Originally posted by killa47
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And as you say, any flow that goes through the ABV will not drop in temperature like the flow through a radiator, so too much flow through an ABV will raise the return temperature, which will be a blend between the hotter water coming through the ABV and the cooler water returning from the radiators, in proportional to how much of the return flow comes either way, just like adjusting a mixer tap.
I think it's also fair to say that setting an ABV too low in pressure is not good for condensing boiler efficiency as you're raising the return flow temperature unnecessarily by having the ABV flowing most of the time. Ideally you only want the ABV to flow when there is only one radiator flowing or a few only slightly open, to limit the maximum pressure and avoid the radiators that are on getting too noisy. But if you have at least two radiators fully open there shouldn't really be any flow through the ABV - that's the point of using a pressure relief valve design instead of the old fashioned slightly open gate valve that passes some flow all the time.
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