Integrating DOAS Into Your Restaurant Project

In the past couple of years, we’ve seen a large increase in the building and operating costs of a restaurant. Now more than ever, every penny counts when developing a restaurant as Owners and developers keep a keen eye on their projects’ finances and return on investment (ROI).

There is also a push for high indoor air quality to have healthy and comfortable working and dining environments for employees and patrons. Air quality indicates how clean or polluted your air is. Poor air quality can lead to unsafe working conditions, coughing, dry skin, and eyes, or mold growth.
A dedicated outdoor air system (DOAS) offers an ideal solution by providing cost savings through reduced installation expenses and long-term energy/utility savings while maximizing air quality through precise temperature control, maintaining comfortable humidity levels, and employing high filtration and air dilution.

Understanding Dedicated Outdoor Air Systems

A DOAS serves as a compact, rooftop packaged equipment solution that supplies large volumes of outdoor air that is fully conditioned, dehumidified, or heated. This outdoor air is required for ventilation, to satisfy exhaust requirements from the kitchen hoods, and to provide cooling to the space while maintaining a positive building pressure. The unit is generally made up of the following components;
• condensing coil for cooling
• a heating module that can be electric or gas
• filters on the intake and return sections
• a blower to move the air through the duct and diffusers
• controls and a space temperature sensor for programming, activation, and scheduling.

Design Considerations

An important consideration when designing a space is ensuring the building is properly air balanced. When you have cooking equipment with a kitchen hood above head, there is grease and smoke that must be exhausted out of the restaurant. The exhausted air must then be replaced with fresh outdoor air to maintain a proper balance, to prevent drafts and doors from being hard to open, for example.

A traditional restaurant design will use a make-up air unit (MAU) on the roof that is ducted into the kitchen with a plenum in front of the hood to deliver the air. This air is intended to be recaptured into the hood so therefore it is not designed to be fully dehumidified and cooled.

However, since the air isn’t fully captured by the hood, it introduces some humidity and hot outdoor air into the space which will in turn add more load to the other rooftop units (RTUs) in the space.

The typical RTUs are only designed to handle about 25% of outdoor air at a time, with the remaining 75% of the supply air being recirculated from the space. Typically, there is a large amount of recirculated air and total supply air to ensure the units can adequately cool the space. Traditional RTUs are not designed to handle the combination of loads from a MAU in addition to the changing space temperature.

This is where a DOAS system could help:

A DOAS simplifies the design and completely removes the MAU and supply plenums. They are designed to deliver up to 100% of fresh outdoor air directly to the space to replace the exhaust air and will fully dehumidify and condition the air as well.

This gives you more control over the air entering the space as the DOAS conditions the outdoor air separately from the recirculated air. There will be a total reduction in supply air as the DOAS is completely handling the cooling of the space and satisfying the ventilation requirements with the minimum quantity of outdoor air required.

How Does A DOAS Improve Air Quality?

Precision

A DOAS uses the latest technology in regards to controls, monitoring, and operation. It is recommended that a comfortable temperature for a space be between 70oF to 75oF with a relative humidity between 40% and 60%. This is precise criteria and the DOAS achieves this by modulating its cooling and heating components to only use the amount of energy that is required at that time to maintain a comfortable environment.

Traditional RTUs typically do not have precise modulating components and may overheat or overcool the space leading to fluctuations in the space temperature, followed by some excessive cycling of the unit to meet the space temperature.

Humidity

Humidity is the amount of water vapor in the air and can cause many issues if not handled. Humidity comes from multiple sources; it is built up through the cooking processes, the people inside, and the outdoor air that is being brought into the space.

If the humidity is too low, you can experience dry skin, burning eyes, or coughing. If the humidity is too high, the environment can feel damp and encourage mold growth, and surfaces such as the duct, ceiling tiles, dripping diffusers, and the floor can condensate and sweat, causing slipping hazards. Think of how it feels outside after a heavy rainstorm, it is not an enjoyable environment. A DOAS is able to provide more accurate humidity control at increased efficiency in comparison to traditional RTUs and can safeguard these conditions from happening to provide a more comfortable environment.

Filtration

One of the most important benefits is that a DOAS typically has very little or zero recirculated air. This prevents contaminants such as viruses or grease from being recirculated into the space which can damage equipment, dirty a space, or get employees and patrons sick. The fresh outdoor air brought in by the DOAS dilutes any contaminants and filters it with multiple layers at a very high minimum efficiency reporting value (MERV) level.

Money Back Into Your Pocket

When it comes to the cost of a traditional RTU vs a DOAS, the DOAS has a higher overall unit cost as it requires a higher amount of capacity and tonnage to cool the outdoor air. This is quickly offset by savings in installation and the efficiency of the unit.

With a DOAS design, you can completely remove the Hood makeup air unit, its roof penetrations, and associated supply plenums and ductwork. That’s a lot of installation costs that will not be needed. The ductwork may be smaller due to a reduction in total supply air, and the test and balance of the restaurant will be more manageable and easier.

A DOAS is built with efficiency in mind. It has fully modulating internal components to conserve energy to provide the precise amount of dehumidification or heating required based on the outdoor air temperature and space requirements. The unit can also be interlocked with a demand control ventilation hood system (this system modulates the exhaust fans based on cooking needs) to modulate the amount of outdoor air to maintain building balance. This in turn means lower utility costs as it eliminates the excessive runtime and cycling from traditional RTUs to meet space demands. This leads to a quicker payback and a longer lifetime with less maintenance and downtime for the unit.

Another advantage is that lead times for a DOAS are generally quicker than traditional RTUs which have seen production delays rise in the past couple of years due to global supply chain issues. You can get units quicker to keep projects on schedule and moving with no delays. No more waiting around for units to deliver or having to order months in advance (hopefully you don’t have any design or ventilation changes within that time that may affect the units!).

For More Information

As we’ve discussed in this post, it’s important that air is delivered at a comfortable and healthy temperature and humidity. This ensures employees and patrons are having an enjoyable experience working and dining in the restaurant. A dedicated outdoor air system (DOAS) is the best solution to provide precise temperature and humidity control through high efficiency and technology, all while meeting the ventilation requirements of the space, and providing long term cost savings benefits.

If you would like to learn more about the details and benefits of a dedicated outdoor air system, please contact us at 407.645.5008 or via email at [email protected].