Buffer Tank with Distribution-Loop Credit
Net buffer tank after crediting the water already in the distribution loop: gross V = on_time x (source_min - zone_load) / (500 x delta_T), loop water = 0.0408 x d^2 x L gal, net = max(0, gross - loop). A fat, long common primary loop already holds gallons that count toward the anti-short-cycle volume, so the tank you actually buy is smaller -- or unnecessary. A sizing aid, not the manufacturer's data.
Formula and source
gross_buffer_gal = min_on_time_min x (source_min_btu - zone_min_load_btu) / (500 x delta_t_f); loop_volume_gal = 0.0408 x pipe_id_in^2 x loop_length_ft; net_tank_gal = max(0, gross_buffer_gal - loop_volume_gal).
ASHRAE / Idronics (Caleffi) anti-short-cycle buffer-tank practice, by name.
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