> I'm not sure if heating with cooler water is more efficient.

Unfortunately I only have a Dutch link which goes into way more detail: https://gathering.tweakers.net/forum/list_messages/2027810.

Dutch energy companies by law have to advise their customers how to save money. The app I use give exactly this advice (lower the temperature), plus various other advices.

> Some places you pay for the joules delivered into your home

That's something different than what I said, no? I'm talking about when you generate the heat in your home. I'm aware of that solution as well, they're efficient because of volume plus part of the heat (energy required) is waste-heat from some industry.

There's still various ways to save energy despite exactly measuring the temperature out and in. E.g. radiator fans.

I know this all seems entirely illogical. Energy in (or required) should stay the same. Practically though, it's probably energy losses that somehow occur and are avoided.

E.g. for the radiator fans people measured if they save energy. They do, though the cost of buying them might outweigh the savings. DIY is cheap though.

IF that is how you are billed it doesn't matter to you.

Someone is paying to heat that water, and that someone would get a bit more efficiency out of the system if the water temperatures were lower.

Yes, but I think it's only because losses during transfer would be lower.

If you generate energy inside your isolated house and transfer it to radiators also inside your house temperature shouldn't matter.