Calculation sequence
Step-by-step method
- Calculate hot fraction from entered temperatures.
- Convert peak mixed demand to stored-hot demand.
- Subtract recovery during the peak and clip negative preliminary storage to zero.
PlumbingAdvancedDomestic Hot Water
Water-Heater Storage and Recovery Balance Calculator
Convert a peak mixed-water draw to stored-hot-water demand and subtract recovery available during the peak interval.
Why it matters: Peak draw, mixing fraction, recovery basis, and usable storage must be visible in one workflow.
Calculated result
Engineering evaluation
Preliminary net storage (Vˢ)90 gal
- Hot-water fraction (xʰ)
- 75 %
- Equivalent stored-hot demand (Vʰ)
- 150 gal
- Recovered hot volume (Vʳ)
- 60 gal
Engineering interpretation
The entered peak requires 150 gal stored-hot equivalent, receives 60 gal recovery, and leaves 90 gal preliminary net storage.
Important limitations
- Preliminary screen only. No equipment selection or acceptance is implied. Confirm demand profile, usable storage, recovery rating basis, stratification, dead volume, heat loss, mixing-valve capacity, scald/health policy, redundancy, code, and manufacturer data.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
xʰ=(Tᵐ−Tᶜ)/(Tʰ−Tᶜ)
Vʰ=xʰ·Vᵐ
Vʳ=Rʰ·tᵖ
Vˢ=max(0,Vʰ−Vʳ)
Current substitution
xʰ=(120−60)/(140−60)=0.75; Vʰ=150; Vʳ=60×1=60; Vˢ=90.
Equation legend
Inputs and calculated quantities
- Vᵐ
- Peak mixed-water demand — gal or L
- tᵖ
- Peak interval — h
- Tᵐ
- Mixed delivery temperature — °F or °C
- Tᶜ
- Cold-water temperature — °F or °C
- Tʰ
- Stored hot-water temperature — °F or °C
- Rʰ
- Recovery at storage temperature — gal/h or L/h
- Vˢ
- Preliminary net storage — gal
- xʰ
- Hot-water fraction — %
- Vʰ
- Equivalent stored-hot demand — gal
- Vʳ
- Recovered hot volume — gal
Applications
How this calculation is used
Building design
Screen preliminary net storage from a user-approved peak profile.
Construction
Check scheduled temperatures, recovery, and storage assumptions.
Commissioning
Document the design relationship before testing heater capacity and usable storage.
Quality controls
Assumptions and limitations
Assumptions
- Entered values describe one documented system and use a consistent unit, energy, pressure, elevation, and measurement basis.
- Measurements are representative and synchronized where the calculation combines field readings.
Limitations
- Constant temperatures, ideal mixing, constant recovery, and one peak interval are assumed; draw profile, stratification, dead volume, standby/distribution loss, and redundancy are omitted.
- Demand profile, usable storage, mixing-valve capacity, health/scald policy, code, and manufacturer ratings require Frank's review.
Field use
Verification procedure
- Confirm the calculation boundary, source documents, formula symbols, and units.
- Record representative field values or approved project inputs and their sources.
- Review the result against the applicable code, criteria, manufacturer data, and measurement uncertainty.
Common engineering mistakes
- Combining values from different system boundaries, operating periods, temperature bases, or unit conventions.
- Treating a calculated screening value as a hidden pass/fail threshold or final design decision.
Technical basis
References and source standards
- International Plumbing Code, 2024The legally adopted edition and local amendments govern water, sanitary, storm, sump, and pumping requirements; protected code tables are not reproduced.
- ASSE Product Performance StandardsASSE 1017 and other applicable standards provide product context for hot-water mixing and temperature-control devices.
- American Society of Plumbing EngineersPlumbing design and engineering-method context; verify the current licensed reference and project criteria during professional review.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement