Hydronic Buffer Tank Sizing (Anti-Short-Cycle)
Buffer-tank volume to stop a boiler or chiller short-cycling.
Example
You enter
- Minimum on-time (min) 10
- Source minimum output (Btu/hr) 60000
- Allowable temperature swing (°F) 20
You get
- Buffer volume (gal) 60
Details, formula, and sources
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.
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.
The buffer-tank sizing relation is standard hydronic design practice; the equipment minimum-cycle data and the manufacturer govern.
Estimate. AHJ and licensed professional govern.
Field names used by the API: min_on_time_min, source_min_btu, delta_t_f, buffer_volume_gal
- Worst case at zero load size at near-zero simultaneous load, where the full source minimum has nowhere to go but the tankIdronics / Caleffi
- 500 factor 8.33 lb/gal x 60 min/hr for water; adjust for glycolASHRAE