All About Pressure Reducing Valves – Hidden Treasures

The water systems in domestic, commercial and industrial properties can present unique problems and challenges as plumbing fittings become ever more sophisticated.

The water supply pressure in the UK can vary from 1bar to 20bar (or even higher in some low usage areas). The water pressure will also tend to vary through the day, for instance at high usage times (typically mornings and late afternoons), the pressure may drop by comparison with low water usage times (throughout the night), when the pressure may increase dramatically.

Such periods of high pressure can cause several problems: excessive noise from high flow velocities, water hammer from quick closing taps or solenoid valves, plus the risk of water wastage is particularly great because higher pressure means higher flow rates.

The water supply usually enters domestic dwellings beneath the kitchen sink so the first effect of high pressure is often experienced at that point; when the cold tap is turned on too quickly or too far, this creates a gush of water which hits the bottom of the sink and bounces back, soaking the user and creating a wet mess!

The best way to control high pressure is by installing a pressure reducing valve. These take a high pressure at the inlet, then the valve reduces it to a lower pressure at the outlet as desired, under both flow and no-flow conditions.

How do PRVs work?

A pressure reducing valve is a valve which takes a high inlet pressure and reduces it to a lower outlet pressure. When it does this under both flow and no-flow conditions, the type of control is known as ‘drop tight’. Reliance’s pressure reducing valves use a balanced spring and diaphragm to control the downstream pressure. This ‘drop tight’ feature is one of the most important criteria for any pressure reducing valves, as this stops the pressure from ‘creeping’ – a term which is used when an increase in the downstream pressure occurs under no flow conditions. A valve which will allow this ‘creep’ cannot be known as ‘drop tight’ or in fact a true pressure reducing valve, as it will eventually allow the pressure to creep up to equal the upstream pressure, which can cause significant problems and essentially negates the point of using a pressure reducing valve in the first place.

The diaphragm effectively separates all of the water contact parts and the pressure from the water supply away from the control spring and associated mechanism. The body is then protected from debris by the use of a stainless-steel strainer.

Under no flow conditions the downstream pressure puts back-pressure on the seat and diaphragm of the valve, which in turn overcomes the spring pressure. This means the seat moves up, forcing it to seal against the diaphragm, therefore not allowing the downstream pressure to increase.

Under flow conditions the back pressure against the seat is reduced thus allowing the seat to open and water to flow through the valve.

How to size a PRV

This is predominantly based on two different criteria: application and flow rates. Application describes the type of property the valve is to be used in: whether it is commercial/industrial or a domestic installation.

Flow rate is the most important factor for sizing a pressure reducing valve. Sizing a valve incorrectly can cause several problems; if oversized the valve seat may open for a very small flow rate, which may occasionally be acceptable but over a long period of time can result in a wire drawing across the valve seat. A wire drawing occurs when the valve disc and seat position operate close to the shut-off point of the valve for extended periods of time. This then means the water flow scores a pathway in the seat material which remains when the valve closes tight to the shut-off position and allows a little flow and pressure to creep through the valve.

To calculate the flow rate you must work out how many outlets are required and what the combined maximum flow rate for these will be. You can then use the provided table to ascertain which size valve is required:

For larger commercial applications various sizing solutions can be used: for example, if the flow rate is lower at some times than at others then using several smaller size PRVs in parallel may be more practical, or use one smaller valve as a bypass thus allowing water to flow easily through the valve when the flow rate is lower than normal, without causing either wire drawings or creating noise across the valve, as previously mentioned.

Reliance UK PRVs

All Reliance UK pressure reducing valves are WRAS approved: this means that they have undergone independent third-party testing to ensure that they comply with the current UK water regulations for pressure reducing valves. It also ensures that all materials used within the make-up of the valves have been verified as safe for potable water systems.

You can find out more about our PRV range in the ‘Flow Control’ section of our website’s product portfolio, by contacting our team via the below form, or by speaking to your local sales representative.

UFH vs Wet Radiators – Which to to choose?


A conventional radiator system uses one or more heat surfaces within a room. These heat the air in their immediate vicinity by radiation and convection, air currents around the room then distribute this air. Doorways and windows, which create their own airflows, will also affect the heat distribution. This results in the colder air being at the floor level and much warmer air at ceiling level. Some products advertised as ‘saving wasted heat’ or even ‘using free heat’ use this effect. These consist of a de-stratification fan that takes the warmer air at ceiling level and discharges it at floor level. Even with extra air circulation the room will have hot and cold spots within it. A further disadvantage of air circulation is that it will distribute dust as well as the heat.

A modern underfloor heating system works almost completely by radiating heat. Furniture will reflect and absorb this radiated heat. By absorbing heat, they also become secondary heat emitters. This results in a much more even heat distribution, and the air at floor level being warmer than that near the ceiling. This type of heat distribution is also more comfortable for the people using the room. People often say that if their feet are comfortable then they are more likely to be comfortable in themselves. However, it is important that one’s feet do not get too hot.


