Dedicated Outdoor Air System

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Illustration of DOAS system providing ventilation and four-pipe fan coil systems providing space conditioning. (Image reproduced with permission from source: Kolderup Consulting)

Dedicated outdoor air systems, commonly referred to as DOAS, provide conditioned outdoor air to the spaces in a building for ventilation, separate from the HVAC system that provides most of the heating and cooling to those same spaces. A DOAS unit always includes fixed or variable speed fans, filters, and a cooling coil but also often includes heating coils and air-to-air energy recovery devices. The air provided by the DOAS is filtered and usually dehumidified to provide the exact amount of outdoor air required by the various spaces in the building. Very often, the DOAS utilizes some type of energy recovery from air being exhausted from the building to minimize energy consumption. The DOAS can be paired with central air handling unit systems but are even more likely to be paired with systems that don’t have ductwork, such as fan-coil units, VRF, water-source heat pumps, radiant heating and cooling, and passive chilled beams.

How It Works

ASHRAE Standard 62.1 “Ventilation for Acceptable Indoor Air Quality” and other applicable codes and standards set ventilation rates by the type of spaces in a building. These requirements, along with decreasing sensible cooling loads in buildings from higher efficiency lights and windows, mean that for many buildings, the majority of the load is due to the outdoor air. Adding the prevalence of buildings in high-humidity climates means that providing humidity control is becoming a more important aspect of any HVAC system design. Dedicated outdoor air systems  are one solution for these design issues by separating the building HVAC system into two distinct systems: one that provides sensible cooling and heating to the spaces and another that conditions the outdoor air before being distributed throughout the building.

The DOAS, often a rooftop system, brings in 100% outdoor air and, at minimum, uses a filter, cooling coil, and fan to condition the air before sending it to the various spaces or other equipment in the building. Because of the high humidity of outdoor air in many climates, the cooling coil may have a sensible energy recovery device before and after the coil that pre-cools the air entering the cooling coil and then reheats the air after, providing even better dehumidification. This can be done with heat pipes, a run-around loop, a plate heat exchanger, or a heat recovery wheel. This system reduces the energy consumption needed to condition the outdoor air and supplies it at an appropriate temperature. In addition, it does not use any exhaust air, so it is simple and has less ducting than more complicated DOAS systems.

More common are DOAS units that do utilize recovery of heat from the air being exhausted from the building. Ducts route air leaving each space to a central point at the DOAS unit, and a heat exchanger is used to recover some of the heat prior to the active conditioning in the DOAS. This provides heat recovery in both heating and cooling seasons. The heat exchanger can be sensible-only: heat pipes, run-around loop, plate heat exchanger, or heat recovery wheel, or it can be a total-energy wheel that recovers both sensible and latent heat from the exhaust air stream. Both of these configurations can reduce the heating and cooling energy consumption needed to condition the outdoor air.

Dedicated outdoor air systems can be combined with demand-controlled ventilation by including a variable speed fan in the air handler and varying the quantity of outdoor air entering and being distributed.

According to the 2020 ASHRAE Handbook Heating Ventilating and Air Conditioning Systems and Equipment, Chapter 51, the air distribution from the DOAS unit into the building comes in several different configurations:

  • Direct supply to each zone - ducting is provided to move the air directly to each zone from the DOAS unit, which means when used in conjunction with central air systems, nearly redundant ducting is running to each space along with twice the number of diffusers. This is an appropriate approach when zonal systems are used or systems without ducting, such as radiant panels, fan coils, VRF, water source heat pumps, or  passive chilled beams. Since the air is supplied directly to the zone, the outdoor air requirements can exactly be met after balancing.
  • Supply to intake to local units - the DOAS supplies conditioned outdoor air to the intake of each local unit. Often used with terminal units in the zone, such as for VRF, fan coils, and water source heat pumps. This system avoids some of the duplicative costs of ducting and diffusers but adds the requirement that the terminal unit fan runs continuously while outdoor air is required, which may be inefficient.
  • Delivery to the supply side of local units - similar to the previous configuration but the conditioned air from the DOAS is supplied to the duct leaving the local terminal unit. It carries the risk of backflow through the unit unless pressure balancing is used, like in VAV terminals.
  • Supply to plenum near local units - similar to the DOAS supplying the intake to local units, the conditioned outdoor air is provided to the plenum that is used as the intake for the local units. Since the conditioned outdoor air is not explicitly provided to a zone, some of the benefit of a DOAS in providing the required outdoor ventilation is not possible with this configuration.

