ASHRAE states: “The Building Guidance clearly encourages building operators to increase their systems outdoor air ventilation to reduce the recirculation air back to the space.”1 System design, outdoor and space conditions will each have a bearing on how feasible it will be to introduce more outside air.
This article is part 2 of a 3-part series that will focus on the design aspects of how to increase the amount of outside air which reduces the level of contaminants in the space. We will discuss some ideas for replacement work as well as suggest some potential retrofits of existing systems by adding energy recovery devices as well as other components.
1(ASHRAE Epidemic Task Force, Building Readiness, 9/30/2020)
One method to improve indoor air quality is to increase the ventilation rate of the outside air and to reduce the amount of recirculated air. This strategy, however, creates tension for the designer trying to balance increased outdoor air (OA) with ASHRAE Standards 55, 62.1, and 90.1. Increasing OA to an existing system can also create problems for contractors and building owners as comfort, cleanliness, and energy efficiency may suffer.
New or Replacement Applications
On page #17 of the ‘Schools and University’ guideline recommends increasing OA when a) retrofitting or replacing an existing AHU or b) if a dedicated outdoor air systems (DOAS) is being replaced. The recommendation is for the designer to consider increasing capacities beyond code minimum for ventilation. This increase in the amount of outside air could drastically increase the capacity requirements of the heating, cooling, and dehumidification systems. This increase could also trigger the requirement of an energy recovery device to satisfy ASHRAE 90.1 standards.
It is H. C. Nye’s recommendation that when considering equipment replacement (AHU, RTU, or DOAS) that the designer or contractor should select equipment components that will allow stable operation at three design points: a) in excess of ventilation code minimum, b) at ventilation code minimum should the situation allow at a future date, and c) minimum operating conditions. Furthermore, energy and comfort standards need to be met.
As can be seen, a very broad operating envelope is required. Such equipment should include the following:
- 2-stage, digital scroll, or inverter driven compressors
- Direct drive fans with EC motors or VFDs
- Hot gas reheat with modulating control
- Energy recovery devices (wheels, plates, coils)
- High turndown modulating gas heat
- Electric heaters with SCR control
Given that all these components are variable and operate both simultaneously and independent of each other (with limits), the control system must be robust enough to accommodate the diverse operating points with diverse operating sequences. We recommend that these controls should be factory installed.
broad range of products can provide all the preceding features, plus many more, including IAQ, special materials, high filtration, and acoustical packages. Aaon’s products are uniquely qualified to increase the amount of outdoor air ventilation while still controlling space temperatures and humidity to ASHRAE 55 standards. Their products are designed with capacity control features that can hold tight temperature and humidity setpoints at the three referenced unique operating conditions. Furthermore, they can be selected to meet present and future ventilation requirements, with and without energy recovery devices. In addition, many of Aaon’s products can incorporate an airside heat pump that will provide energy savings and superior temperature control when there are mild ambient temperatures.
Evaluating Existing System’s Ability to Increase OA in Lieu of RA
For existing systems where the OA and return air (RA) mix, an evaluation should be made to determine whether there is excess cooling and heating capacity available to accept an increase in the amount of outside air. A “back of the envelope” calculation yields the following results and observations:
Download these evaluations in text format.
Increased Capacity with the Same Footprint
As expected, increasing the amount of outside air results in larger capacity requirements. But what does that mean if you have existing equipment that you want to replace, is there any way to minimize replacement costs? One thing to keep in mind is that oftentimes HVAC equipment comes in a few different capacities for the same cabinet size or footprint. Some examples would be a 1000 cfm unit ventilator (U)V might be the same size as a 1250 cfm unit; a 200 cfm fan coil unit (FCU) may be the same size as a 400 cfm unit. Another example would be Aaon’s AAON’s model #RN packaged RTU, ERU and DOAS units that were designed with 6 different cabinet sizes. Within the same cabinets there are units of multiple tonnages that all have the same dimensions. As an example, Aaon’s AASON’s ‘D’ size cabinet has the same footprint and height for a 26 ton unit all the way up to 70 tons. Some of the models that are close in size have very little difference in weight and electrical requirements between them. Please note that any of these replacements would require a proper engineering evaluation to determine what other potential problems these changes may cause. Please work with your H.C. Nye Sales Engineer to determine equipment capabilities and any impact on an existing system.
Retrofit with Energy Recovery
Another option available to provide additional outside air for replacement work or retrofits would be to install an energy recovery (ER) device. ER wheels, plates, and coils recovery waste energy (sensible only as well as total) to pre-treat the outside air. This device would allow increases in the outside air quantity, would assist in maintaining comfort conditions and would help minimize the additional energy usage.
H.C. Nye represents well know energy recovery manufactures,, such as AirExchange, NovelAire Technologies, Renewaire, and Semco. Each of these manufacturers have a variety of different ER products that are AHRI 1060 certified for performance and are available for retrofits, replacements, and new construction.
In instances where the amount of outside air can be increased, the ‘Building Readiness’ guideline on page # 18 also notes that building and space pressurization should also be evaluated. Existing exhaust and relief air systems might have to be changed to prevent additional problems, , such as doors not closing, excessive noise, and reverse of intended pressure required for a space.
H.C. Nye represents Paragon Controls Incorporated, a leading designer and manufacturer of airflow and pressure sensing systems for commercial and industrial applications. All of Paragons products are ISO 9001 certified for quality management, and are available for retrofit work as well as for new construction. Their airflow sensing elements and transmitters are available for installation for outside air, fan inlet as well as duct mounting. Their pressure sensing elements and transmitters are available for rooms and spaces, for outdoors, as well as duct mounting.
In conclusion, ASHRAE’s Epidemic Task Force has determined that increasing the amount of outside air to a space would increase the indoor air quality in these times of the COVID-19 crises. They have addressed some of the methods that are available, while also emphasizing that temperature and humidity conditions must still be maintained. For almost 40 years H.C. Nye has been a strong advocate for indoor air quality and has many products available to assist the HVAC community.