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The challenge of conforming to ASHRAE Standard 62.1-2004 in an energy efficient manner has led some HVAC practitioners to the conclusion that using dedicated outdoor air systems (DOAS) in conjunction with sensible cooling at terminal units offers the best solution.

This web site takes the position that there are 4 main pillars that under gird the Dedicated Outdoor air approach. They are:

  1. The delivery of ventilation air must be separated from the space conditioning systems for proper ventilation air distribution. Go to: Reasons Engineers Are Choosing Dedicated OA Systems section, to see why this is true.
  2. Conditioning the OA to handle all of the space latent load and much of the space sensible load employing energy recovery as specified by ASHRAE Std. 90.1-2004. Go to: Decoupling the Building Latent and Sensible Loads Using 100% OA systems for more information.
  3. Meet the balance of the space sensible loads with a parallel system. Go to: Parallel Building Sensible Cooling Choices for details.
  4. Integrate the fire suppression and energy transport systems as permitted by NFPA 13. Go to the ASHRAE Paper: Integration of Hydronic Thermal Transport Systems with Fire Suppression Systems.

Reasons Engineers Are Choosing Dedicated OA Systems:

The following four major problem areas, common to all air systems, are easily overcome with properly designed dedicated OA systems working in parallel with sensible cooling equipment.

First, Ventilation Air Distribution In All Air VAV Systems. Engineers cannot be sure where the ventilation air, in all air systems, is distributed once brought into the building and mixed with return air at the air-handling unit. Proper distribution is a function of VAV boxes minimum settings, space sensible loads, local exhaust and exfiltration, short circuiting paths and interzonal air transfer (Mumma, Lee 1998). Complete knowledge of all of these factors at design is not possible, much less in real time. Therefore engineers cannot really defend compliance with ASHRAE Standard. 62.1-2004 when using all air VAV systems. On the contrary, dedicated 100% OA systems place the proper ventilation air quantities into every space.

Second, Excess OA Flow And Conditioning Required For All Air VAV Systems. When the multiple spaces equation of ASHRAE Standard 62.1-2004 is used, generally from 20-70% more outdoor air is required in an effort to assure proper ventilation air distribution in all air systems than is required with a dedicated outdoor air systems. Cooling and dehumidifying the high OA quantities in the summer and humidifying and heating the air in the winter is an energy intensive proposition.

Third, VAV Box Minimum Settings Must Be Surprisingly High. Perhaps contrary to current practice, VAV box minimums must reflect both the ventilation requirements of the space and the fraction of ventilation air in the supply air. For example, a space requiring 200 scfm of ventilation air and served with supply air that is 40% ventilation air, will require a box minimum setting of 500 scfm (i.e. 200/0.4) rather than the conventional practice of 200 scfm. When the box minimums are properly set to satisfy the ventilation requirements, the potential for considerable terminal reheat becomes an issue. Therefore, properly operating all air VAV systems will always use more terminal reheat than dedicated outdoor air systems supplying air at the same temperature.

Fourth, Inability Of Most All Air VAV Systems As Currently Designed To Decouple The Space Sensible And Latent Loads. The inability to decouple the space sensible and latent leads to high space relative humidity at low sensible loads in the occupied spaces. Properly designed dedicated OA systems can accommodate 100% of the space latent loads and a portion of the space sensible loads, thus decoupling the space sensible and latent loads. A parallel sensible only cooling system is then used to accommodate the sensible loads not met by the dedicated OA systems. When all of the OA load, all of the space latent load, and up to 30% of the space sensible load is accommodated by the dedicated OA system, only about 40% of the design chiller load must be borne by the parallel sensible only cooling system. There is therefore a strong incentive to control the space latent loads independently of the space sensible loads to avoid moisture related IAQ problems.

