Hydronic Buffer Tank Sizing Calculator
Estimate the buffer-tank volume required to achieve a target heat-source run time without excessive short cycling.
Net heat into tank
48 MBH
Minimum active volume
48 gal
Metric volume
182 L
Turn the calculation into a complete design workflow
Use the free result as a quick engineering check, then continue into MEPFlow for connected system design, sizing and project calculations.
How the Hydronic Buffer Tank Sizing Calculator Works
Buffer tank volume depends on how much net heat the tank must absorb during the minimum acceptable heat-source on-cycle. Subtract the concurrent system load from heat-source output, multiply by desired run time, then divide by the water heat-capacity factor and allowed tank temperature swing.
For a modulating heat source, the relevant source capacity can be the minimum stable output rather than full rated output when evaluating short-cycle risk. Use the operating condition that actually creates the minimum-load problem.
The calculation estimates active water volume. Final tank selection should also consider connection arrangement, hydraulic separation, sensor placement, stratification, glycol fluid properties and manufacturer minimum volume requirements.
Calculation basis
V = t × (Qsource − Qload) ÷ (500 × ΔT)
When to Use This Calculator
- Air-to-water heat pumps
- Highly zoned boiler systems
- Chiller minimum-volume checks
- Short-cycle mitigation
Engineering Checks Before Final Design
- Use net source output minus concurrent load.
- The 500 factor assumes water.
- Manufacturer minimum system volume can govern even if the formula gives less.
Frequently Asked Questions
Why does a longer run time require a larger tank?
The tank must absorb net heat for a longer period before the heat source is allowed to turn off.
What load should I subtract from heat-source output?
Use the minimum concurrent load actually served while the tank is charging under the short-cycle design condition.
What delta-T should I use?
Use the allowable change in average tank temperature during the selected on-cycle, consistent with controls and terminal requirements.
Can the same equation be used for cooling?
Yes for a preliminary water-volume calculation, using cooling capacity and the allowed tank temperature change.
Engineering References
MEPFlow uses public engineering references to document the calculation basis. Always verify the final design against the standard, code edition and manufacturer data applicable to your project.