Improved designs for Heat Recovery Ventilation

Air throw & Coanda effect

The ‘Coanda’ effect refers to the entrainment of fluid or gas particles in a jet flow. If a solid surface is placed close to a jet, this jet flow will be pulled in close to the surface. This is the effect used by AirMaster units in placing the air supply grille close to the ceiling surface.

Care must be taken to ensure that the ceiling surface is flat over the maximum air throw distance from the unit. There should be no interference from beams or light fittings. If there are light fittings in the dispersion area, these need to be recessed into the ceiling or suspended below the air flow line.

Videos of air throw revealed by smoke tracer have been included below:

How to get a longer throw:

AM 800 heat recovery ventilation unit shown at full throughput with smoke tracer. Using the Coanda effect, air supply stays close to the ceiling and thus enables a longer throw (approx 7m), even with reduced supply air velocity. Air falls towards the back of the room, and then begins its return journey back to the extract fan. It means that air change applies to the whole of the room, not just a part of it.

This is an excellent visual representation of the Coanda effect, which has been harnessed effectively by the AM 800 test unit. The video shows inlet air remaining close to the ceiling for virtually the whole length of the test chamber.air-throw

The turning vanes of the air handling unit, just inside the air supply grille can be used to direct flow against an adjacent wall, if the ceiling cannot be used for any reason. The Coanda effect again helps to distribute air laterally just as effectively as for ceiling flow.

AirMaster aerodynamic designs ensure as far as possible that the whole room volume is swept with replacement air.

By projecting supply air using the Coanda effect, the possibility of stale air pockets is greatly reduced.

Throw lengths for air travelling at 0.15m/s, based on either 1300m3/h or 1000m3/h

Design features for integration in ceiling

The horizontally segmented design gives the wall-mounted AM units a light and streamlined look. The casings appear less dominant, and helps to make them an architectural element in their own right. The horizontal edges of the segments blend in unobtrusively with the ceiling line.

Wall-mounted AirMaster heat recovery units can be placed below ceiling level or partly recessed in the ceiling void space. The depth of recess can be either 1/3 or 2/3 of unit height, depending of the space available above the suspended ceiling. Alternatively, units can be concealed completely within the void space. If any part of the unit is visible, the panel with the air supply grille to the room must always be positioned next to the ceiling. This enables the Coanda effect to be maximised.

Ducted options are available for direct ventilation units installed below ceiling level (ie, where they remain fully visible) and for those which are fully concealed in the void space. These are known as ‘Combi’ models, and can be summarised as follows:

1. Combi Model 1: Ducted extraction for an air handling unit installed below ceiling level. Inlet air supply is through the normal inlet grille.

2. Combi Model 2: Ducted inlet for units below ceiling level. Extraction is directly through the unit grille, as normal.

3. Combi Model 3: For units concealed completely above ceiling level, ducting is used for both air supply and extract.

By placing air access points along the length of the ducting, direct ventilation can be arranged for more than one room at a time. This approach also lends itself to the ventilation of rooms located away from any outside wall. Combi solutions can be used for the AM 300, AM 500 and AM 800 versions.

Specific fan power (SFP) is the measure of all power consumed by an air handling unit in moving a given volume of air within a unit of time. The SFP value for AirMaster heat recovery ventilation units takes account of both the fans, electronic controls and the damper actuators.

Specific Fan Power (SFP)


However, as intake and exhaust connections are usually very short, AirMaster SFP values do not need to take account of any significant duct loss. In addition, by having drive motors designed for electronic commutation, fans are able to modulate with lower power consumption than would be required for fixed or 2-speed units.

Across the whole range of AirMaster direct ventilation units, SFP is kept to within 0.7 – 1.2 W / ltr /sec.

Condensate Removal

Condensate production is a natural part of the heat recovery ventilation process. Condensate is produced in the extract side of the heat exchanger and makes its way by gravity to the collection tray. Normally the extract air passing over the tray is sufficient to keep condensate build up in check.

Build up of condensate in AirMaster heat recovery ventilation units is monitored by a float sensor. On a ‘level high’ signal being generated, the exhaust fan is automatically taken to maximum speed for a period of 10 minutes. This increases the rate of evaporation from the tray, bringing the condensate level down again. Having an automatic process to disperse condensate (ie, exhaust fan speed increase), makes it much less likely that operator involvement will be required to deal with condensate blockage problems.
If the ‘level high’ signal continues, the exhaust fan stays in service for 2 further periods of 10 minutes. Should the signal still persist, the condensate alarm is triggered and the heat recovery ventilation unit shuts down. This is done to prevent damage to either the unit or to the building furnishings.

Where heavy condensate build up is anticipated (eg, as for sports facilities), the air handling unit specification should include a condensate pump. Operation of this is controlled automatically by the float sensor.

Document Downloads

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  • 1. AirMaster Brochure
  • 2. AirMaster Technical Data

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