The buffer is an absolute necessity. The European blokes I know (who only do rads and floors, typically) tell me that 30L/kw (25G/ton?) is the right size to prevent issues. Those issues can be based on multiple zones and therefore multiple flow rates (which can cause freeze ups in the HP), but are as much an issue of trying to maintain the closest dT across the HP to keep the running current down. If the suction and discharge stays at a close ratio, the unit will last longer and have less start/stops. ALL my discussions with the dudes over there come to this conclusion, and their COPs are better because of it.
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