The Difference Between a TMV and an RPZ Valve
- Marc Fitzpatrick
- 21 hours ago
- 3 min read
When it comes to water safety and compliance, not all valves serve the same purpose. Two of the most commonly specified devices in plumbing and mechanical services are TMVs (Thermostatic Mixing Valves) and RPZ valves (Reduced Pressure Zone valves). While both are critical for protecting end users, they work in very different ways and address very different risks.
At Titan Water, we’re often asked: what’s the difference between a TMV and an RPZ valve? In this article, we’ll unpack the key distinctions, explore where each is used, and explain why compliance with both scalding and contamination protection standards matters.
What Is a TMV?
A Thermostatic Mixing Valve (TMV) is designed to deliver water at a safe, consistent temperature by blending hot and cold supplies.
How TMVs Work
Inside the valve is a thermostatic element that constantly reacts to changes in water supply. If the hot or cold feed fluctuates, the TMV adjusts the blend to maintain the set temperature at the outlet. Crucially, if the cold water supply fails, the TMV shuts down the hot flow, preventing scalding.
Why Use Them?
Scalding prevention: TMVs are a legal requirement in many healthcare and care settings. High-risk groups such as children, elderly people, or patients with reduced mobility are especially vulnerable.
Comfort: Consistent water temperature improves user comfort in showers, taps, and baths.
Compliance: In the UK, Building Regulations and NHS Estates guidance set out strict requirements for the installation of TMVs in specific environments.
Typical Applications
What is an RPZ Valve?
An RPZ (Reduced Pressure Zone) valve is not about temperature control at all – it’s about protecting the drinking water supply from contamination.
How RPZ Valves Work
An RPZ is a backflow prevention device. It contains two internal check valves with a relief valve in the middle. This creates a “reduced pressure zone” between them. If back pressure or back siphonage occurs – for example, if contaminated water tries to flow back into the mains – the relief valve opens and safely discharges the water.
Why Use Them?
Protecting potable water: RPZs stop harmful or dirty water re-entering the mains supply. This includes everything from chemicals and fertilisers to bacteria.
Legal compliance: Under the Water Supply (Water Fittings) Regulations 1999, certain high-risk systems require backflow prevention devices. RPZ valves are one of the highest levels of protection available.
Risk management: In industrial and commercial environments, the consequences of backflow contamination can be severe, from health risks to business interruption.
Typical Applications
Commercial boilers.
Laboratories.
Irrigation and horticultural systems.
Processing plants where chemicals are in use.
Key Differences Between a TMV and an RPZ Valve
Although both are valves, their functions are not interchangeable. Here’s how they compare:
Feature | TMV (Thermostatic Mixing Valve) | RPZ (Reduced Pressure Zone Valve) |
Primary purpose | Controls water temperature to prevent scalding | Prevents contaminated water from flowing back into clean supply |
Risk managed | Excessively hot water | Water contamination via backflow |
Typical use | Showers, taps, healthcare outlets | Boilers, irrigation, industrial systems |
Safety mechanism | Thermostatic shut-off if cold fails | Relief valve discharges backflow water |
Regulatory context | Building Regulations, NHS guidance | Water Supply (Water Fittings) Regulations |
Why You Might Need Both
In many commercial and institutional settings, both risks exist simultaneously:
Scalding risk at outlets where people interact with hot water.
Contamination risk in plant rooms, heating systems, or irrigation systems.
For example, a hospital may install TMVs at every patient washbasin to control temperature, while also protecting the main incoming water supply with an RPZ valve. Both devices serve vital but different roles in ensuring safety and compliance.
Common Misunderstandings
One of the biggest misconceptions is that a TMV “does it all.” Some facilities managers believe that because they have thermostatic protection in place, their system is safe. The truth is that a TMV won’t stop backflow contamination, and an RPZ won’t prevent scalding.
Another common issue is neglecting regular servicing. Both TMVs and RPZ valves are safety-critical. Over time, scale, debris, or wear can affect their performance. Annual testing and maintenance are not just best practice – in many cases, they’re a legal requirement.
Conclusion
While TMVs and RPZ valves are very different devices, both are essential in modern water systems. One protects people from temperature hazards, the other protects the entire community from contamination hazards. Understanding the distinction – and ensuring both are correctly specified, installed, and maintained – is key to compliance and safety.
If you’re unsure whether your system requires a TMV, an RPZ, or both, our team at Titan Water can help. We provide expert advice, installation, and ongoing maintenance across the UK to keep your water systems safe and compliant.
