Saturday, November 3, 2012

Zoned HVAC Systems and Design Issues

HVAC systems are designed to condition air in our homes. The bigger the home, the bigger the system to condition that air. As system size increases so does the amount of air it moves and energy it uses. There are new modern systems that have modulating speed that the flow increases and decreases depending on load needs. The fan can increase and decrease in speed which is balanced between the return and supply air of the system.

Traditional systems that are installed in many homes have a on or off method.  The thermostat calls for conditioned air and turns on reaches the temature setting of the thermostat and turns off.
In each case modulating and traditional system air flow is key for the units operation. Without proper airflow systems do not work as designed.

While zoned systems can be an excellent choice for energy savings and comfort in homes the design and implementation must be correct for the system to work properly.   The idea is that the system can separate zones and only condition the area that you are in.  As we turn on the thermostat in one zone the unit turns on. We are now only pay to condition the space we are occupying saving energy and money with only one HVAC unit. This saves upfront and operation costs.

When using traditional HVAC Systems for a zoned system, the ducting is inadequate to handle the air flow when only one zone is being called for.  The air being produced now in the unit is causing static pressure to rise in the system.  The air must go somewhere so either the system chokes down because the static pressure air is resisting the air flow out and will compensate by resisting airflow in causing a host of problems or the pressure or flow must be diverted to maintain cfm flow.

There are two strategies for handling this problem in a traditional HVAC systems. One would be to “dump” the supply air into the space of the home into a large area such as a hallway.  The other method will use a “by pass duct” which as the pressure rises the air is diverted to the return via this by pass duct. Here are some reasons that make the bypass duct a poor design in any system

1)      By diverting the conditioned air during the cooling season we are pushing colder air into the return. This makes the coil colder and reduces the efficiency of the system. Our cooling capacity is then significantly reduced
2)      Our air conditioner and heaters are designed to have the supply and return separate. By combining the two in this manner air can be detrimental to proper flow. Air can be described as being lazy; it will follow the path of least resistance. Even if all four zones are open having a large hole to escape into right at the point of the most pressure the unit itself, some air will cheat to the bypass duct and system will not have proper return flow.
3)      By having the colder conditioned air directly to the return end of the system and restricting flow from the supply part of the system we now have risk of the system freezing up or icing up.  

The bypass duct systems have two methods for controlling the bypass air. The dampener for the bypass duct is controlled by either static pressure and or mechanical.

The mechanical is always preferred as it acts in concert with the supply dampeners and as the system calls for more or less air it adjusts it position mechanically. This can be tested and the flow balanced and set.

A static pressure controlled dampener  also known as a barometric controlled dampener works by pressure and balance. As the static pressure builds the dampener opens to the return for the bypass air such as during the operation of one zone.  As the system calls for more air the static pressure drops and so does the dampener closing the air to the return. The dampener needs to be completely level and the weight on the dampener must be adjusted perfectly and tested to ensure that the flow is correct. For proper operation the perfect level and balance must be maintained.

While used as a solution the barometric controlled dampener is difficult to install correctly and for system operation must really be perfect.

I would never recommend a system that combines the return air to the supply air for many reasons that are listed above and if one were installed I would recommend only a mechanical dampener be installed

In conclusion zoned systems can work quite well.  Zoning systems that are designed from the beginning are state of the art and can increase comfort and reduce cost by conditioning only the space you occupy. There are some key components that are lacking in this system to make it state of the art.

1)      A modulating system that can increase and decrease flow depending on demand.
2)      Mechanically dampened supply and return to isolate zones
3)      Separate returns for each zone for proper temperature control


  1. There is currently a central forced-air system.
    I’m considering having two zones, one for upstairs and one for downstairs. I don’t mean having two air con units, one for each floor. I’d really like three zones as one room upstairs gets extremely hot.
    Lets see what technicians suggest me.

    Lauren Bacall.

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  3. Zoned HVAC systems are an absolute must... I think you'd be pretty foolish to install anything else.

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