In larger homes, unvented hot water cylinders are fast becoming, if not already are, the most popular and practical choice to serve hot water to outlets around the property.
However, these systems must be installed with safety in mind, and tempering valves are a key part to ensuring that water arrives at the outlet at a safe and stable temperature for users. In this blog, we’ll look at how tempering valves work, what their function is, and why you should install them.
Why choose an unvented hot water cylinder?
Before anything else, we should first consider where and why an unvented hot water cylinder makes sense.
Sometimes known as pressurised cylinders, unvented hot water cylinders work well where there are multiple users in the building. Their key benefit of an unvented hot water cylinder is that they concurrently deliver hot water at mains pressure to multiple outlets.
This means that unlike systems that run off combi-boilers, occupants can run baths and comfortably have showers at the same time with no loss of pressure or hot water. These pressurised systems store hot water within the cylinder and are fed directly from the cold mains water feed.
Safety devices in pressurised cylinders – how they work
So, the homeowner or project calls for an unvented hot water cylinder to be installed – what do you do next? Well, as an installer, it’s about making sure it is fitted correctly and is safe for the user. Also check that you have the right qualification before accepting the job – unvented hot water cylinders must be installed by a G3 competent installer.
Thankfully, many modern pressurised cylinders are already pre-fitted with lots of safety and failsafe devices. This includes expansion vessels and thermal and pressure relief valves.
These components work by being pressurised with air, which is separated from the cylinder’s water by a membrane. As water heats up in the cylinder it expands and pushes the water from the cylinder into the expansion vessel. The air in the expansion vessel is compressed to allow for this expansion, ensuring that the cylinder is not damaged from the water pressure. Some cylinders trap air at the top to form a buffer instead of using a separate vessel.
Thermal and pressure relief valves
Cylinders are fitted with other failsafe mechanisms too, including pressure relief and thermal relief valves. Typically, the thermal relief valve is set at 95°C, while the pressure relief valve is set between 7 bar and 10 bar. These are complemented by a thermal cut out and a cylinder thermostat which is typically set to 60-65°C in order to kill off any Legionella bacteria.
Unvented hot water cylinder problems
If there is a malfunction in the cylinder, due to incorrect installation, lack of maintenance or a component failure, it is possible that the internal water pressure and temperature could rise.
Why do you need a tempering valve?
If the water in the pressurised cylinder exceeds the values of the temperature or pressure relief valves, the valves will open in order to drain water from the cylinder, through a tundish and into a discharge pipe in order to relieve the pressure. Cold water is fed into the cylinder again to replace the discharged water and this reduces the temperature. The cold water relief valve also discharges down the common pipe feeding the tundish in the same way the temperature and pressure relief valve does.
The cold water supply to the cylinder also requires a separate pressure relief valve. Cold water pressure can increase as ambient temperature rises cause the water in the pipework to warm and expand. This expansion within the supply can increase the overall cold water pressure, therefore a relief valve is installed should the pressure increase dramatically. High cold water pressure could lead to major damage to a cylinder. Surges in mains pressure, as well as system debris causing a restriction in the supply to the cylinder can also cause a rise in system pressure, so also installing a relief valve on the cold water is a must within the G3 regulation.
Pressure reducing valves
Often looked upon as a control valve, a PRV also acts as a safety valve for unvented cylinders. The valve maintains a set pressure for the cold water feeding the cylinder – protecting it from potential fluctuations in pressure to the cylinder from the mains supply.
Lack of maintenance can cause scalding
However, even though the above safety measures will protect the cylinder, the higher temperatures and pressure can create a much higher scalding risk for users long before the temperature and pressure relief valves are triggered.
The lack of maintenance mentioned earlier is often the root cause of higher scalding risks. Even though unvented cylinders must be serviced annually to stay compliant with building regulations, homeowners and landlords often don’t comply with the requirements.
If not serviced, the air contained by the expansion vessel membrane is slowly lost over a period of time. This reduction in expansion capacity allows the pressure and temperature to build up in the system. However, as the process is gradual, it may be months before it rises enough to trigger either the temperature or pressure relief valve.
For instance, an unvented cylinder with a pressure and temperature relief valve arrangement set to 8 bar and 95°C respectively, can allow for pressure and hot water within the system to reach 7 bar at 75°C without the pressure or temperature relief valves having to open. This pressure is much higher than normal, and the temperature is hot enough to cause scalding, yet high enough to ensure harmful bacteria doesn’t breed within the pipework. And fitting a tempering valve in essence allows you to run your system at such disinfecting temperatures, while effectively limiting the maximum temperature going to the taps and reducing the risk of scalding – particularly dangerous for young children, who experience scalding at much faster rates than adults, as water at 60°C can induce injuries in less than three seconds.
Tempering valves – an additional safety measure for unvented cylinders
Tempering valves, such as the Heatguard Tempering Valve from Reliance Valves, are fitted to the hot water outlet of the pressurised cylinder and cold mains water to regulate the outlet temperature to the taps. Any excessively hot water exiting the cylinder is mixed with cold water to ensure a safe and stable operating temperature. The valve is pre-set and locked to 55°C, which significantly reduces the chances of scalding. The water in the cylinder is still at the minimum recommended temperature to prevent Legionella breeding in the system.
The Heatguard tempering valve comes complete with 22mm compression connections, making installation quick and easy. With a tamperproof cap, the temperature cannot be changed without special tools either, so the maximum temperature reaching users never exceeds 55°C.
Futureproofing against scalding
Installing a tempering valve takes little time, but the benefits are clear. Scalding incidents have been on the rise recently, with an increase of 9%, according to reported figures from CIPHE last year.
With a simple tempering valve, many of these incidents could be avoided. At RWC, we offer a range of tempering valves with a number of innovative features to help make installation quick and simple. Along with this, you can speak to our technical team for help and best practice. For more about our temperature control valves, please see here.