Diffusers - Variable Volume
  • No electric requirements
  • Reversing change-over
  • No routine maintenance
  • ARI certified performance
  • Fully self-contained


The RICKARD VARIABLE GEOMETRY THERMO-DISC Ceiling Diffuser is a thermally powered VAV diffuser which contains its own temperature sensing and volume control mechanism. In appearance and construction the Thermo-Disc is similar to the popular electronically controlled vari-disc diffuser but has the advantage that it requires no external wiring or power supply. The Thermo-Disc is available in two basic options. The change-over between control modes is automatic and is carried out by means of a unique patented change-over mechanism.

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


The first consideration when designing a system is to calculate the required air volume and temperature to satisfy room conditions at maximum heat loads. It is recommended that ducting is sized using the static-regain method. Velocities in branch ducts should be in the range 650 - 1200 ft/min. Thereafter the following should be considered:
This is the distance from the diffuser at which the air velocity has slowed to 50 ft/min when measured 1 in. below the ceiling (or from the wall) with the diffuser in the fully open position. Coning occurs when two airstreams traveling in opposite directions meet and result in a downward moving cone of air. Ideally diffusers should be spaced such that the distance between two diffusers is slightly less than the sum of their throws. Throw remains essentially constant throughout the range of air flow rates, a feature of the variable geometry diffuser.
Diffuser performance has been established using diffuser neck total pressure, although what is normally known and controlled is duct static pressure. What happens between the duct and the diffuser depends on the length and type of flexible duct. For simplicity, it can be assumed that duct static pressure approximates diffuser neck total pressure. This is a valid assumption for systems where flexible duct lengths are not excessive. The above can be explained briefly as follows: The static pressure loss due to friction in the flexible duct (±0.05 in.w.g.) would normally be about the same as the velocity pressure in the diffuser neck and since total pressure is the sum of static and velocity pressures, we can say that neck total pressure is approximately the same as duct static pressure. Although the tables give diffuser performance for neck total pressures ranging from 0.08 - 0.40 in.w.g., caution should be exercised when selecting diffusers outside the range of 0.16 - 0.32 in.w.g. At lower pressures air movement and induction may be insufficient and at higher pressures drafts and excessive noise may result. Best results are obtained when diffusers are selected at pressures of 0.20 - 0.30 in.w.g. Bear in mind that all diffusers on a particular branch will operate at the same static pressure as controlled by the pressure control damper. Diffusers which are able to supply more air than required will be driven partially closed by the temperature controller and the system will self balance. NOTE: Avoid placing restrictions (such as manually operated dampers or pinched flexible ducts) at any point in branch ducts. The result is that at maximum flow these restrictions result in a significant static pressure loss (which for some cases may be desirable) but at minimum flow conditions offer virtually no resistance, which will result in the static pressure at the diffuser being too high.


Room temperature is controlled by varying the supply air volume in accordance with demand. No other system is more energy efficient, since the volume of air is controlled to match exactly the requirements of the space served by the unit, no more or less. Volume control is achieved by moving a disc, known as the control disc, vertically up and down in the diffuser so as to vary the aperture through which air passes. This results in the “Variable Geometry” concept which effectively maintains constant air movement in the room throughout the range of control, from 100% down to approximately 30%. The position of the control disc is varied by means of a wax filled thermal element which responds to changes in sensed temperature. The wax contained in the thermal element melts at the formulated temperatures 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 inside an air induction channel above the appearance panel trim-plate. Adjustment of the room temperature setpoint is achieved by rotating the blue adjustment knob. To access this knob, unscrew the support pins, unhook the hook from the support post so the trim disc & thermal actuator can be removed from the diffuser. The adjustment knob has calibrated markings and the setpoint may be adjusted to suit individual requirements. It is advisable that no changes are made to the factory setpoint of (22°C) 72°F until the space is occupied. This will give realistic operating conditions, after which individual units may be adjusted to suit individual requirements.
The heating/cooling Thermo-Disc change-over pill which automatically changes the action of the diffuser when in the system is changed to the heating mode. This allows full VAV control of room temperature when warm air is supplied to the diffuser. When the supply air temperature exceeds 75°F the changeover is initiated and at 84°F the change-over to heating mode is completed. Now the diffuser will open as sensed room temperature decreases, and vice verse. Change-over back to cooling mode is completed when supply air temperature drops to 71°F. For best results the supply air temperature should be maintained above or below the change-over temperature range although operation within this range is feasible.
The Thermo-Disc controls its own VAV zone, with each diffuser adjusting between 66°F - 77°F. The Thermo-Disc will sense and control the temperature within 3°F of set temperature. There is no need for thermostats, which are often situated incorrectly i.e. in incorrect room. This is because sensing occurs at the diffuser itself.
The Thermo-Disc is a self powered system, which requires no other power source i.e. electrical or pneumatic. There are numerous savings on cabling/piping, thermostats and other electrical/mechanical equipment.
The Thermo-Disc is reliable and requires no routine preventative maintenance. The absence of electrical or pneumatic equipment and accessories also means less maintenance and fewer potential problems. All moving components are easily accessible from below the ceiling. The entire control mechanism can be removed from below in seconds. The only items ever likely to need attention are the thermal elements and these can be easily replaced.