Rooftop Packaged Air Conditioning Systems and Split Systems
What are they? Split systems have an indoor unit which provides heating and cooling and an outdoor unit which houses the heat rejection coil and compressors. Refrigerant is circulated between the indoor and outdoor units. These systems are most common in small buildings. Rooftop Package Unit has heating, compressor, heat rejection and compressors all in one unit, and are typically mounted on the roof and connected to the occupied spaces via a duct through which the heated or cooled air is provided. Water cooled package units have heating, cooling and compressor mounted in a single unit mounted in the ceiling space of the occupied area, and reject heat to a condenser water loop which is cooled by a cooling tower and may also be heated by a boiler.
All three of the above types may be [reverse cycle", whereby the refrigerant system can provide heating or cooling. Alternatively, the refrigerant system may provide cooling only while heating is provided by electric duct heaters or hot water coils.
Advantages / Disadvantages - Low install cost. These system types are typically the cheapest to install - Moderate energy efficiency. These units tend to be designed for price rather than efficiency, although some quite efficient units exist. Water cooled package Air Conditioner have quite limited options for efficiency. - Simple maintenance requirements. These are stand-alone systems designed for operation in a low maintenance environment - Poor heating efficiency if water cooled or direct electric heating units are used. - Limited lifespan. Split systems and packaged units typically have a life span of 10-15 years only, and less than this if poorly maintained. - Refrigerant costs. Some of the common refrigerants used in these systems , most notably R22, are being phased out through legislation, because of their negative impact on the environment. This will mean that the costs of this refrigerant will rise markedly and ultimately many R22 split and packaged systems may need to be replaced before the end of their operational life. - Refrigerant leakage is common and badly affects both the capacity of heating and cooling provided and the efficiency of the unit. However, it is often not detected in standard maintenance. - Very sensitive to blocked air filters. Blocked filters lead to a reduction in airflow which will lead to excessively low and excessively high supply air temperatures which drastically reduce effectiveness and comfort.
Energy efficiency Split and package systems are available in a large range of efficiencies. Smaller systems are rated using the appliance rating system and should always be selected for the highest star rating available in the right size. For most air cooled systems it is possible to source a unit with an inverter controlled compressor which reduces energy use by upwards of 20%, and should always be selected where available. Running costs The age and condition of split and packaged air conditioner will significantly affect running costs. Older systems tend to have higher rates of temperature controller failure, refrigerant loss, coil damage as well as less efficient underlying hardware, all of which can increase costs. As with all systems, efficiency will be improved by good management, especially in relation to the calibration of temperature sensors and the maintenance of filters, coils and refrigerant charge. Retrofit / improvement opportunities The major areas for improvement for split and package units systems re: - Ensure filters and coils are not blocked - Ensure control set-points are consistent and appropriate. - Ensure that refrigerant charge is correct - Ensure that there is good airflow around the outdoor units so htat heat can be rejected and does not [pool" around the outdoor unit. - Replace older units with new more efficient units preferably with inverter driven compressors. - For water cooled package unit systems, install valves to prevent condenser water flow through the unit when the compressor is not operating, and improve condenser water pump control. - Install free cooling so that outside air can be used for free cooling when conditions are appropriate.
Applicable buildings Air cooled split systems and packaged units are common in smaller buildings, for which they are a reasonable solution. Water cooled packaged units are used in a wider range of building types, including retail and office developments of significant size, owing to the convenience of condenser water distribution as a means of moving heat. Floor plate implications Split and package units systems are ceiling and / or roof mounted and so do not take up floor space. Temperature control / Occupant comfort Split systems and ceiling mounted package units systems are generally considered a lower grade solution for office Air Conditioning due to fan noise. Roof mounted package unit systems avoid this problem. All of these system types have limited control of supply air temperature which can produce occupant comfort issues. Maintenance implications These system types are reasonably low maintenance but nonetheless require regular maintenance to maintain efficient and effective operation. Filters, coils and refrigerant charge must be well maintained. Identification Split systems have an outdoor compressor unit which is typically mounted at ground level or on a roof. Indoors, these systems may use wall mounted terminals or have a ducted indoor unit within the ceiling space. Air cooled package units are generally roof mounted, and are recognisable by the presence of ductwork leading to and from the unit and the operation of compressors within the unit. Water cooled package units are ceiling mounted and have a cooling tower on the building roof (typically). Maintenance or operational staff will be able to advise whether a building uses Ducted Split Unit and package unit.