Why Have Two Thermostats on VAV Diffusers?

Why Have Two Thermostats on VAV Diffusers?

Why Have Two Thermostats on VAV Diffusers?


When selecting thermostats for thermally powered diffusers it is necessary to take into account the effects of stratification, radiation and conduction. The limited range of a single thermostat will not extend over a comfort range which must be biased lower for cooling and higher for heating.

In order to overcome this dilemma, the TVAD-SS VAV diffuser is fitted with two room thermostats providing VAV control over the complete comfort range in both cooling and heating. The cooling thermostat has been selected for a lower temperature range than the heating thermostat because of the effects of stratification, radiation and conduction.



The widest setpoint adjustment range available from a room temperature wax thermostat is 4.4°C. In principle this range is sufficient to cover the winter/summer design conditions specified in the General Design Criteria, Table 1, pg.32 of the 1999 ASHRAE Application Handbook. However, this does not allow for the influences of stratification of room temperature between the occupied zone and the ceiling nor the effects of radiation and conduction of supply air temperature through the diffuser. Temperature sensed when cooling is around 1.1°C lower and around 2.2°C higher when heating.

Thus, a single thermostat with a 21.1-25 .5°C range would actually control at a 22.2-26.6°C range when cooling and an 18.9-23.3°C range when heating. Neither fully covers the 21.1- 25.50C range ASHRAE design range. More importantly the actual control temperature varies above and below the selected setpoint depending upon the mode of operation. For example, if the diffuser operates when the wax is 23.3°C then the corresponding room temperature in cooling would be 24.4°C whilst in heating the corresponding room temperature would be 21.1°C.

Most Common Solution


These effects are taken into account by most manufacturers and consequently the wax in ‘single thermostat’ models is selected for a slightly lower range thus providing the expected the 21.1-25.5°C range when cooling. However, this results in a 17.8- 22.2°C range when in heating mode. Consequently, in the heating mode at room temperatures above 22.2°C the diffuser is not responsive to room temperature; it remains fixed and has no VAV characteristic.

Similarly, if a 20.0-24.4°C range is selected for cooling then the heating range effectively becomes 16.7-21.1°C. In this instance the diffuser would remain fixed, with no VAV characteristic, when in the heating mode at room temperatures above 21.1°C.



The Royal TVAD-SS VAV diffuser module has two room thermostats (and one changeover thermostat). Each has a different wax. The waxes of the separate heating and cooling thermostats are individually selected to achieve the same 21.1-25.5°C setpoint range allowing for the bias requirements described above. Having two room thermostats enables accurate control fully within the normal design criteria and also facilitates the use of different setpoints for either heating or cooling to suit individual needs.

For more information visit our VAV Diffuser Product Page, call us on 1300 770 908 or Contact Us.