HVAC Zoning: How Zone Dampers Work and When Zoning Fits

What a zoned system is made of

Carrier describes a zoning system as one that divides a home into separate areas, each with its own thermostat. It says motorized dampers inside the ductwork direct air to specific areas. When a zone calls for cooling, the central unit starts and the dampers open or close to send air where it is needed.

The California Energy Commission lists what an inspector looks for in a damper-based system. There are motorized dampers on the supply ducts, either one or more large dampers near the supply plenum (the box where the main ducts start) or one damper for each outlet. There are multiple thermostats or temperature sensors in the area served by one system. And there is a control board near the air handler, with low-voltage wires running to the thermostats, the dampers and the main equipment board.

That control board is the zone control panel. It collects each zone's call and decides which dampers open. Two-zone systems are by far the most common, according to the Commission, and the most common use is a two-story home on one forced-air system. Why the upstairs runs hotter in the first place is covered in the two-story uneven cooling page, so this page starts where that one leaves off.

The airflow problem when only one small zone calls

The blower in an air handler is built to move a set amount of air. Static pressure is the resistance that air meets in the ducts. When dampers close and only part of the duct system stays open, the Commission says excess pressure builds at the air handler fan.

The Commission describes three ways to deal with it. One is to let the pressure rise, which substantially reduces the airflow across the cooling coil. A second is a bypass duct, which short-circuits supply air back to the return side and sends excessively cold air back into the system in cooling mode. A third is a dump zone, which sends the extra air into a part of the house, usually an unoccupied one, that then gets over-conditioned.

Equipment makers frame the same problem as a minimum-airflow rule. Carrier's design guide says each constructed zone should support the minimum airflow of the equipment selected. A maker of EcoNet zoning says the same in its installation instructions and adds that a system should not depend on over-conditioning or a bypass for large volumes of excess air. Another maker, EWC Controls, tells designers to assume the smallest zone may be the only one calling at any given time.

Too little air across the coil is also one of the two main causes of a frozen coil. Our reading: that is why a zoning design has to show how it protects the coil, a point covered in why an AC freezes up.

Bypass dampers, supply-air sensors and variable-speed equipment

A bypass damper is a motorized or weighted damper in a duct that joins the supply side to the return side. EWC Controls calls it the most common tool for managing airflow and static pressure in a zoned system. It advises routing the bypass air to the return duct or to a rarely used part of the house, and adding a hand damper in the bypass duct to restrict it.

The makers disagree on how much a bypass should do. EcoNet's instructions say there is a time and a place for a bypass but it is not unlimited, and suggest limiting it to no more than 15% of the smallest zone's maximum airflow. EWC Controls also suggests letting closed zone dampers leak 10 to 20% of their air volume, with smaller zones set lower. Carrier's design guide for its communicating system goes the other way and says a bypass must not be used with it.

EWC says a supply air sensor, a temperature sensor on the supply side, protects the coil from freeze-up and the heat exchanger from overheating. Carrier's communicating system instead relies on a variable-speed blower and modulating dampers, which are dampers that open part way. The Commission adds that bypass ducts, or large airflow cuts across the coil, can make a cooling system use more energy.

Variable-speed and multi-stage equipment gives the system room to turn down. Carrier's guide recommends equipment with more than one stage of capacity so the zoning system can stage the equipment down when fewer zones call. How those blowers and compressors work is covered in the variable-speed blower page and the compressor stages page.

Zoning, a second system or a mini-split

The Commission notes that zone control can also come from two separate systems, each sized for its own zone. They act independently, need no zone dampers, and need no bypass or other way to handle excess air. In its two-story example, one system with zone dampers or one small system per floor are both possible. It says that where a single large system can serve the house, the damper approach generally costs less.

A mini-split is a ductless system with one outdoor unit and one or more indoor units, covered in ductless mini-splits. Our reading: it is a third way to give one area its own control, because each indoor unit has its own controls and no ducts are shared. The tradeoff to weigh is that a room gets its own indoor unit rather than a damper on an existing duct.

Our reading on choosing:

1. Zoning on existing ducts fits when the ducts are sound and the equipment is the right size, and the problem is that one thermostat cannot serve areas with different loads.
2. A second system fits when the two areas are large and very different, or when the single system and ducts are already at their limit.
3. A mini-split fits when one area has no ducts or its ducts cannot be extended, such as an addition.

EWC Controls adds a precondition: a load calculation should confirm that the equipment and ductwork are sized correctly before a zoning system is installed. Sizing is explained in the Florida AC sizing guide.

How many zones, and which rooms go together

Carrier's planning guide says to group rooms with similar use, occupancy and heat load, and to make different levels separate zones. It adds that in a retrofit, the existing ductwork may limit the zone choices.

EWC Controls says not to create numerous small zones, and that two to four large zones work best. It also says to avoid dampers on branches serving bathrooms, laundry areas and foyers, which should keep constant airflow.

Our reading: a single small bedroom zone is the hardest case, because it is the zone that may call alone for long stretches and cannot take the full airflow the blower is built to move.

Tampa Bay and Florida notes

The Commission's text is a California rule, so it does not bind a Florida project. We cite it because it is a public description of what an inspector checks, and it states the same airflow concern the makers do.

Equipment changes in Florida usually go through the local building department, a process the Florida HVAC permits page walks through. Our reading: adding zoning to existing equipment is a change worth raising with the building department, since rules vary by city and county.

Our reading: because Florida cooling seasons are long and humid, ask how the design keeps the equipment running long enough to remove moisture without starving the coil of air. The answer should name the equipment's minimum airflow, not just say the system is zoned.

Six questions to ask before you agree to zoning

1. What is the equipment's minimum airflow, and can the smallest zone take it? Ask for the number from the maker's literature.
2. Is a bypass planned? If so, where does it return air, and how is the amount limited? If not, what protects the coil?
3. Is there a supply air sensor or an equivalent protection against coil freezing?
4. Is the equipment compatible with the panel? Carrier's and EcoNet's systems work only with their own compatible equipment.
5. Was a load calculation done, and are the ducts big enough for the airflow with some zones closed?
6. Why was a second system or a mini-split ruled out for this house?

Ask for the answers in writing, along with the maker's installation instructions for the panel. Leave the panel, damper wiring and equipment connections to a licensed contractor.