Optimal boiler flow temperature for a condensing combi boiler

fixxation

Hi, we've got a Viessmann Vitodens 100 condensing combi boiler in our house. We recently had external insulation applied to the house, so I thought we could turn down the "flow temperature" from the boiler to the radiators, as the heat should be retained in the house better now.

The default for the flow temperature was at 75C - and I've turned it down to 50C now. I'm trying to determine what the optimal flow temperature should be... I've read a few different blogs online, some saying 60C, some saying 50C and others saying as low as possible, as long as it can keep up. The hot water is on a differenet setting, so I'm just looking at the temperature for the radiators here. Does anyone know what the most optimal temperature would be, for energy efficiency but also keeping the house warm? I've been testing with 45C and it appears to keep up, is there any harm in running it this cool... or is it better running it as low as possible?

Thanks.

John.G

As low as you like as the heat exchanger is stainless steel.

Ush1

The only problem is it can cause boiler cycling if the minimum output of the boiler is greater than the current heat loss, resulting in the return coming back too warm and cutting off the flame.