Design Considerations

Planning and Installation

Designing a zone system for a new installation is slightly different than designing a zone system for a retrofit or existing structure. However, the guidelines are no more difficult to apply than those for existing duct layout and design. A little common sense and preparation will resolve most problems before they occur.

While zoning can offer considerable savings in energy and equipment function costs, the main goal in zoning a home or structure is to provide greater comfort to the home owner or occupants than is achieved through a single thermostat system. When designing a new system, the following considerations should be addressed.

What are the different areas of occupancy or usage? Establishing areas of load or occupancy allows the installer to focus on “ZONE” conditioning where people gather at different times of the day or night. For example: The living room, dining room, and kitchen are usually occupied during the day. The bedrooms are occupied in the evening. By establishing these distinct areas as two separate zones, this format not only allows for maximum comfort by matching areas of conditioning with structure occupancy, but it also establishes definite usage patterns effectively shutting down or lowering the demand for conditioning in areas of low occupancy resulting in energy savings. This format also maximizes the performance of setback thermostats.

Are there any areas that cause abnormal loads? Great rooms, glass walls, cathedral ceilings, hot tub enclosures, etc. - These features can put unusual strain on the comfort system. Be sure when creating your zones and sizing your equipment and duct work, you “have a handle on” the usage and loads created by such additions. Depending upon the application, it could be more beneficial to put in two smaller systems and zone them, instead of putting four or five zones on a larger system.


DuroZone offers two distinct types of zone control panels:

Relay Based
The JTZ and the SMZ 2, 3, and 4 systems use a primary thermostat for Zone One that has a sub base that designates the mode of operation (Heating or Cooling). This thermostat also controls constant fan operation. Zones 2 and higher require only a three-wire thermostat with R, W, and Y terminals. These panels feature mechanical relay operation and have switches so you can determine whether
a zone will or will not participate in constant fan mode. The SMZ is also available in a “first come, first served” auto-changeover design called SMZ-AC. The SMZ-AC is a three-zone system and is compatible with virtually any thermostat currently on the market.

Microprocessor Based
The ED3 and ED4 are microprocessor control panels which can be programmed by the installer for conventional heating / cooling, heat pump, dual fuel, geothermal, or hydronic systems. The panels are compatible with most low cost heat / cool thermostats as well as more expensive programmable and automatic changeover types. Other features of ED series panels are; an optional purge cycle, limiting second stage heating when only one zone is calling, fresh air intake, built in diagnostics, and much more.


“How do I size my Ductwork?”

This is probably the number one question asked of contractors when discussing zoning. There are no hard and fast rules regarding this. There is no magic formula that always works. What we can provide are some guidelines -“rules of thumb” and alert you to some common pitfalls.

The primary objective is to maintain constant airflow through the HVAC system when only one zone calls and still being able to provide sufficient airflow if all zones call.

On two and three zone systems, adequate airflow can be maintained by sizing the trunk line to each zone to be able to handle 60 to 70 % of the available cfm. If you then run five 6-inch takeoffs from these trunks, adequate airflow is maintained. Below is a simple chart to guide you for systems of 800 to 2000 cfm.

System CFM
Trunk Duct
Branch Duct
12 x 8 or 12” round
5 - 6” round
14 x 8 or 12” round
5 - 6” round
16 x 8 or 12” round
5 - 6” round
18 x 8 or 14” round
5 - 7” round
20 x 8 or 14” round
5 - 7” round
22 x 8 or 16” round
5 - 8 ” round

When designed this way, a by-pass damper may not be necessary but it never hurts to have one. We recommend installing one even if it is only in anticipation of future alterations and/or for balancing purposes.

On systems of four or more zones the 60% rule will not work. For systems of this configuration lay out your duct work as if it was not zoned. Now, increase each trunk to handle 20% more of your designed cfm. For example, if you determine that Zone One would require 500 cfm under normal conditions, install a trunk duct capable of 600 cfm. Repeat this for each zone.

On systems of four zones or more a by-pass damper is almost always required. The by-pass damper should be sized to “dump” the difference between the total available cfm and the smallest zone.

The biggest pitfall in designing four or more zones is not keeping all the zones approximately the same size. Try to avoid having one zone of 100 cfm and another of 600 cfm. Also try to keep all zones within 20% to 30% of each other. If this is not possible, install adequate by-pass or consider splitting the system into two smaller zoned systems.

What do I do with by-passed air?

The by-passed air can be ducted into non-critical temperature areas such as entryways, basements, recreation rooms, cathedral ceilings, etc. Do not by-pass this air into attics or crawl spaces as this may cause a negative pressure situation and/or condensation in the house.

Ideally the air should be ducted back into the HVAC system through the return air. If this procedure is followed, the air should be ducted into the return duct as far from the air handler as possible to allow adequate mixing of the airstreams. If space does not allow this, controls such as an anti-freeze-up control (FRP) should be installed to protect the equipment.






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