DOAS, VAV Systems

How Can A DOAS and Radiant System Save You More Compared To A VAV System?

Change is not always easy. When it comes to HVAC systems, this sentiment certainly holds true. Conventional variable air volume (VAV) systems have been used for a very long time, which gives them the ‘tried and tested’ value.

However, technology advances quickly, and HVAC systems are no exception. While a conventional VAV system may seem appealing, it is outdated. In the field of HVAC, developments have produced “DOAS + Radiant” systems, which far outperform conventional VAV systems in almost every way.

While the enhanced comfort provided by a DOAS and Radiant system (discussed in a previous blog) may not be enough of an incentive to move away from VAV systems, I’m confident we can agree that considerable monetary savings will swing the balance.

Why a “DOAS + Radiant” system will save you more compared to a VAV system

 

There are four main areas that incur monetary savings when using “DOAS + Radiant” compared to VAV. These four areas include costs, energy usage, health benefits, and design advantages. As you will see through each of these points, using “DOAS + Radiant” rather than VAV saves money at every turn, but has the added benefits of being energy conscious, modern, healthy, and comfortable.

 

1. Energy efficiency

 

“DOAS + Radiant” systems are far more energetically efficient than conventional VAV systems. Energy savings vary depending on the particular design and components of the system, as well as the climate conditions, but studies have shown that they can be upwards of 40 %. The best performing systems are considered to be “DOAS + Radiant” fitted with an enthalpy or desiccant wheel to recover sensible and latent energy. As an added benefit, these systems have also been shown to perform better regarding humidity and temperature control, as well as to regulate indoor air quality (IAQ).
Energetic efficiency is obviously good for the environment and in keeping with contemporary ‘green’ building designs and codes. That makes using energy efficient systems fashionable, modern, and environmentally conscious. However, perhaps most importantly for business, it saves a lot of money through forming part of the significantly reduced operational costs (see below).

 

2. Health, comfort, and performance

Health comfort and performance

The saving offered by energetic efficiency is easy to visualize. However, the savings just from how “DOAS + Radiant” systems improve the indoor environment should by no means be underestimated.
Essentially, “DOAS + Radiant” systems provide far superior indoor environmental conditions compared to VAV. This happens via a number of pathways: Firstly, radiant cooling/heating systems provide more evenly distributed and better controlled thermal comfort than all-air VAV systems; secondly, DOAS systems provide better humidity control, which controls the growth of fungi, dust mites, or other microbes that cause allergic reactions or illness; and thirdly, DOAS provide 100 % outdoor air (filtered if necessary) to occupied spaces via accurate and precise ventilation, which eliminates issues of recirculating and spreading contaminants (allergens, or pathogens) which can occur with VAV systems.
What the improved environmental quality does is reduce the prevalence of ‘sick building syndrome,’ reduces absenteeism, and significantly improves productiveness (Figure 1 below).

Combined, these can elicit massive monetary savings and advantages. Fisk et al. (2011) estimated the cost-benefit of improved indoor environmental quality to be approximate US$ 20 billion annually in the US alone.

predicted relationships between ventilation rate and the prevalence of‘sick building syndrome ventilation rate and performance

Figure 1: The predicted relationships between ventilation rate (a proxy for IAQ – higher ventilation rates correspond to improved IAQ) and the prevalence of ‘sick building syndrome, ventilation rate, and performance, and temperature and performance. These show how improved indoor environmental quality (IAQ and thermal comfort) reduce illness, and improve performance. Taken from Fisk W, Black D, Brunner G (2011) Benefits and costs of improved IEQ in US offices. Indoor Air 5 (21):357-367.

 

3. Space

 

“DOAS + Radiant” systems use considerably less space than conventional VAV systems, because of greatly reduced size and quantity of ductwork and plenums. This is achieved by the fact that reaching the required ventilation rates to occupied zones, DOAS need a 20 – 70 % lower flow rate, and 25 – 30 % less air volume, than conventional VAV systems. The use of an enthalpy wheel also reduces the size of the refrigeration and humidification plants by up to 40 %, saving more space.
Using less space provides new opportunities for building design and floor space utilization, but also reduces installation and building costs (see below).

 

4. Installation and operating costs

 

This is often the primary concern for people considering moving to new, “DOAS + Radiant” systems. People want to know how much they have to initially put down to install a new system.

The common perception used to be that “DOAS + Radiant” systems were far more expensive to install than traditional VAV systems. However, Mumma (2002) showed that both the initial installation cost and the operating costs of a “DOAS + Radiant” system are reduced compared to a traditional VAV system. Subsequent studies have shown that operating a “DOAS + Radiant” system compared to a conventional VAV system can amount to annual operational cost savings of between 30 % and 40 %. That means that over a short period (a year or less), a “DOAS + Radiant” system will pay back the cost of installation, and after that begin accruing significant monetary savings.

Conclusion

 

These four areas demonstrate the immense savings offered by “DOAS + Radiant” systems compared to conventional VAV systems. These savings apply to new building designs, as well as to retrofitting buildings with existing outdated HVAC systems.

In today’s society, it is increasingly difficult to compete on the global market. Added to that, there is the growing drive to develop and implement ‘green’ technologies to combat the dire threat of climate change. Using a “DOAS + Radiant” system allows a building to meet both these challenges.

 

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