Thermal Variable Volume Ceiling Diffuser (VCD, VRD, VSD 4/5) | rickardair

Thermal Variable Volume Ceiling Diffuser (VCD, VRD, VSD 4/5)

The Rickard Variable Volume Thermo-Disc Ceiling Diffuser is a thermally powered VAV diffuser which contains a built in temperature sensing & volume control mechanism. In appearance the construction of the THERMO-DISC is identical to the popular electronically controlled Vari-Disc range of diffusers, but has the advantage that it requires no external wiring or power supply. The Thermo-Disc is available in two basic options—the Series 5 is a cooling only VAV diffuser while the Series 4 will provide full VAV control in both cooling & heating modes. Change-over between cooling & heating control modes is automatic & is carried out by means of our unique PATENTED thermal mechanism.

Supply air is discharged horizontally in UNIFORM 360º radial pattern while the Variable Geometry feature ensures that adequate room air movement is maintained throughout the full range of volume variation.

All diffusers are manufactured from sheet steel & finished in a chip resistant epoxy powder coating available in a wide range of colours.

Rickard VAV Thermally powered diffuser-Cut away



Rickard VAV Diffusers control Room Temperature by adjusting the volume of air at the diffuser outlet. By changing the diffusers exit geometry, Coanda, Air Velocity and Throw is maintained at minimum and maximum volume. This technology prevents cold air from dumping at minimum, ensures excellent ventilation, air mixing, Air Change Effectiveness (ACE) and therefore thermal comfort (ADPI).

Rickard VAV diffusers reduce pressure loss in the system due to their aerodynamic design and the absence of restrictions in the duct work.

Rickard VAV Thermal Diffuser Operation-MorningRickard VAV Thermal Diffuser Operation-Midday


Every THERMO-DISC diffuser controls its own zone by using forced induction technology to ensure its temperature sensing element is reacting within 0.15°C of room temperature for every 1°C difference between room and supply air temperature i.e. the Temperature Difference (TD) between room and supply air. By using on-board sensing, inaccuracies caused by incorrect wall thermostat placement, layout changes or walls affected by external loads can be eliminated.


Rickard VAV Thermal Diffuser Induction


Unique temperature set point adjustment with heating offset adjustment. Energy can be saved by offsetting the set point lower in winter. Since occupants are conditioned to a colder environment in winter and are dressed more warmly a lower control temperature feels more comfortable. It is possible to offset the set point higher or lower -2 to +2°C (-4 to +4°F).

Receive Management, Indoor Environmental Quality and Energy Efficiency Credits by using Rickard VAV Diffusers.


The THERMO-DISC is a self-powered Variable Geometry VAV diffuser which requires no outside power source or wall thermostats.

Low diffuser height (100mm) can reduce a buildings overall cost by reducing the height of the ceiling void.


Since the THERMO-DISC is a self-contained unit, it can be easily moved or additional diffusers added.


The Rickard range of ceiling diffusers offers a clean uncluttered look. The design hides the internals, is pressed to lie flush with the ceiling and comes in a range of colours and styles to satisfy different tastes.


VAV diffusers are inherently simple to commission and Rickard THERMO-DISC diffusers make it even easier. The internal damper (control disc) is manually opened by detaching the diffuser face, unhooking the control disc springs and rotating the diffuser face back into place.

Adjustable minimum control disc limits allow designed airflow volumes to be achieved.


Included Jubilee Clamp saves time and material when attaching the flex.

Included plastic packaging can be used to protect the Tile once installed.

Light weight Diffuser.

Tile can be installed separately to the active sub assembly if required.


The Rickard THERMO-DISC does not require regular maintenance. Working components are all accessible from below the ceiling and skilled labour or special tools are not required. Life cycle testing exceeds 20 years of operation.


Rickard offers a 3 year manufacturer’s warranty on the thermal element and a 10 year warranty on the actuator. Please see Terms and Conditions below for a full description of our Warranty.


Working plastic components are moulded in glass reinforced Makrolon – Makrolon is flame retardant and chlorine and bromine free when burnt. The Rickard Thermo-Disc and Electronic actuators are moulded in Makrolon and are UL Certified.

Stainless steel safety cable supports the working sub-assembly when detached from the back pan.


The RICKARD VARIABLE GEOMETRY THERMO-DISC CEILING DIFFUSER is designed for general building zones where uniform radial discharge is the most suitable and desirable supply air distribution pattern.  The basic diffuser is available in a wide range of options to suit every individual requirement.

Optimum performance in terms of uniform air distribution and low noise levels have been combined with simple construction and aesthetically pleasing appearance to provide a unit which is both functional and reliable.  All diffusers are of steel construction and are finished in a chip resistant baked epoxy coating which is available in a wide range of colours to suit architectural requirements.

Room temperature is controlled by varying the supply air in accordance with demand. No other system is more energy efficient since the volume is controlled to exactly match the requirements of the space served by the diffuser.

Volume control is achieved by moving the disc, know as the control disc, vertically up & down within the diffuser so as to vary the aperture through which the air passes. This results in the “Variable Geometry” concept which effectively maintains constant air movement throughout the range of control from 100% down to as little as 25%.

The position of the control disc is varied by means of a wax filled thermal element which responds to changes in sensed room temperature. The wax contained in the thermal element melts at the formulated temperature to expand or contract. With a rise in sensed temperature the expanding wax extends the plunger, causing the amplifying lever-arm mechanism to move the control disc such that the supply air volume is changed to the extent that is required to maintain constant Room temperature. When the wax cools down the plunger retracts under the action of the return spring, causing the control disc to move in the opposite direction to counter the change in sensed temperature.

