Function
TRVs, fitted to individual radiators, are accurate set-and-forget room temperature controls that generally help minimise heat generation and enhance comfort levels. They also make additional savings by independently maintaining lower temperatures in unoccupied and other appropriate rooms.
Requiring no external power to operate, TRVs sense room temperature changes – including extraneous ‘free’ heat from sunlight, appliances, body heat, etc. – and adjust the flow of heated water through the radiators accordingly. Conventional room thermostats alone provide just single-zone temperature control based on one sensing point and one temperature setting.
Construction
A TRV comprises two basic sub-assemblies – a valve body and a sensor - that are easily fixed together.
Valve bodies regulate the flow of heated water through radiators. They are available in straight, vertical angle and horizontal angle configurations to suit various applications. There are also special valves, e.g. presetting types and high flow versions for single pipe systems.
Sensors monitor room temperatures and adjust valve operation. They incorporate wax, liquid or gas charged capsule-and-bellows assemblies within the sensor head. Wax has the slowest response to temperature change with liquid being currently the most usual option. Gas-charged sensors have the quickest response to temperature changes and hence provide the optimum comfort control and energy saving.
In use, these easily settable units expand and contract with room temperature changes, operating a piston that acts on the valve pressure pin (spindle). As room temperature increases, the valve throttles back the flow. A spring returns the piston promptly when contraction (cooling) occurs.
The TRV’s control action is modulating, i.e. it makes appropriate slight adjustments to the valve spindle position to increase or reduce the volume of heated water passing through the individual radiators. The desired control temperature is easily set by rotating the setting dial (sensor head) until the appropriate icon or number coincides with the setting mark. Setting ranges vary with different manufacturers and types.
Types
Today’s most popular TRVs for fully-pumped (2-pipe) domestic systems are 15mm bi-directional, reversible, angle types with built-in (integral) sensors. These allow total installation flexibility, i.e. sensor mounted vertically or horizontally in either flow or return.
Many alternative versions are produced to suit other systems and situations where this type is not appropriate.
Valve Bodies. Valve bodies for domestic systems exist in sizes from 3/8” to 1” BSP (Imperial) or 8, 10 and 15 mm (metric) for use in 2-pipe systems.
TRV valve bodies can be either threaded or provided with compression fittings. Also available are push-fit fittings for plastic pipe. Special high capacity bodies are available for 1-pipe systems.
Others, such as presetting valves, are also obtainable. These help installers to hydraulically balance heating systems to give even distribution of heat, better comfort and lower energy consumption.
Standard bi-directional valve bodies are claimed to allow flow in both directions, thus removing the possibility of system noise. However, intensive research found it physically impossible to produce a completely universal bi-directional valve that would eliminate noise under every set of conditions. Consequently, a flow selection mechanism was introduced that enabled flow direction to be completely reversed simply by rotating a metal collar through 180°. Thus, any noise problems encountered could be easily and quickly overcome without draining down and reinstalling.
Sensors. Built-in (integral) sensors may be secured to valve bodies by Allen screws, screw collars or snap-lock mechanisms.
For installations where integral sensors are not practical, remote adjusters are readily available. These sealed units connect to the valve-operating unit by a length of thin capillary tubing.
Also, there are advanced battery driven electronic programmable sensors that enable temperature setting changes to occur at pre-set times.
Installation
Installation involves a straight-forward plumbing job for the valve body plus ensuring the sensor head is fixed correctly. No wiring is necessary. However, care is required with valve siting and sensor mounting.
To monitor air temperatures representative of the room as a whole, the best installation position is in the radiator flow with the sensor mounted horizontally away from the hot valve body and pipes. Mounting the sensor vertically above the hot valve can prevent the room attaining the desired temperature as and when the flow temperature fluctuates.
If sensors must be mounted vertically, due to space limitations or for other reasons, they are best fitted at the return where temperatures are lower.
Also, sensors positioned behind curtains or furnishings, or in tight corners alongside hot pipes, will not allow satisfactory control and remote sensors should be used.
In all domestic heating systems, including those controlled predominantly by TRVs, an overriding room thermostat should be provided to meet the Building Regulations requirement for boiler interlock circuitry that prevents wasteful boiler ‘dry’ cycling.
Explain use
It is important that installers explain to end users how to operate TRVs so that all optimum savings are made, e.g. turning down the setting when ventilating a room or when rooms are not being used for a longer period of time. It is although important to get the message across to the user that a TRV is overall a set-and-forget control and there is no need to operate the TRV as a manual valve. The opening and closing will be handled by the sensor according to the set temperature.
Commercial TRVs
Also available are excellent ranges of radiator thermostats designed to withstand the accidental and deliberate abuse frequently encountered in commercial and public premises. They high quality units are designed to give extra strength to resist heavy loading (e.g. children standing on them) and with more robust fixings.