If an underfloor heating system is run so that the floor surface temperature goes above 29°C it will feel uncomfortably hot. With wet radiator systems the radiator surface temperature is the same as the circulating heated water. This can be as high as 80-85°C, but lower surface temperature radiators are available which protect the room users from contact with high temperature surfaces. The fact that there is much less air movement with underfloor heating systems has already been mentioned, as well as the resultant effect on dust distribution. This can be very important for people who have some of the more common dust allergies.

Underfloor heating also has the major advantage of depriving the common house dust mite of the one thing it needs to survive and reproduce: moisture. Without moisture the house dust mite will simply die. Underfloor heating maintains a much higher relative temperature in carpets and consequently reduces the amount of moisture available.


All underfloor heating systems work on a lower temperature than a radiator system. Normally a radiator system will have 82°C hot water for the flow, the underfloor heating will run at much lower temperatures in the region of 35 to 60°C, depending on the floor construction and the building. The tempering or blending valve is the heart of an underfloor heating system. It blends colder water from the underfloor heating system return with hot water from the heat source to supply the correct temperature of water to the underfloor pipework. The required temperature will vary depending on the type of flooring and the sub-floor structure but as these valves are adjustable the correct temperature can be easily obtained. RWC has been supplying a variety of high quality UFH Blending Valves for use in underfloor heating systems for many years. As the floor area served in domestic applications has increased so the required flow rate has also increased.

Reliance UK has an active interest in the fitting dimensions of these valves as they are now supplied in pre-plumbed kits. We currently supply the underfloor heating industry with several different underfloor heating packs, details of which can be found in our Underfloor Heating Brochure. In addition, Reliance UK has developed for many years a range of bespoke valves for specific OEM customers to sell on as part of their own product range.

All About The TMV3 Scheme & NHS Model Engineering Specification D08

History of the NHS’s D08 Specification

In 1996 the UK market for thermostatic mixing valves (TMVs), was very similar to the rest of Europe and North America: regulations and standards were either non-existent or hopelessly full of holes. The Department of Health soon realised that the issues of scalding within NHS properties was a serious threat to both patients and staff, and that the tempering valves being fitted were woefully inadequate to protect end users from scalding. Part of the problem was that the tempering valves used at the time were mainly of continental or North American design and they could not cope with the vagaries of UK supply conditions such as low pressure from gravity fed systems or having a mixed pressure system (ie high pressure mains cold and low-pressure gravity hot water). Another far bigger problem was that the UK standard at the time, BS 1415 part 2, was a self-certification standard so anyone could claim compliance; the standard was also very weak in terms of how much of the functionality of the valve was tested and virtually everything was done at low, equal pressures.

To address this problem the NHS Estates enlisted the help of WRc and industry, including Reliance Water Controls, to discuss what could be done to make sure that the thermostatic mixing valves fitted in hospitals and nursing homes were:

(a) of suitable quality and built in an ISO 9001 accredited facility,
(b) were capable of functioning under extreme supply conditions that were and are still common in the UK,
(c) were identifiable and traceable after installation, and most importantly
(d) were independently third party tested by an approved test house and not self-certified.

From these initial discussions the 1997 NHS model engineering specification D08 was born. The key to why the D08 specification has worked so well is because they were written together with the UK regulators, test houses, industry and NHS Estates who understand that the unique qualities found in UK plumbing systems create challenges that are different from anywhere else in the world. The specification was written to cover performance at both high and low pressures, and with mixed high and low pressures and to cover a multitude of other requirements never before considered, such as thermal shut down on hot or cold supply failure (ie the failsafe test), testing for hot spikes, testing for temperature control at equal and unequal pressures, testing for temperature stability under changing inlet temperatures as well as many others.

In the years since the D08 was first written it has had some updates, but it has proven its validity and quality as the core of the specification has remained largely unchanged from the 1997 version and it has become a benchmark for many countries upon which to base their own national standards for thermostatic mixing valves.

The TMV3 Approval Process

In order for a manufacturer to get a valve approved to the D08 specification and market it as a TMV3 scheme approved product, there is a very difficult approval process to go through. It starts with the application by the manufacturer in writing that they would like to submit the valve for testing to TMV3. All the relevant information is submitted, including material specifications, drawings, marketing information, instructions etc. Once the application has been processed they will then ask the company to submit samples of the valve for testing. Once the valve is on the test rig it is subjected to a battery of tests including but not limited to:

  • Endurance testing of the thermal element
  • Temperature overshoot on start up (the hot spike test)
  • Temperature response on temperature adjustment
  • Thermal shut off
  • Temperature stability on changing pressures
  • Temperature stability on changing inlet temperatures
  • Temperature stability on reduced flow-rates.

As the above list shows, TMV3 is very comprehensive in its performance testing, and necessarily so for a valve that is really a safety valve.

Reliance UK TMVs

Reliance UK are one for the market leaders for Thermostatic Mixing Valves (TMVs), providing high quality solutions for a variety of applications & sectors. The Ausimix, Heatguard Dual and Heatguard Style as just some of the key products we supply to a variety of customers and you can find out more about them in the Products area of our website or by contacting our friendly customer services team using the short online form provided below.