gas usage - non condensing boiler

[desktop987456]

I had an old non condensing boiler (with pilot flame) with a rotary thermostat knob and sensor installed near heat exchanger. I had a timer fitted next to it and it will turn it on at 0500-0800 and then 1600-2100 hrs. I have logged its gas usage so I know how much it was consuming. I used to keep the thermostat between 55-60C.
This winter, with increasing gas prices I decided to save on pilot flame which was always on to an auto ignition kit and a digital thermostat. The temperature sensor is installed on return pipe and I have set the return temperature to 42degC. In simple terms the boiler will fire when return sensor sensed temperature at 42C and will cut out at 47C to maintain the return temperature between 42 and 47C. I don't have any TRV or room stat installed and I haven't changed any configuration except that I have cleaned all 6 radiators manually and have balanced the radiators.
I have compared my gas usage from Nov 2021 and it has gone up by 20-25% in Nov 2022 (I thought I'll be saving gas on pilot flame). My question is that in old scheme the thermostat was controlled by heat exhcanger temperature and in new configuration by return temperature.
- Does this mean that boiler is staying on longer after upgrade this year?
- Should sensor be installed on flow side? Is there any benefit in doing this?

I am not an expert on heating but understand heat loss calculation, sizing and balancing the radiators through online reading etc. (I am not un UK and in my area temperature stays around 5C) Thanks in advance.

 

[Notch7]

I’m not sure the professionals on here will be terribly keen on advising as here in the UK what you are doing is not legally allowed unless you are a member of Gas Safe the professional standards body.