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Single Zone VAV and ASHRAE 90.1-2010 – Part Load Control and direct expansion systems implications

May 6, 2013

ASHRAE 90.1-2010 is a substantial updated standard that is in effect and has implications across the HVAC industry.

This blog post will deal with the impact that ASHRAE 90.1-2010 has on Single Zone Variable Air Volume (VAV). Specifically, we attempt to address issues of dehumidification and Sensible Heat Ratio, while also integrating energy savings and equipment longevity considerations.

The specific part of the updated standard (referred to as Single Zone VAV) addresses Direct Expansion (DX) systems 10 tons and larger serving single zones. The standard demands that all these DX systems have their supply fans controlled by either two speed motors or variable speed drives (thus Variable Air Volume serving a Single Zone). A significant change to the prior standard, specifies that at low load conditions below 50%, the supply fan controls will be able to reduce the airflow down to 2/3 of full fan speed or at least down to a minimum volume of outdoor air to meet space ventilation requirements.

The nature of this change is expanding the role of these systems to operate at part load more often than originally intended, or expected. This requires that the refrigeration circuits of these AC systems be more flexible, able to modulate capacity to match changing load conditions.

Sensible Heat Ratio

Engineers often reference Sensible Heat Ratio (SHR) which expresses the ratio between sensible heat and total heat {= sensible + latent heat}. There is much discussion of how, during the system design phase, the sensible heat ratio of the direct expansion equipment should match the sensible heat ratio of the space/building to be conditioned. But as we know, the SHR of a building changes regularly with occupancy, ventilation, and all the other factors that influence the load conditions. While the SHR of the equipment is also changing, it is doing so primarily in response to indoor humidity level. The updated ASHRAE standard exacerbates the conflict of asking the equipment to match the constantly changing needs of the building. Nonetheless, it’s clear that this reciprocal relationship can only be addressed at the equipment level.


Dehumidification will be improved by providing the air conditioning system with Variable Air Volume capabilities. As airflow through a cooling coil is reduced, the benefit is more latent cooling. Essentially, the direct expansion evaporator coil gets proportionally larger in relation to the airflow, thus more dehumidification can be performed. This results in an increased need for system capacity control. To extend the run time of the unit to allow for more latent moisture removal, and without overcooling the space or freezing the coil, the refrigeration circuit must be able to modulate along with the airflow. Tremendous flexibility and greatly improved latent performance is available by the combination of the variable airflow offered in the VAV control and the modulation provided by managing the refrigeration circuit operation.

Costs and Other Benefits

Along with these benefits to improved dehumidification, there are potential costs. Reducing airflow and related cooling load on the evaporator coil can cause significant stresses on the compressor and refrigeration circuit that will result in reducing the expected life span of the equipment. Part of the problem can result from attempting to use unimproved constant volume systems with either two-speed airflow evaporator fan control or Variable Air Volume (VAV) control which leads to lowering the coil temperature below acceptable limits. This often causes liquid slugging and freezing the evaporator as a result of inadequate heat transfer across the coil.

Balancing the benefits to IAQ with the concerns for equipment longevity will need to be continually monitored and addressed by contractors, engineers and facilities personnel.

The implementation of ASHRAE Std. 90.1-2010, which is being adopted nationwide as an energy efficiency standard (for state codes), will result in a double digit reduction of commercial building energy consumption when compared to design by earlier editions of the standard (according to the DOE). And while we are currently only aware of application to new systems, there is a good chance that in the future the standard may be interpreted to include existing/retrofitted systems.

Not all improvements are to be gained from the HVAC systems, but the feeling is that these systems can contribute substantially when one considers that they account (on average) for more than a third of a building’s energy consumption. Energy savings are relatively easy to observe and quantify, better explained most directly by the fan and fan motor manufacturers, as energy inputs directly relate to fan speed. But additional benefits can be gained from reduced airflow across the coil, as well. Installing an APR Control in the circuit to maintain the refrigeration circuit and evaporator coil in proper balance can result in subcooled liquid, lower head pressure and in general a lower compression ratio that can further reduce overall systems’ energy consumption.

APR Control for Continuous Capacity Modulation…

Rawal Devices, Inc. APR Control offers a simple enhancement to the refrigeration circuit that will provide modulation to improve single zone VAV system’s functionality through the APR’s Continuous Capacity Control. The APR Control allows the system and space to operate in harmony while protecting the system’s operation and ensuring compressor longevity.

When one combines the benefits to protecting systems and the way the operation further enhances the performance goals sought by the new standard…The APR Control is clearly the cat’s meow!

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