Dedicated outdoor air system

A dedicated outdoor air system (DOAS) is a type of HVAC system that consists of two parallel systems: (1) a dedicated outdoor air ventilation system that handles latent loads and (2) a parallel system to handle sensible loads.^[1] The graph on the left shows the schematic of a dedicated outdoor air system. Depending on the environment and the parallel system involved, the outdoor air system will handle some of the sensible load in addition to the latent load and the parallel system will handle the rest of the sensible load. The main point of a DOAS system is to provide dedicated ventilation rather than ventilation as part of conditioned air. DOAS is a term given to a system that has been used extensively in Europe and in various forms in the US. This page outlines the basics of DOAS, including the advantages and disadvantages of such a system.

DOAS System Overview

S. Mumma suggests that there are four main problems with conventional all air overhead mixing VAV HVAC systems.^[1] These issues of VAV systems highlight the corresponding advantages of DOAS systems. However, some disadvantages of DOAS include: potentially higher first costs, lack of use in the United States, and potentially higher complexity.

1. Ventilation air in all air VAV HVAC systems
   • Designers and building engineers are unable to know exactly how the ventilation air that is mixed with the return air in a typical VAV system is distributed throughout the building. Issues such as air leakage, control setpoints, minimum air volume settings, and short-circuiting (e.g. exhaust air mixing with intake fresh air) can all effect the amount of ventilation air that reaches a space.^[1][2] A DOAS system solves this problem by providing a dedicated supply of 100% outdoor air.
2. Need for excess outdoor air flow and conditioning in 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 room air distribution in all air systems than is required with a dedicated outdoor air systems. Cooling and dehumidifying the high outdoor air quantities in the summer and humidifying and heating the air in the winter is an energy intensive proposition.^[1] The DOAS system is sized to meet the requirements, and does not require oversizing.
3. VAV box minimums have to be set high to account for ventilation requirements
   • 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 5663 slpm (200 scfm) of ventilation air and served with supply air that is 40% ventilation air, will require a box minimum setting of 14158 slpm (500 scfm) (i.e. 200/0.4) rather than the conventional practice of 5663 slpm (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.^[1]
4. No decoupling of latent and sensible space loads
   • The inability to decouple the space sensible and latent loads leads to high space relative humidity at low sensible loads in the occupied spaces. Properly designed dedicated outdoor air 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 outdoor air systems. There is therefore a strong incentive to control the space latent loads independently of the space sensible loads to avoid moisture related IAQ problems.^[1]

Parallel sensible cooling system choices

For a typical DOAS ventilation system, the outside air system can accommodate around 0-30% of the space sensible load. In order to create a comfortable indoor environment, the balance of the space sensible loads must be accommodated by many other optional equipment choices as follows:

1. A parallel all air variable-air-volume (VAV) system
2. Packaged unitary water source heat pumps
3. Fan coil units
4. Radiant ceiling panels

Radiant system as parallel cooling system

Compared to other sensible cooling systems, radiant ceiling cooling panels are the best parallel system choice for use with the DOAS. Because the DOAS only accommodates the space ventilation and latent loads, it provides an opportunity to reduce the required floor-to-floor height by reducing the size of the duct system and the required fan power^[3]

There are numerous advantages of a radiant ceiling cooling system coupled with a DOAS. The general evaluation section in 2008 ASHRAE Handbook gives a brief description as follows^[4]:

• Advantages
  • Because radiant loads are treated directly and air motion in the space is at normal ventilation levels, comfort levels can be better than those of other air-conditioning systems.
  • Meet the requirement of supply quantities for ventilation and dehumidification.
  • Due to the reduced outdoor air quantities, the DOAS system can be installed with smaller duct system.
  • Radiant ceiling cooling panels can eliminate wet surface cooling coils and reduce the potential for septic contamination.
  • The automatic sprinkler system piping can be applied into radiant ceiling cooling panel systems.
• Disadvantages
  • Potentially higher first costs

Besides the advantages presented above, parallel radiant cooling panels offer other advantages as well, such as compact design, vertical shaft space area savings, and quick accommodation of dynamic controls. Energy savings in DOAS/radiant ceiling cooling panel system can by linked to: cooling coil load reduction, chiller energy reduction, pumping energy consumption and fan energy consumption reduction. In general, due to the total energy recovery and small supply air quantity of DOAS, the chiller energy consumption can be reduced significantly compared to the conventional VAV system. In a study of a pilot DOAS/radiant ceiling cooling panel system, hourly energy simulation predicts that the annual electrical energy consumption of the pilot DOAS/radiant panel cooling system is 42% less than that of the conventional VAV system with economizer control.^[5]

Air-based system as parallel cooling system

There are two main ways to design a DOAS when using an air-based system as the parallel system^[6]:

1. Two separate systems with different ductwork. In this setup, there is an outdoor air system that dumps preconditioned air (accounting for latent load and partial sensible load) directly into the space in its own duct/diffuser. There is a separate system (e.g. fan coil unit) that takes air from the space and conditions it to meet the remaining space sensible load.
   • Advantages
     1. Easier to measure the outdoor air flow rate into the space
     2. Easier to measure airflows and balance system
     3. Avoids imposing ventilation loads on space HVAC equipment (fan coil units)
   • Disadvantages
     1. Separate ductwork for parallel paths can increase first costs
     2. Separate diffusers for outdoor air and recirculated air may not provide adequate mixing
     3. Separate parallel paths for airflow increases overall airflow to the space which can increase overall fan energy consumption
2. A combined system in which conditioned outdoor air is ducted to the terminal unit in the space. In this setup, the preconditioned outdoor air is ducted into the fan coil units directly, mixing with the return air from the space. This system is similar to a chilled beam setup.
   • Advantages
     1. Combined ductwork leads to lower first costs
     2. Combined airflow reduces air volume and consequently fan energy
     3. Thorough mixing of outdoor air and return air from space
   • Disadvantages
     1. Local terminal unit must operate whenever ventilation is required, regardless whether or not the sensible load has been met
     2. Balancing airflow may be more difficult

References

1. ^ ^{a b c d e f} http://doas.psu.edu/doas.html Accessed 11/15/2010
2. ^ Mumma, S; YP Ke (1998). "Field testing of advanced ventilation control strategies for variable air volume systems". Environment International Journal 24 (4): 439–450. 
3. ^ Conroy, C.L.; S. Mumma (2001). "Ceiling Radiant Cooling Panels as a viable Distributed Parallel Sensible Cooling Technology Integrated with Dedicated Outdoor Air Systems". ASHRAE Transactions 107: 5778-585. 
4. ^ 2008 ASHRAE Handbook-HVAC Systems and Equipment, ASHRAE, Inc, 2008.
5. ^ Jeong, J.W.; S. Mumma, W. Bahnfleth (2003). "Energy Conservation benefits of a Dedicated Outdoor Air System with parallel Sensible Cooling by Ceiling Radiant Panels". ASHRAE Transactions 109: 627–636. 
6. ^ Morris, W. (May 2003). "The ABCs of DOAS". ASHRAE Journal: 24–29.