Been in contact with viessmann and all their boilers are capable of running a weather compensation sensor. I've purchased one via Screwfix today and shall be fitting it Monday.
My flow rate has been down at 55 degC so Hopefully by the end of January I should have a comparison Vs December. I've downloaded the outside terms for December and will do the same for January to give a more comparable reading.
As Arnie once said. I'll be back.
Ok a bit of a update.
I fitted the weather compensator to my boiler at the beginning of January
I have the readings from 8th December to 6th January,
compared with 7th January to today. To make it easier Ive used December and January for the figures..
Plus as a comparison I recorded the AVG temperature for both months.
December figures
Gas total usage 2349kwh
Highest daily usage 104.5kwh
Lowest daily usage 43.8
AVG temperature 6.3C
High 14C
Low -5C
January figures after compensator
Gas total usage 1911kwh
Highest daily usage 89.6kwh
Lowest daily usage 38.5
AVG temperature 5.8C
High 13
Low -3
The temperature in both Dec and Jan are very similar.
I only use the gas for heating and hot water.
I made a slight correction to the room temperature after the compensator was fitted and upped the temp by 1degC. Plus I set it to turn on at 5am not 5:30am as in december.
From this short experiment the weather compensator looks like it's saved about 18% though if I didn't up the temp and bring the heating on 1/2 hour earlier it would have been closer to 23%
