The primary function of an HVAC system is to generate and maintain comfort for occupants in a conditioned space. The best way to achieve comfort for everyone in an office, restaurant or home is through zoning. It is important to make the distinction between HVAC equipment zoning capabilities and building zoning requirements.

A multizone HVAC system consists of one air handler/furnace with zone dampers, thermostats and a zone control panel.  This type of configuration will allow for different set points in each zone. The thermostat will measure the temperature in a room and relay the information to zone control panel. The zone control panel receives a message from each thermostat and will command the air handler/furnace to turn on or off and the zone dampers corresponding to each thermostat to open or close.

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The advantage of multizone HVAC system is greater zone temperature control and energy savings.

For example, in the evening when you are going to bed, it is not necessary to cool both the living area and bedroom to the same temperature.  The living area zone can be adjusted to a higher temperature set point since it will not be occupied during the night and the bedroom can be maintained at the temperature you prefer.   The comfortable temperature is maintained and you are not paying to cool an area that is not occupied.

As amazing as this sounds, zoning a single HVAC system does have its cons, especially when done with a constant volume air handler/furnace.


Zone dampers are installed on each branch duct to feed the air to their respective zones.  The first problem appears when there are several zone dampers and only a small percentage of their corresponding zone dampers are open. This produces a higher static pressure, quicker velocity and a potentially noticeable noise difference in the air traveling through ductwork.  The faster velocity, which leads to additional noise, may be unpleasant to the owner, but the main mechanically concerning issue arises when dealing with the high static pressure.

A bypass duct is a relative cheap and easy way to theoretically deal with the extra static pressure. However, it can easily create additional mechanical problems.  A bypass duct can redirect air to the return ductwork entering the air handler/furnace even when the zone dampers are open and calling for air, lower the average temperature entering the return ductwork of the air handler/furnace and make it less efficient and the evaporator coil could potentially freeze up. Keep in mind there are alternatives to dealing with the excessive static pressure that don’t involve a bypass duct.  However, some of these methods are not as price friendly or easy to install.  A bypass duct is shown below.

 


For offices, restaurants, and larger homes, zoning can be achieved by using multiple air handler/furnace units.

Each zone will only have one thermostat and should contain similar rooms.  In a residential application, a basement would experience a much different thermal load than a second floor bedroom.  These two rooms would not make a good zone.  A good zone could consist of rooms on the same floor that have are subjected to a large solar gain because they have south facing windows.

In a commercial application, it is good to separate heat generating machines onto their own zones. For office buildings, IT/server equipment produce heat and may require cooling all year long. Therefore, these rooms are often on their own zone.  In restaurants, cooking equipment can put off a large amount of heat.  Although the majority is exhausted out of the space, kitchens have different cooling requirements than the dining area of the restaurant. A multilevel building zoning plan is shown below.

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These examples are relatively straight forward in illustrating when different zones are needed. Yet, there can be a fine line between the number of zones and the cost of purchasing and installing all the necessary zoning equipment.


Allegheny Design Services

At Allegheny Design Services, we pay attention to the details of our projects to ensure that comfort is achieved and maintained while keeping in mind the associated construction costs. Whether you are seeking mechanical, electrical, plumbing or structural services, ADS understands the construction process is as important as delivering construction documents.

ADS is the link between the architect’s creativity and the efficient construction of the project.  Learn more here or click the button below:

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Thanks for reading,

Alex

Written by Alex Clarkson, EIT, Junior Mechanical Engineer

Alex-ClarksonAlex Clarkson joined the Allegheny Design Services team in March of 2015 as an engineering Intern. He graduated magna cum laude from West Virginia University with his bachelor’s degree in mechanical engineering in May 2015. While at WVU, Alex served as the treasurer of the student branch of ASHRAE during the fall 2014 semester and president during the spring 2015 semester. Prior to joining ADS, Alex worked as an MEP engineering Intern at Miller Engineering. He is now a junior mechanical engineer at ADS, primarily focusing on the design of mechanical and plumbing building systems.