Hydronic Buffer Tank Sizing (Anti-Short-Cycle)
Buffer-tank volume to stop a boiler or chiller short-cycling: V = on_time x (source_min - zone_load) / (500 x delta_T). The worst case is at nearly zero load, when the full minimum output has nowhere to go but the tank -- sizing at the design load badly undersizes it. The same formula sizes a chiller buffer. A sizing aid, not the manufacturer's data.
Formula and source
V_gal = min_on_time_min x (source_min_btu - zone_min_load_btu) / (500 x delta_t_f); no buffer when the zone load meets or exceeds the source minimum.
ASHRAE / Idronics (Caleffi) anti-short-cycle buffer-tank practice, by name.
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