Blend 2–3 mixed streams to their outlet temperature, or work back the hot/cold flow split needed to hit a target blend temperature (TMV / tempering-valve sizing).
Advisory only. Always verify anti-scald compliance to project spec and the applicable standard.
Anti-scald advisory only, not a compliance check. See the note in the results panel. For design guidance only. Always verify with a qualified engineer.
About this blend temperature calculator
This free blend temperature calculator has two modes: blend two or three streams to find their combined outlet temperature, or work back the hot/cold flow split needed to hit a target blend temperature, the core sum behind sizing a thermostatic mixing valve (TMV) or tempering valve. It is aimed at building services and plumbing engineers designing hot water, heating and process mixing. Enter each stream’s flow, temperature and fluid (water, glycol mix or custom cp), or in split mode the hot and cold source temperatures and your target, and it returns the blend temperature or the required mass-flow split. Everything runs in your browser. Nothing is uploaded.
How the blend temperature is calculated
Blend mode is an adiabatic energy balance: Tmix = Σ(ṁ·cp·T) ÷ Σ(ṁ·cp) across the streams, so a hotter or higher-flow stream pulls the mix towards its temperature. 1 L/s of water is treated as ≈ 1 kg/s (ρ ≈ 1000 kg/m³), and glycol specific heat is interpolated from a percentage-by-volume property table. Split mode inverts the same balance: the hot-stream heat-capacity-rate fraction is (Ttarget − Tcold) ÷ (Thot − Tcold), converted to a mass-flow split via each stream’s cp. A target outside the hot–cold range is flagged as not achievable, since no two-stream split can reach it.
Frequently asked questions
Is the anti-scald temperature a compliance check?
No. The tool shows an advisory note on the practical ~43–46°C band that UK guidance converges on (Approved Document Part G, BS EN 1111/1287 TMV2/TMV3 set points, HSE scald-risk guidance and NHS HTM 04-01), but it does not enforce any of them. Anti-scald compliance is out of scope. Treat the blend temperature as a design figure and verify the set point against the applicable standard.
Do I need to enter a specific heat for glycol?
No: pick the glycol type and concentration and the calculator interpolates the specific heat from its property table (percentage by volume). For water, leave cp as the default 4.2 kJ/kg·K. If you are mixing a fluid the presets do not cover, enter a custom cp per stream and it feeds straight into the energy balance.
Why is my target blend temperature ‘not achievable’?
Because it sits outside the range between your hot and cold source temperatures. Mixing two streams can only produce a temperature between the two sources, so a target above the hot source or below the cold one is impossible with that pair. The figures still show how far off the requested split would be, so you can adjust a source temperature or add a third stream in blend mode.