HVAC Zoning

Big common rooms with vaulted ceilings - sometimes stretching to two stories, a lot of windows, an open air feeling... These are features very much in demand in today's American home. But these features create special challenges when it comes to designing an energy-efficient air conditioning system that will keep your home comfortable year-round.

Central air conditioning has been a standard feature in the average newly-constructed American home since the 1960s. Before that time, window units were the norm. The typical central air conditioning system is controlled by a thermostat placed in a centrally located area, usually one of the home’s common rooms, like the living room or dining room. For the most part, non-custom homes were built with a "one thermostat handles everything" philosophy.

The problem with the "one-thermostat" approach is that with the advent of today’s most popular architectural features like vaulted ceilings, lofts, walls of windows and atrium areas, the heating and cooling needs in one area of the home (like an expansive living room with a lot of windows and a vaulted ceiling) can be markedly different from the needs in another, equally-important area, like a study or a children’s bedroom. If the single thermostat that controls the whole house is in the living room, it is almost a sure bet that in the summer, the study and children’s bedrooms will be over-cooled. The one-thermostat approach ensures that only the people in the area near the thermostat will have the desired comfort level.

Fortunately, air conditioning technology has created a solution to this problem — Zone Control. Zone Control is the division of a home into separate areas, or zones, that can be independently heated or cooled, based upon the demand in that zone. The beauty of Zone Control is that a single heating and cooling system is divided, by means of its ductwork, to serve multiple zones with an individual thermostat control in each zone.

Components of a Zone Control system

A standard Zone Control system is comprised of the HVAC equipment, a control panel, ductwork, dampers that are built into the ductwork (usually one damper for each zone, but multiple dampers may be used), and thermostats.

The way the system works is that thermostats in each zone are wired into a zone control panel. The control panel is, in turn, wired to the HVAC equipment and to zone dampers located in the system’s ductwork. The thermostats tell the control panel which zones need conditioning. The control panel then opens or closes the dampers in the specified areas, controlling the flow of heated or cooled air to those areas.

The Aprilaire Zone Control system we featured in our story allows for the creation of up to four heating or cooling zones in the home.

In addition to increasing comfort throughout the home, zoning can lead to significant savings on your home’s energy bill. For example, if you live in a two-story home, with bedrooms on the second floor, you may want to only heat the upstairs bedrooms on a cold winter night. With zoning, this is possible.

If there’s an area of the house that remains unoccupied for long periods of time, a zone control system allows the homeowner to save money by not heating or cooling that area when nobody is there.

The "Pressure Relief" Issue

Additional pressure on the system can be created when the full capacity of the heating and cooling equipment is directed to less than all zones. As dampers close to restrict airflow from non-calling areas, the equipment will attempt to deliver its full capacity, even though only a percentage of it is really required. Some method of pressure relief is required in order to avoid problems like air noise and freezing of the coil. Common solutions are oversizing the ductwork (in which all zones are capable of carrying 70% of the system’s airflow) and using a bypass damper.

Aprilaire has developed a proprietary "Controlled Pressure Relief"® method. This method allows a specified amount of "bypass" air to be delivered into a closed zone, an amount that will not affect the zone’s temperature by more than one degree.