Balance with a % of the flow temperature or absolute value?

[ama]

First off. A big thankyou to all of you who post here for your time and expert advice. I have successfully dismantled my system, cleaned every radiator, installed deaerator on flow and dirt separator on return added new radiators. Well done to all of you that have posted through the years as I trawled extensively through the posts to decide on best practice. All working well touch wood!

I am now balancing the system and have trawled through the posts re balancing. My question is for the more technically minded really.

Does anyone know where the value of 12oC comes from? Surely the drop in temperature between the flow and the return should be a relative value rather than an absolute value?

For example I have set my boiler to heat the CH circuit to 57 degrees (trying to get it as low as poss to save on heating bills) and so 12 degree drop in temperature is higher relative to this flow temperature when compared with a temperature drop in a system set to a flow temperature of 80 oC?

It is a combi system (Vaillant ECO TECH 837 Plus).

 

[twgas]

The greater the temp dif between rad and room the more heat will be given to the room. If you set a flow temp of 57°c the mean temp of rad will be about 51° so a relatively small temp dif (rad to room) will mean you need larger rads and have to wait longer for the rooms to reach required temp.
I would suggest setting the flow at around 70°c.
If you have a condensing boiler the temp drop should be 20ish°.