Its the boiler return temperature that determines the boiler efficiency, most boilers just control the flow temperature and the return temperature will obviously be lower than this, by 10C to 20C, so if your flow temp is actually 55C then the boiler will be pretty efficient as the return temp will be 45C to 35C. Outside temperature compensation will reduce the flow temperature automatically leading to greater efficiencies. You can also run a Vaillant in return temperature control but I see very little examples of this on any heating sites.
The Vaillant Ecotech 418, for one, I asked someone in, I think, another heating forum to c/o to return temperature control, here is a extract of my comments on that.I've not heard of return temp control before, which boiler offers this please?
(Vaillant Ecotec 418) briefly c/o to Return Temp control, it had been set to Flow control with a SP of 70C which gave a return temp of 55C, the target temp was then set to 55C on c/o and the flow/return temps then fell to 47C/37C, with a observed d.05 of 36C which is the "Determined heating flow set target temperature", presumably, this was now the calculated return temperature required?. Unfortunately, as there were more pressing problems at the time, the test was not continued, maybe there's a volunteer or two out there?.
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