I'm no expert but I would have thought that hot water priority is only useful if you have a "quick heat" cylinder which has a proper plate heat exchanger in it, so that the heat to the radiators is only disabled for a short time. (10-15 mins at most) Otherwise you might as well have a combi boiler...
My cylinder is an old one with (presumably) just coils of copper pipe and as such the heat up time is quite slow (about 30 minutes) so for that reason I don't use hot water priority.
Although I think this is probably non standard, to make this work well I also have a separate ABV in series with the hot water indirect loop set to 0.2 bars so that even when both cylinder indirect loop and radiators are circulating at the same time there is sufficient pressure to supply the radiators enough to keep them hot and even heat up, at a reduced but useful rate. (Albeit at 0.2 bars instead of the 0.5 bars the main ABV regulates to when the cylinder loop is cut off) Without that the very short and easy flow path through the cylinder indirect loop completely starved all the radiators and effectively caused a hydraulic hot water priority...
How long does a full reheat of your cylinder take if you use all the hot water, and is your system configured with a flow temperature boost during reheat ? (Mine isn't)
My cylinder is an old one with (presumably) just coils of copper pipe and as such the heat up time is quite slow (about 30 minutes) so for that reason I don't use hot water priority.
Although I think this is probably non standard, to make this work well I also have a separate ABV in series with the hot water indirect loop set to 0.2 bars so that even when both cylinder indirect loop and radiators are circulating at the same time there is sufficient pressure to supply the radiators enough to keep them hot and even heat up, at a reduced but useful rate. (Albeit at 0.2 bars instead of the 0.5 bars the main ABV regulates to when the cylinder loop is cut off) Without that the very short and easy flow path through the cylinder indirect loop completely starved all the radiators and effectively caused a hydraulic hot water priority...
How long does a full reheat of your cylinder take if you use all the hot water, and is your system configured with a flow temperature boost during reheat ? (Mine isn't)
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