The room temperature sensing element is located behind the induction cap in the diffuser appearance panel, known as the trim plate. This is the ideal position to sense room temperature owing to the high rate of induced room air across the trim-plate.


The Series 4 Cooling/Heating Thermo-Disc Diffuser is identical in appearance to the cooling only version, the only difference being the addition of a second thermal element within the mechanism which automatically changes the action of the diffuser when the system goes into the heating mode. This allows full VAV control of the room when warm air is being supplied to the diffuser.

The cooling/heating change over thermal element is similar to that used for sensing room temperature allowing heating change over to be initiated when the supply air temperature is elevated to 24ºC (75ºF) and is completed when the supply air temperature reaches 31ºC (88ºF). Under these circumstances the diffuser will open as sensed room temperature decreases & vice verse. For best results the supply air temperature must be maintained 2ºC (3.5ºF) above or below the change over range.


Adjustment of the room temperature setpoint is achieved by rotating the blue temperature adjustment ring to the desired set point temperature. Adjustment of this ring sets the cooling set point in the Series 5 (Cooling only) actuator and sets the heating and cooling set point in the Series 5 (Heating and Cooling) actuator. It is possible to offset the heating set point from the set point by adusting the green temperature adjustment mechanism in a positive or negative direction.

To access the actuator, rotate the diffuser trim-plate counter clockwise & remove the entire trim-plate/mechanism assembly. The adjustment ring has calibrated temperature markings to suit individual occupant requirements.

NOTE: It is advisable that no change be made to the factory 22ºC factory setting until the space is occupied. This will provide for realistic operating conditions, after which individual diffusers may be adjusted to suit each individual occupant.

VAV Thermal Actuator

The first consideration when designing a system is to calculate the required supply air volume and temperature to satisfy room conditions at maximum heat loads. It is recommended that ducting is sized using static regain design principles. Supply air velocities in branch ducts should be between 3.5 and 7.5m/s (650 and 1500ft/min).


This is the distance from the centre of the diffuser to the point at which the supply air velocity has reduced to 0.25m/s (50ft/min) when measured 25mm (1 inch) below the ceiling and the control disc in the fully open position. Coning occurs when two airstreams travelling in opposite directions meet and result in a downward moving cone of air. A similar effect is experienced should a diffuser be positioned at a distance from the wall that is less than its throw. The air will strike the wall and flow in a downward direction such that the point at which the air reaches a velocity of 0.25m/s (50ft/min), the sum of the horizontal and vertical travel of the air is equal to the diffuser throw. Throw remains at acceptable levels throughout the range of air flows, a feature of the variable geometry VAV diffuser concept.


The published diffuser noise level must be checked to ensure it is within the project specification. Published diffuser noise levels represent only the noise generated by the diffuser and do not take into consideration any duct-borne noise.

Noise Criteria ratings are taken for a standard office environment 2 m (6’) from the diffuser.


Diffuser performance has been established using diffuser neck TOTAL pressure, although that which is normally know or measured is duct STATIC pressure. What happens between the duct and the diffuser depends on the length and type of flexible duct being used. For simplicity, it can be assumed that the duct STATIC pressure is approximately equal to the diffuser neck total pressure. This is a valid assumption for systems where flexible duct lengths are not excessive and can be explained briefly as follows:

The static pressure loss due to friction in the flexible duct (±10Pa or 0.04ins wg) would normally be about the same as the velocity pressure in the neck of the diffuser and since total pressure is the sum of static and velocity pressure, we can say that neck total pressure is numerically approximately the same as duct static pressure. Although the tables reflect diffuser performance for neck total pressures ranging from 20-100Pa (0.04-0.40ins wg), caution should be exercised when selecting diffusers outside the 40-80Pa (0.08-0.32ins wg). At lower pressures air movement and induction may be insufficient and at higher pressures draughts and excessive noise may result.

Best results are obtained when diffusers are selected at pressures of 50-70Pa (0.20-0.28ins wg). Bear in mind that all diffusers served by a common duct will all operate at the same static pressure as controlled by the pressure control damper. Diffusers that are able to supply more air than required will be driven partially closed by the thermal element and therefore the system becomes self-balancing.

NOTE: Avoid upstream restrictions such as manually adjusted dampers or squashed flexible ducting. The reason being that at maximum flow, any restriction will result in a significant static pressure loss (which for some cases may be desirable) whereas at minimum flow conditions offer virtually no restriction, which will result in the static pressure at the diffuser being too high at minimum flow conditions causing over-cooling/heating.

Diffusers are factory set for an approximate minimum air quantity of 33% of the maximum levels reflected in the performance data section. It should be noted that the minimum air settings are only approximate and may require to be reset on site to compensate for the actual site system pressures.

Total Pressure can be calculated as follows:

P total = P static + P velocity

P velocity = constant * (volume/1000)²

150 1921.350
200 607.927
250 249.007
300 120.084

The drawing below describes how to determine the maximum ceiling height that can be achieved from a diffuser. Please see the diffuser performance data page for airflow, throw, noise and pressure information.

VSD4/5 Large Cone
150 to 300mm
S595 and 603
VSD4/5 Medium Cone
150 to 300mm
S495 only
VSD4/5 Small Cone
S325 only
VCD4/5 Medium Cone
150 to 300mm
S495, 595 and 603
VRD4/5 Medium Cone
150 to 300mm