Decoupling the Building Latent and Sensible Loads Using 100% OA systems:

In order to decouple the space latent and sensible loads using the small volumetric flow rates of OA (approximately 20% of the normal VAV design flow), the supply air dew point temperature must be suppressed more than is typical with an all air VAV system. If the bulk of the latent load is from the occupants (approximately 205 Btu/hr-person), and each occupant receives 20 scfm of OA, then the change in humidity ratio is approximately 15 grains/lbmDA. Clearly, the supply air dew point must be at least 15 grains/lbmDA less that the space target humidity ratio.

This approach to meeting all of the space latent loads requires that the building envelope be tight, and in conformity to ASHRAE Standard 90.1-2004. Large and unpredictable variations in the envelope transmission of moisture, either thru infiltration or movable sash, will lead to a loss of space design conditions. Such a loss of control could result in unwanted condensation at the distributed parallel sensible cooling equipment and an associated distributed microbial problem.

The dedicated OA must be cooled and dehumidified in the summer, and humidified and heated or cooled at other times of the year. In the simplest form, the preconditioning could be achieved with a preheating coil, a cooling coil, a reheating coil, and a humidifier. However ASHRAE standard 90-2004 in section: G3.1.2.10 Exhaust Air Energy Recovery requires the following. "Individual fan systems that have both a design supply air capacity of 5000 cfm or greater and have a minimum outside air supply of 70% or greater of the design supply air quantity shall have an energy recovery system with at least 50% recovery effectiveness. Fifty percent energy recovery effectiveness shall mean a change in the enthalpy of the outdoor air supply equal to 50% of the difference between the outdoor air and return air at design conditions". Consequently, since dedicated OA systems are by definition 100% OA systems, energy recovery is required to supplement the simplest form of preconditioning equipment mentioned above.

Further ASHRAE Standard 90.1-2004 places the following limitations on reheating (defined in the standard as: raising the temperature of air that has been previously cooled either by mechanical refrigeration or an economizer system) as it relates to preconditioning the OA. Dehumidification. Where humidistatic controls are provided, such controls shall prevent reheating, mixing of hot and cold airstreams, or other means of simultaneous heating and cooling of the same airstream. Exceptions to (e) At least 75% of the energy for reheating or for providing warm air in mixing systems is provided from a site-recovered energy source.

The ASHRAE Standard 90.1-2004 requirements clearly mandate that a total energy recovery device, i.e. an enthalpy wheel, also known as a passive desiccant wheel be used to reduce the load on the cooling coil. Further, if the supply air is to be delivered at a temperature above the required supply air dew point temperature, a sensible energy recovery device is required.

More in-depth coverage of this topic is presented in the ASHRAE paper: Achieving Dry Outside Air in an Energy Efficient Manner.

Parallel Building Sensible Cooling Choices:

As a general observation, the ventilation air may be supplied with temperatures ranging from 75-45F (24-7C). In this range of supply temperatures, for typical ventilation air flow rates, from 0-30% of the space sensible loads can be accommodated by the OA. The balance of the space sensible loads must be accommodated with many optional equipment choices. Those choices include:

    A parallel all air VAV system,
    Packaged unitary water source heat pumps,
    Fan coil units,
    Packaged unitary equipment,
    Radiant ceiling panels.

This site takes the position that radiant ceiling cooling panels are the best parallel system choice for use with the DOAS. The small ventilation air ductwork and parallel radiant cooling panel terminal equipment offer a significant opportunity to reduce the required floor-to-floor dimension. For more in-depth coverage of this topic, go to the ASHRAE paper: Ceiling Radiant Cooling Panels As A Viable Distributed Parallel Sensible Cooling Technology Integrated With Dedicated Outdoor-Air Systems.

Role of Economizers in Dedicated Outdoor Air Systems, Click Here to read more

Please visit the Papers section of this site for more information on
      Dedicated Outdoor Air Systems and Radiant Ceiling Panels.

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Dedicated Outdoor Air Systems & Radiant Ceiling Panels
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Copyright 2001 Bob Hedman.