Run Around Coil 3-Way Mix (RAC3MX)

Run Around Coil 3-Way Mix (RAC3MX)
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Air handling units incorporating run around coils and 3 way mix or air-mixing plenum are important elements in saving.

Heat Recovery Efficiency

The correct efficiency is a subjective decision and depends on the economic calculation and written guidelines, i.e. Ecodesign Commission Regulation (EU) No 1253/2014, on operating data such as energy prices, useful life, running times, temperatures, maintenance costs, and interest rates. With regard to (EU) No 1253/2014, profitability and environmental protection the heat recovery efficiency should be no less than 68%

Dependent on the right conditions heat recovery efficiencies can reach 70%

Air handling units incorporating plate heat exchangers and 3 way mix or air-mixing plenum are important elements in saving energy in Hotels, Corporate, High End Housing, Sports & Leisure, Education, Offices, Industrial, Data centre and Health.

This system works very well in buildings where the occupancy rate can vary greatly throughout the day, or seasonably. Additionally, when outside air conditions are such, typically mid-season weather conditions, it may be that ambient temperatures are suitable for free cooling purposes. In such conditions the mixing damper will set to close and the system uses full fresh air for optimum energy efficiency. Where fresh air is not required, such as early morning pre heat or pre conditioning periods, the mixing damper can be automatically set to full recirculation, again for optimum energy efficiency.

There is also a further added benefit of utilising a 3 Way Mix, such as frost coils are made redundant which saves costs in many areas including AHU coil, reduction in boiler and pump size, less pipe work and installation time on site.


It is common for the supply and extract air handlers containing the run around coil system to be joined together, either in a side-by-side or a stacked configuration. However if required the units can be separated and up to 15m apart.

A run-around coil system comprises two or more multi-row finned tube coils connected to each other by a pumped pipework circuit. The pipework is charged with a heat exchange fluid, water, which picks up heat from the exhaust air coil and gives up heat to the supply air coil before returning again. Thus heat from the exhaust air stream is transferred through the pipework coil to the circulating fluid, and then from the fluid through the pipework coil to the supply air stream.

From this process the specific heat output capacity depends on the temperature difference between the two air streams. Hence the coil is suitable for heat as well as cool recovery, i.e. for winter and summer operation.

The AHU air-mixing plenum combines two air streams. The configuration includes three sets of dampers: one for the fresh air, one for the exhaust air, and a mixing damper between the two air streams. The mix of fresh air and recirculated air can thus be adjusted to suit the needs of the building's occupants. Typically as the fresh air and exhaust air dampers are driven from 0% open to 100% open, the mixing damper will in turn be driven from 100% open to 0% open, so as to always ensure a constant volume of supply and extract air.

Ancillary Items

Air filtration is always present in order to provide clean dust-free air to the building occupants.

Direct heating, placed directly in the air stream, are direct heat exchangers and include those for gas-fired fuel-burning heaters or electric air heater batteries (EAHB). 

Indirect Heating and Cooling coils use hot water or steam for heating, and chilled water for cooling. Heat pumps can be used as well. (Prime energy for heating and cooling is provided by central plant).

Humidification is often necessary in colder climates where continuous heating will make the air drier, resulting in uncomfortable air quality and increased static electricity. Various types of humidification may be used as part of our air handler and include evaporative, vaporizer, spray mist and wetted medium.

If dehumidification is required, then the cooling coil is employed to over-cool so that the dew point is reached and condensation occurs. A heater coil placed after the cooling coil re-heats the air to the desired supply temperature and humidity level. This is often used for chilled beam applications.

Smart Control System

Either our Smart Control System or the clients BMS may monitor return air quality, energy available to recover and CO2 concentration for optimum energy efficiency whilst maintaining desired fresh air requirements.


Our complete range of AHUs can be tailor made to suit all customer requirements including controls and refrigeration.

Air volume ranges from 0.4m3/s to 35m3/s