What is the maximum bath water temperature under Part G?

Building Regulations Approved Document G Section G2 limits bath fill temperature to a maximum of 48°C in new dwellings and conversions. This requires a TMV (thermostatic mixing valve) fitted to the bath hot supply. This regulation aims to prevent scalding, particularly for children and vulnerable people.

What is the difference between TMV2 and TMV3?

TMV2 valves are approved under the TMV2 scheme (formerly D08) for domestic use. TMV3 valves meet the more stringent NHS requirements under HTM 04-01 for healthcare premises. TMV3 valves have tighter temperature control, faster fail-safe shutdown, and must be tested more frequently.

Do I need a TMV on a kitchen sink?

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.

How do I calculate the mixing ratio?

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.

What flow rate should I allow for a bath?

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.

TradeCalculator

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.

Outlet Type

Type of outlet being supplied

TMV Class

TMV2 domestic, TMV3 healthcare

Hot Supply Temperature

Hot water supply temperature (typically 60°C)

Cold Supply Temperature

Cold water supply temperature (UK avg 10°C)

Required Flow Rate

Desired flow rate at the outlet

Desired Mixed Temperature

Target mixed water temperature

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: Q_total × (T_mixed − T_cold) ÷ (T_hot − T_cold)
Cold flow: Q_total − Q_hot
Part G 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

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.