Hydraulic stability
A buffer tank can provide hydraulic separation or additional water volume where the system design requires it. It can help reduce short cycling in some installations, but it should not be added automatically without a hydraulic reason.
Sizing considerations
Sizing depends on heat-pump minimum output, system water volume, emitter characteristics, control strategy and desired operating cycle. Oversizing can also create unnecessary thermal losses and cost.
Hot water is different
Domestic hot-water storage is not the same function as a heating buffer tank. The required storage temperature, hygiene controls, heat exchanger arrangement and draw-off profile must be considered separately.