Mixed Hot Water and Heating Cost Calculator
Estimate hot/cold water volumes and ideal heating input for an entered mixed-water demand and temperature.
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Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Balance hot and cold water at equal density and specific heat to reach the entered use temperature. Calculate the heat added above the cold supply and divide by heater efficiency for purchased energy.
Hot fraction=(use−cold)/(hot−cold); delivered heat=water volume×1000 kg/m³×4186 J/(kg·K)×temperature rise
Worked example
100 L/day mixed from 15°C cold and 60°C hot to 40°C uses 55.556 L hot and 44.444 L cold. Ideal delivered heat is 2.90694 kWh/day; at 90% efficiency the heater input is 3.22994 kWh/day.
Assumptions and input details
Ideal liquid-water mixing with approximate constant density 1000 kg/m³ and heat capacity 4186 J/(kg·K). All converted temperatures must be 0–100°C; hot must exceed cold and use must lie between them. Entered temperatures are scenarios, not thermostat or safety recommendations. Pipe loss, storage/standby loss, heater cycling, water-volume expansion, tariffs and fixed charges are omitted. Add losses only through a consistent entered efficiency, without counting them twice.
References: OpenStax: heat and temperature change
Unit definitions
Unit selection is independent of language. Each selector contains only units for its physical quantity. Calculations use defined base units internally.
The 36 volume definitions distinguish US liquid, US dry and Imperial measures. Cups carry an explicit definition. Japanese shō is 2401/1331 L and a gō is one tenth of it; the explicitly labelled 180 mL rice cup is separate.
Regional measures are not interchangeable merely because their names look similar. This is a documented selection, not a claim to include every historical unit.
NIST measurement tables · UK Imperial definitions · Japanese traditional units · Mainland China and Hong Kong unit distinctions
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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