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TEMPERING VALVES – Getting Hot Water Under Control
Tempering valves for use in potable hot water systems and in wet central heating systems have been around for many years and are used extensively in the UK and many other parts of the world to control hot water temperatures. Tempering valves come in many different shapes and sizes; also, they work under many different principles of operation. Some use thermostats in contact with the water, as the Reliance range does, whilst others rely on temperature sensors feeding back information to an electronic proportional type valve, which then opens and closes depending on the temperature transmitted by the sensor. Others are just pure blending valves and blend water proportionally but have no ability to control or modulate the temperature should system pressures or temperatures change.
 Tempering valves are the predecessors to the thermostatic mixing valves that we see marketed today under the TMV2 and TMV3 banners, but there is one big difference. Both TMV2 and TMV3 type valves are designed to be fitted at or as near as practically possible to the point of use i.e. the tap, to give extremely sensitive and fast response times to changing system conditions. Tempering valves are not designed for these types of applications where very precise temperature control and rapid failsafe shutdown are required. Instead, they are designed to control the water temperature in the whole system, be it heating or sanitary hot water. Because of this they are designed and manufactured to meet a different set of criteria from the point of use thermostatic mixing valve.
The current UK TMV2 and TMV3 schemes are of little use in terms of testing for the correct performance of a tempering valve as they look at a valve’s ability to compensate for changes in pressures and temperatures over very short periods of time. In such a scenario, achieving this kind of performance usually means a reduced flow rate; whereas with a tempering valve, extremely fine temperature control is less important than good temperature control but with high flow rates.
Because these valves are so widely used, it was decided several years ago that a European Standard should be developed so that the valves are built and tested to ensure their fitness for their intended purpose. The European Standard being prepared has been given the number prEN 15092 and has had a huge amount of input from experts from across Europe. It is due to be published sometime in late 2007.
The Reliance range of tempering valves are designed for many different applications. These include controlling sanitary hot water temperatures in domestic and commercial distribution systems using the Masterguardâ series of valves. They also control hot water temperatures in distribution systems that use solar heat as a source of energy, as well as controlling and stabilising temperatures in underfloor heating systems.
Sanitary Whole System Control
Masterguard®840 4in1 Blending Valve |
There are many benefits to using a tempering valve in a hot water system. The obvious one is the anti scald protection they provide while at the same time allowing the hot water heater to be run at a high enough temperature to kill any legionella bacteria that might be present. |
In most countries were the control of hot water temperatures in domestic properties has been regulated by the government, the decision of the governing authority has been that a thermostatic tempering valve should be installed at the point of the hot water generation i.e. on the hot water heater itself. The hot water thermostat should be set so that the water in the heater is at a minimum of 60°C in order to control the growth of legionella, while the tempering valve should be set to supply hot water to the sanitary outlets (those used primarily for ablutionary purposes) at a temperature not exceeding 50°C. Neither how the water temperature is controlled nor where the valve is to be installed is specified: this is left to the installing plumber to decide upon, as long as the requirements of the installation standard are achieved. The governing authorities’ decision was based on a number of factors:
1. The water heater is the single biggest risk area for the growth of bacteria because of the volume of water it contains, so it must be set at a high temperature to kill the bacteria.
2. Although water at 50°C is still sufficient to cause a scald injury, the time that it takes for such an injury to occur changes from 2-3 seconds at 60°C to 5 minutes at 50°C. Therefore, there can be considerably more time for the user to react to the situation where a thermostatic control is fitted than in an installation where delivery temperature is uncontrolled. A temperature of 50°C is also hot enough to deliver a hot bath, so that users will not feel as though the “nanny state” is dictating they must have a cold bath or shave in tepid water. Setting a temperature too low would be self defeating, as people would start boiling kettles and pots of water to heat up a bath that is too cold for their liking, probably causing more scald injuries from people tripping over with a kettle than the valve would prevent in the first place.
3. Fitting individual TMVs on each outlet was thought to be a far too costly and difficult operation. It was felt that the public would be more likely to accept the fitting of one valve rather than five or six in each house. Cost was and is a major factor to be considered: one tempering valve fitted to a water heater may cost as little as £40, whereas in a two or three bathroomed house, the cost of fitting valves at each point of use could run into many hundreds of pounds. It was further felt that a more palatable, less costly and less invasive regulation would have a wider take up and therefore would ultimately keep the public safer than if multiple valves - that never actually got installed - were specified.
Underfloor Heating Control Valves
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Reliance first began supplying tempering valves to control heating system temperatures in the early 1980s. Since that time underfloor heating has become increasingly popular in the UK and has been a major growth area for the use of tempering valves.
Because these valves are capable of maintaining a stable temperature under fluctuating supply temperatures and supplying a high flow rate at the same time they are ideal for use in underfloor heating systems. |
Heatguard® UFH 28mm blending valve |
Tempering valves in this type of system are fitted so that they blend together the flow water coming from the boiler at high temperature and the return water which has transmitted its heat to the floor, to maintain an even temperature which can be set by the installer depending on the type of floor being used. There must also be a provision built into the valve or the pipework to return a portion of the return flow water back to the boiler to be re-heated.
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The beauty of this system is that as the floor heats up and the return water temperature increases, the valve will automatically compensate and throttle down the incoming flow water from the boiler, while at the same time opening the return port and increasing the rate of flow on the return side. Eventually, when the return water and flow water temperatures equalise, the flow port will be virtually closed and the system will be circulating return water only until such time as the floor cools and hot water is needed again.
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Heatguard® Solar HF
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For further information on Tempering Valves, click here.
Eric Winter, Technical Manager
February 2007
© Reliance Water Controls 2007
Heatguard and Masterguard are registered trademarks of Reliance Water Controls
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