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Mixing Valve Temperature Calculator

Work out mixing valve temperature instantly with clear inputs, formula shown and shareable results.

About Mixing Valve Temperature Calculator

A thermostatic mixing valve is a simple energy balance: the fraction of hot water needed is the gap between target and cold divided by the gap between hot and cold. That fraction, multiplied by the blended flow, gives the hot demand the cylinder and hot water pipe must actually deliver.

Formulas

Hot fraction

Hot fraction = (target - cold) / (hot - cold)

Flows

Hot GPM = total x hot fraction; cold GPM = total - hot GPM

Worked example

With the default values below, Mixing Valve Temperature Calculator returns Hot water flow needed of 3.059. Change any input above and every figure here is recalculated in your browser.

Inputs used

Hot supply temperatureminimum 1 °F
140 °F
Cold supply temperatureminimum 1 °F
55 °F
Target blended temperatureminimum 1 °F
120 °F
Total blended flowminimum 0.1 GPM
4 GPM

Results

Hot water flow needed
3.059 GPM
Cold water flow needed
0.941 GPM
Hot water share
76.5% %
Hot to cold ratio
3.250

Step by step

  1. 1Values used
  2. 2Hot fraction
  3. 3Flows
  4. 4Hot water flow needed= 3.059 GPM
  5. 5Cold water flow needed= 0.941 GPM
  6. 6Hot water share= 76.5 %
  7. 7Hot to cold ratio= 3.250

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Scald protection settings and valve selection must comply with local plumbing code and the valve manufacturer's instructions.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Scald protection settings and valve selection must comply with local plumbing code and the valve manufacturer's instructions.

Frequently Asked Questions

How is the Mixing Valve Temperature figure worked out?

Hot fraction = (target - cold) / (hot - cold). A thermostatic mixing valve is a simple energy balance: the fraction of hot water needed is the gap between target and cold divided by the gap between hot and cold.

Why does a mixing valve increase apparent cylinder capacity?

Storing at a higher temperature and blending down means each stored litre serves more than a litre at the tap, so the effective draw-off is larger than the cylinder volume.

What values do I need to enter?

This calculator takes 4 inputs: Hot supply temperature, Cold supply temperature, Target blended temperature, Total blended flow. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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