When I started the OTGW boiler tuning I hoped to save 5% gas by letting the boiler run more efficiently. Now that I have a full year gas consumption from July-July, I was able to compare it with previous years.
We actually used 40% less gas than in previous years. WOW!
Not all savings are due to tuning the boiler, but introducing OTGW on my Homey surely was a huge factor. These are the measures that enabled the 40% less usage:
- we had a relatively warm winter
- awareness by all residents that gas prices were enormous
- awareness forced by the shower timer I built with Homey (see OP)
- automatically lowering the boiler temperature to 40 degrees after 5 minutes of showering (and back to 58 when shower finished)
- installing a thermostat valve in the bathroom
- installing a thermostat valve in the hallway, and setting it to frost prevention mode
- installing radiator fans on all convectors in the living/kitchen area
- installed a new, better insulated, door between living room and hallway
- lowering the thermostat setting with 1 degree (to 19) and wearing extra sweater
- lowering the thermostat to 15 degrees an hour earlyer than before (9pm now)
- tuning the boiler:
- lowering max water temp to 45 degrees
- setting boiler heater power to minimum value
- fine tuning the max pump speed
- so that the return water temp is minimized, and boiler stays in low burner modulation as long as possible
This summer I found a cheap (18 euro) laser guided thermal temperature measuring gun at Lidl. I will use it when winter starts to fine tune the valves on each convector so that ‘delta T’ is equal everywhere. This will further optimize (=lower) the return water temperature, and thus squeeze maybe even a bit more efficiency out of the boiler.
Fine tuning the Central Heating system was very time consuming, but I never imagined we would save this amount of gas!
Hope I can inspire others to do the same.
PS: I read a lot online, discussed on forums, and watched youtube clips about boiler tuning. I have seen/read enough to know I am by far an expert on the subject, so please forgive me if some of the measures I described above are suboptimal, or even plainly wrong, but for me the result is there! 
