Would it be more oil efficient to fill a bath halfway with very hot water then the rest with cooler water?

so that it balences out and the hot water isnt running the whole timeWould it be more oil efficient to fill a bath halfway with very hot water then the rest with cooler water?
I'm not quite sure what the other option is. Does ';all the time'; mean for the duration of the bath, or just until the bath is full? Would you be running the hot ';part throttle';? Or in periodic installments to reheat the bath? Slow long-duration or frequent running suggests you'll have more heat losses in pipes but you might enjoy it longer. If you can give that up you might consider switching to showers; the water's just the temp you want it at and you only use 3-6 gal of hot instead of 20.


But if bathe you must, as one answerer said, do the hot water last so it isn't sitting there cooling while you run the cold. (Not overthinking at all.)


If you have a tankless system there's a scenario where filling up with ';hot'; is more fuel efficient. Suppose you start with a cold boiler, and as soon as you've run the water you want you turn off the burner. (You are trying to economize, right?) Then compare two scenarios.


1. You let the boiler heat up to bath temp, say about 110 F, and run hot-only until the tub is full, never letting the boiler temp go above 110 F.


2. You wait until the boiler has heated to its set point, say 150 F, and run a smaller amount of hotter water with cold to get the same volume and temp.


Both scenarios take the same amount of heat from the boiler, but:


More heat remains in the boiler, wasted, in scenario 2. Also, the heat transfer from flame to boiler is more efficient when the boiler is colder as in scenario 1. So the all-';hot'; fill is more efficient than the hot + cold fill.Would it be more oil efficient to fill a bath halfway with very hot water then the rest with cooler water?
Interesting thought, but remember that while that hot water is running you can also have the cold running which = the same thing. When the water leaves the hot water tank it is 100% hot, the cold is added after. So you can mix the hot and cold at the valve or after it is in the tub, same result
I am not into science but common sense tells me that when you put the hot water in first, it will be cooling. Then you run cold. It seems more energy effiecient to run cold first, then hot. Or am I over thinking?
I think what counts in the end is the final overall Temperature.