 degrees while it's cold & nasty outside. I was just more curious as to whether other folks had found sweet spots in the operating range. As stated, I'm operating at 165-180. I turn my stove on first of October, & run it until late April, or early May. So a full 7 months, & am using approx 15 cord/year on average the last 3 winters. Even a 10-15% improvement would be noticeable but, it's not the end of the world for the time being to continue as I am.
I believe my wood consumption would 1/2 or less if I wasn't running to my garage. So far, it's worth the extra work though. My tractor, motorcycle, & small engines love it too.