Information Needed for the Model

Many but not all building energy modeling programs can have two systems serving a single zone. Some workarounds may be required if the two systems are both based on central air handling units and instead, simulation of either zone DOAS or zone heating and cooling system may be required. Even for simulation software that does have DOAS components, be aware that the parameters may be spread over a number of different groups of data since DOAS features have often been added to the software after the initial architecture was created. Common inputs for DOAS include:

  • Space or zone outdoor air ventilation requirements, exhaust rates, and schedule of thermostat setpoints and occupancy (variations are especially important for demand-controlled ventilation with DOAS).
  • Where the mixing of the  DOAS air and heating and cooling system occurs (before or after equipment, for example) or if the DOAS air is supplied directly into the spaces.
  • DOAS supply fan design volume, efficiency, pressure rise, motor efficiency, fan placement, and part load performance if variable speed.
  • Minimum and maximum supply air temperature and humidity for the DOAS and whether DOAS supply air temperature tracks the supply temperature of the heating and cooling system or if it is a neutral temperature and if it is fixed or reset with outdoor air temperature.
  • Special DX or chilled water cooling coil capacity and performance for 100% outside air coils; this might include different latent performance and extra heat pipe (or other) heat exchanger impacts.
  • Design temperature after each heating and cooling coil.
  • Heat recovery type (sensible wheel, total enthalpy wheel, or another sensible heat exchanger), effectiveness at a variety of conditions, frost control, humidity setpoint, humidity control mode, and bypass control.
  • Optimized staging of the DOAS and heating and cooling systems that reflect the actual operation.

Make sure that ventilation air has been eliminated from the conventional heating and cooling system, which often have defaults that would be providing outdoor air.

Energy Modeling Guide for Very High Efficiency DOAS recommends that the modeler:

  • Verify ventilation rates and size of the DOAS unit
  • Control zone heating and cooling systems for off-hour temperature control and keep ventilation off.
  • Ensure any zone fan coil system does not introduce ventilation air directly.
  • Proper staging of zone ventilation and conditioning systems

This guide goes on to state:

“The following principles are recommended.

• Input zone heating/cooling systems separate from the DOAS unit.

• Schedule the operation of the DOAS on a separate unique schedule from the zone heating/cooling systems.  In general, the DOAS should operate continuously during the occupied periods, and be de-energized completely during unoccupied periods.

• Control zone heating/cooling system fans using a separate schedule that aligns with the thermostat schedules and defines the occupied period. 

• Control zone heating/cooling systems based on a thermostat schedule that includes unoccupied period setback/set-up of temperature setpoints.

o Allow fans to cycle on and off during the occupied period. (Note that the

DOAS will be operating continuously during the occupied period.)

o Allow the heating/cooling systems (in their entirety) to cycle on during

unoccupied period to maintain unoccupied period thermostat setpoints.

o This may require specifying the temperature drift allowed before cycling a system on.

o This may require defining how long a system should then operate, either

until the setpoint is maintained or for a set duration of time, such at 15

minutes in software that allows for sub hourly calculations.

In general, the energy model should be defined to match the control design intent, if known at the time of developing the model.”

In general, given the distribution of input parameters for DOAS and the conventional heating and cooling system across many groups of inputs, it is good to double-check the inputs and make sure they correspond to the correct system. Good naming habits are especially important when trying to interpret outputs (i.e., make sure the DOAS components include the word “DOAS” in their name). Automatic sizing of systems is very common, but please ensure that the component sizes are reasonable and reflect the amount of outdoor air and design conditions as expected.

Common Measures

Dedicated outdoor air systems are often a measure being considered over a baseline conventional system. By separating the often high latent load of the outdoor air from the mostly sensible space load, they can aid in providing an overall system that is optimized for both. Some of the most common measures to apply to DOAS are simply the range of possible configurations. Using building energy simulation to explore which options make more sense based on typical annual weather is often superior to rules-of-thumb or using the same configuration as the last building. Also, once DOAS has been selected, other heating and cooling system options that don’t normally provide ventilation can be considered, like radiant panels, passive chilled beams, VRF, or water source heat pumps.

After the specific configuration is decided upon, some common measures that can be considered include:

  • Increased fan efficiency
  • Increased heat recovery sensible or latent effectiveness
  • Various control options, often related to the supply temperature or humidity setpoint

Common Control Options

According to the 2020 ASHRAE Handbook Heating Ventilating and Air Conditioning Systems and Equipment, Chapter 51, the basic DOAS control modes are:

  • Dehumidification and cooling
  • Sensible cooling
  • Ventilation only
  • Heating

And each of these corresponds to regions on a psychrometric chart:

While DOAS systems do not have an “economizer” when the temperature and humidity are in the “ventilation only “ range, often the only energy needed to condition the space is the fan.

Humidity control, often to cool the supply air temperature to the dew-point temperature for the desired humidity level. When the space latent load is lower than normal, the dew-point temperature can be reset upward, but this requires humidity sensors in each space or representative space. A building automation system is often used to determine this.

The supply temperature may help with providing cooling to many spaces but can inadvertently cause spaces to be over-cooled during low-load situations such as lower occupancy or cooler periods of the year. One solution to this is demand-controlled ventilation that decreases the volume of air from the DOAS based on occupancy sensors. Another approach is to provide some level of reheat or to use a sensible heat exchanger on both sides of the cooling coil. Resetting the supply air temperature from the DOAS based on outdoor temperature is a common strategy but may compromise humidity control in certain months.

