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TMV Sizing Calculator — Part G 48°C Compliance
Size thermostatic mixing valves for Building Regulations Part G compliance. Calculate mixed flow rates, hot/cold ratios, and TMV2/TMV3 selection.
Hot water supply temperature (typically 60°C)
Cold water supply temperature (UK avg 10°C)
Desired flow rate at the outlet
Target mixed water temperature
Adjust supply temperatures and flow rates above for precise TMV selection. The calculator automatically limits bath temperature to 48°C per Part G3(4).
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How We Calculate This
This calculator sizes thermostatic mixing valves in compliance with Building Regulations Approved Document G. It calculates the hot and cold water flow rates needed to achieve the desired mixed temperature at the outlet.
Mixing formula
- Hot flow: Qtotal × (Tmixed − Tcold) ÷ (Thot − Tcold)
- Cold flow: Qtotal − Qhot
- Part G3(4) limit: Bath outlets maximum 48°C
- TMV3 (healthcare): Outlets typically set to 41°C or lower
Always check the TMV manufacturer's data for minimum and maximum flow rates, pressure requirements, and connection sizes. The TMV must be accessible for annual testing and maintenance.
Frequently Asked Questions
Building Regulations Approved Document G, Requirement G3(4), limits the hot water that can be delivered to a fixed bath to a maximum of 48°C in new dwellings and material changes of use. This is normally achieved with a TMV (thermostatic mixing valve) on the bath hot supply. The requirement aims to prevent scalding, particularly for children and vulnerable people. (G3(3) is the separate rule keeping stored hot water below 100°C.)
TMV2 is the BuildCert scheme for domestic and commercial premises; certified valves are tested to BS EN 1111 / BS EN 1287. TMV3 covers higher-risk healthcare premises under the NHS Model Engineering Specification D 08 (HTM 04-01) and BS 7942. TMV3 valves have tighter temperature control, faster fail-safe shutdown on cold-supply failure, and a stricter commissioning and annual-test regime. D 08 belongs to the TMV3 (healthcare) scheme — it is not the TMV2 standard.
No. Building Regulations Part G does not require TMVs on kitchen sinks, as higher temperatures are needed for cleaning and food preparation. However, if vulnerable persons are present (care homes, schools), a risk assessment may recommend TMV protection on all outlets.
The mixing ratio uses the simple heat balance equation: hot flow = total flow × (Tmixed - Tcold) / (Thot - Tcold). For example, with 60°C hot, 10°C cold, and 44°C mixed: hot flow = total × (44-10)/(60-10) = total × 0.68.
A typical bath fill rate is 15–20 litres per minute. This allows a 150-litre bath to fill in 8–10 minutes. Showers typically need 8–12 L/min and basins 4–6 L/min. The TMV must be rated for the required flow rate at the design pressure drop. Check your available pressure with our Water Pressure Calculator.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.