In-line Electric Heating | rickardair

In-line Electric Heating

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Rickard’s VAV diffusers are designed to modulate the volume of warm or cold air to meet the zones demand. In some cases, typically in particularly cold zones, the warm supply air is insufficient to heat the zone adequately. To satisfy this shortfall, Rickard has developed a range of in-line top-up electric duct heaters.  


These heaters have been designed to be fitted to the take-off of the main duct, upstream of the diffuser. By positioning the heater between the main duct and the diffuser flex, the heater is able to meet the building codes that are required for some markets. Any Rickard MLM diffuser supplied with a round duct can be fitted with an in-line heater and controlled. This applies to ceiling diffuser types VCD1, VSD1, CCD3, CSD3, VSW1 and CSW3’s. A 2m ribbon cable is supplied to connect the in-line duct heaters triac to the diffusers main controls.


If used correctly, electric heating in VAV diffusers can be considered an energy saving device. By using them in offices that are typically colder than the building average allows the central plant to produce less heating in winter than is otherwise possible.

The most efficient scenario in heating is for the central plant to supply sufficient heated air to allow most of the zones to be in control when the diffusers damper is close to minimum position. Zones that are colder are controlled by the diffuser opening further. Zones that cannot be satisfied by the diffuser supplying warm air at full volume are toped up with supplementary heating.

The most efficient scenario in cooling is for the central plant to supply sufficient cool air to allow most of the zones to be in control when the diffuser dampers are close to minimum position. Zones that are warmer can be controlled by the diffuser opening further. Zones that cannot be warmed sufficiently by reducing the cold air supply can be controlled by heating this reduced volume of air.

If the room temperature were to fall by 0.5°C below set point, the Triac Controller will commence energizing the heater proportionally and will fully energize the heater when the room temperature is approximately 1.5°C below set point.

Integration of the Rickard VAV diffuser system with the central plant BMS is possible by using our MLM Interoperable BMS Compatible Controls.

For accurate control of room temperature, the electric re-heater is controlled on a step-less, proportional basis.  In addition to having a proportional output signal for cooling control, the temperature controller also has a proportional output signal for heating.

This is done by means of a triac switching set (current valve) which varies the heater output capacity by cycling the power supply to the heater on and off – Pulse Width Modulation (PWM).   This switching takes place over a cycle of approximately 2 seconds and always occurs at zero voltage to avoid radio frequency interference and voltage spikes.  The “on” and “off” periods are varied in proportion to the amount of heating required, i.e. a required heating capacity of 75% will result in an “on” period of 1.5 seconds and an “off” period of 0.5 seconds.

In a situation where multiple diffusers are controlled from a single controller, each diffuser will be fitted with its own triac that will receive a heating signal from the Master controller.  The heating signal transmitted by the controller is a 9 Volt DC signal.
From the table “Maximum Recommended Heater Output (Watts)”, it will be noted that for each neck total pressure there is a specific heater output quoted and for each diffuser size a standard heater capacity is referenced.  For example, in the case of a VCD 250 diffuser, the re-heater sleeve would be factory fitted with a 1500 watt heater, which by utilizing the RICKARD MLM or MLM Interoperable BMS Compatible Controls, can be electronically set for any output from as little as 100 watts to 1500 watts to match the design engineer’s requirements for minimum cooling mode supply air flow and desired leaving air temperature. Therefore, if the diffuser neck total pressure were to be set at 50Pa and the minimum desired air flow was 30% of maximum with 15°C air temperature rise, the heater output for a VCD 250 should be set to 1300 watts. Kindly refer to the help section in the MLM software program for more detailed information.
IMPORTANT ELECTRICAL INFORMATION: Electrical reticulation should be designed to have the capacity to manage the heaters full capacity e.g. when a heater is set to 50%, the heater element draws the same current as it would when set to 100% but it is drawn for 50% of the time.

When calculating heater capacities for VAV diffusers, please keep in mind that heating in the cooling mode takes place when the diffuser is supplying minimum air flow and care must therefore be taken to ensure that an excessive temperature rise in the diffuser is avoided.  Discharge temperatures in excess of 32°C are likely to cause stratification within the room.  As a guide-line, the temperature of the air leaving the diffuser should not be more than 10°C above actual room temperature.  Kindly refer to the appropriate products table giving the “Maximum Recommended Heater Output (Watts)” on page 3 for each diffuser size.  These heater output ratings have been computed on the basis that minimum air flow is 30% of maximum and the maximum capacity of the fitted re-heater are set electronically for an air temperature rise of no more than 15°C, a standard feature of the RICKARD MLM and Interoperable BMS Compatible Controls.

IMPORTANT: These maximum capacities do not take into account limitations of the triac which are rated at 12A maximum. This reduces the capacity of the triac at low voltage supply.


Every in-line Duct Heater Module is fitted with a coiled Electrical Element inside a Mill Galvanised Sheet metal enclosure. The Heater Elements are “black heat” having a heat density of 3.2W/cm² and are constructed from an Incaloy material that does not glow red when energised. This element is selected to reduce the risk of combustible materials igniting should they come into contact with the heater element itself. No combustible materials are used in the construction of a Rickard in-line Duct Heater Module. The in-line Duct Heater modules are fitted with their own Triac or Heater driver and receive a proportional signal from the diffuser controls when additional heating is required to bring the room into control. The Triac receives its power from a seperate power circuit. Dedicated plug tops can be fitted to the heater module on request.

The in-line Heater Duct Modules Triacs are fitted with a number of safeties to reduce the risk of failure. The Triac is fitted inside an electrically grounded metal enclosure that is physically attached to the Heater module Enclosure. This safety increases the electrical safety of the device should a short circuit occur. A fuse offers additional protection against large current surges and shorts. A Transient suppressor prevents the Triac from failing closed and therefore driving the heater permanently after a voltage surge has occurred.

In all cases an auto-reset 65°±5°C (10 000 cycles) and power-reset 85°C±5°C (300 cycles) overheat safety cut-out is fitted as standard. The reset temperatures indicate the air temperature inside the overheat safety cut-out casing at which it operates. Rickard heater modules are designed so that the overheat safety cut-outs trigger when the neck Total pressure is 30Pa or below. The trigger point can vary depending on a number of factors namely, excessively squashed or bent flex, neck size, heater size and damper position. Rickard controls do not activate its heaters below 20% flow damper position, thereby reducing the likelihood of the overheat safeties not triggering in the range described. The power reset cut-out is reset by turning the power supply off momentarily. If a power reset is required, an investigation into the cause should be made. Push-button type manual reset safeties are not recommended in conjunction with diffuser re-heaters.


All electrical wiring associated with the re-heater is carried out in the factory and all units carefully tested for correct operation.


Heaters are available in various capacities, ranging from 0.5kW to 2.5kW.

For additional safety, RICKARD offer an Airflow Switch to interrupt power to the re-heater controls when there is insufficient airflow across the heater element.

To limit stratification in heating Rickard recommends that the heater outputs be limited to the values published in the tables above. The calculated values will ensure that the heat rise is no more than 15°C in VAV diffusers and 10°C in CAV diffusers. Please note that these values are a guide and are calculated at 30% volume for VAV diffusers and 100% volume for CAV diffusers. By adjusting the diffuser damper position down, a smaller volume will create a larger heat rise and therefore increase the likelihood of stratification. The Fit column indicates the maximum fitted heater size recommended, the Adjust value indicates the maximum heater setting recommended to achieve a 15°C (VAV) or 10°C (CAV) heat rise and the Set column is the MLM Heater Output % value required to achieve a 15°C (VAV) or 10°C (CAV) heat rise.