Total energy wheels that transfer both sensible and latent heat from the exhaust air stream to the supply air stream of the DOAS can be controlled by either enthalpy sensors or just temperature. The following figure from ASHRAE Design Guide for Dedicated Outdoor Air Systems, Second Edition, shows a common approach:

Common Applications

Many different types of buildings are good applications for DOAS, but they are specifically valuable in high-humidity climates and for facilities with high or complicated outdoor air requirements. They are also commonly used to simplify ventilation design and control since it is independent of the main heating and cooling system. DOAS is also commonly paired with heating and cooling equipment that is not capable of providing ventilation, such as passive chilled beams, water or ground source heat pumps, variable refrigerant flow systems, and radiant systems.The combination of DOAS with these low-energy systems often provides a high-efficiency building for a relatively low cost. In addition, it is common for DOAS to be combined with systems that may provide no or limited dehumidification.

Model Output Checks

According to the Energy Modeling Guide for Very High Efficiency DOAS, (the Guide), some simple validations that can be performed to verify that the energy model is correct, include:

“1. Check the ventilation total airflow rate. In many commercial buildings such as offices, ventilation may be between 0.1 cfm/sf and 0.2 cfm/sf. In buildings with more occupants per floor area, such as schools or assembly spaces, ventilation tends to be between 0.3 cfm/sf and 0.5 cfm/sf. In the case where a model is auto sizing or generating ventilation requirements between 1.0 cfm/sf or greater and the building is not a laboratory or other process-intensive space, the model is most likely not configured correctly.  2. Check the DOAS fan power. In some buildings, the exhaust fan is not necessarily located in the DOAS unit, so ensure you count the power of both supply and exhaust fans. The total power should be between 0.25 W/cfm and 1.0 W/cfm3  in most commercially designed DOAS units. Several systems may have higher power thresholds and the operational power should be confirmed with any designer if indicated to be above 1.0 W/cfm.”

In addition, the Guide encourages the evaluation of the seasonality of HVAC energy use and performing parametric verifications to help the modeler understand the impact of changing specific model input parameters. The questions the Guide suggests answering include the following:

  • Why would a building need more or less heating or cooling?
  • Why would a building use more or less fan energy?
  • Are there HVAC controls that could drastically change energy use?

Related Energy Code Requirements

For many applications, what is driving the use of a dedicated outdoor air system is the level of outdoor air required by each type of space in the building, which is often based on local energy codes or on ASHRAE Standard 62.1 Ventilation for Acceptable Indoor Air Quality. Table 6-1 from that publication shows the minimum ventilation rates in the breathing zone, and a small portion is reproduced below:

In addition, 62.1 also defines the distribution effectiveness of various supply configurations in the building spaces as well as all the equations and methods needed to determine the amount of air needed. Some special spaces may also have separate ventilation requirements, such as certain hospital spaces and special manufacturing facilities.

ASHRAE Standard 90.1-2019 includes tables of requirements for packaged DOAS systems that use DX cooling coils in Table 6.8.1-13 and Table 6.8.1.13. These tables set the minimum efficiency for air cooled, air source, heat pumps, water cooled, and water source heat pumps in dehumidification mode and heating mode using efficiency metrics ISMRE and ISCOP from AHRI 920.

Based on a search through the ASHRAE Standard 90.1-2019 user manual, many other HVAC requirements in ASHRAE Standard 90.1 can impact DOAS systems even if they do not explicitly mention DOAS in the standard itself, including:

  • Section 6.4.3.1.2 Deadband control
  • Section 6.4.3.3.4 Zone Isolation
  • Section 6.5.2.6 Ventilation Air Heating Control
  • Section 6.5.3 Air System Design and Control
  • Section 6.5.3.8 Occupied Standby Controls

ASHRAE Standard 90.1-2022 also includes an option of using HO6: Dedicated Outdoor Air System with Zone Fan Control as a way to gain energy credits in the new additional efficiency requirements.

AHRI 920 Performance Rating of Direct Expansion-Dedicated Outdoor Air System Units, which was referenced in the 90.1 tables above, includes several different metrics in the test procedure, including:

  • Rated Supply Airflow, scfm
  • COPDOAS at various conditions, heating, W/W
  • EATR - Exhaust Air Transfer Ratio
  • ISCOP2 - Integrated Seasonal Coefficient of Performance
  • ISMRE2 - Integrated Seasonal Moisture Removal Efficiency
  • MRC - Moisture Removal Capacity
  • MRE - Moisture Removal Efficiency
  • Power voltage, frequency, and phase
  • Supply Air temperature in heating and dehumidification modes
  • Total Heating CapacityDOAS
  • Water flow rate

Similar or Related Systems

Other system configurations include heat recovery components that are not dedicated to conditioning outdoor air, and unit ventilators often provide all the outdoor air.

Additional Resources

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References

2020 ASHRAE Handbook - Heating, Ventilating, and Air Conditioning Systems and Equipment. Chapter 51 - “Dedicated Outdoor Air Systems.”

AHRI Standard 920 (IP) with Addendum 1 - 2020 Standard for Performance Rating of Direct Expansion-Dedicated Outdoor Air Systems.

Bulger, Neil. Energy Modeling Guide for Very High Efficiency DOAS. NEEA. February 21, 2023.

ASHRAE Design Guide for Dedicated Outdoor Air Systems, Second Edition. Peachtree Corners. Georgia. 2022.

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