HVAC Zones

Photo courtesy of Residential Control Systems, Inc.

HVAC Zone Design

HVAC zoning is very difficult to add to an existing HVAC system, so for the most part, zoning is a new construction application. Not many home automation firms take on the job of designing the actual HVAC system, the ductwork, or the location of the airflow registers, so leave this part of the job to the HVAC and mechanical engineering people.

Once the HVAC system is designed, the design of the HVAC zoning can be done using the following steps:

1. Establish the number of zones.   Typically, a two- (or more) story house has at least two zones, but the goal is to establish HVAC zones that are roughly the same size in area. The zones shouldn’t be too small because they will be difficult to control, or too large because they may be subject to cold or hot spots. A zone should be created for each separate climatic environment the homeowner desires, typically defined by the activities of the area. For example, a family room where the occupants may be more active can be slightly cooler than a living room where the occupants are less active.

2. Adjust the HVAC duct plan to fit the zone design.   Ductwork must be designed so that each zone is a separate branch of the main HVAC duct system. The damper for each zone will be installed just after a zone’s ducts branch from the main duct. The volume of the ductwork branches may need to be increased to handle an increased airflow volume. Work with the HVAC specialists to design the volume of the ductwork for a system that is open all the time. Each duct in the system should be able to handle the entire airflow volume of the HVAC system. This allows the system to function normally, even when a damper is closed, without putting too much back pressure on the HVAC system and possibly damaging it. The general rule-of-thumb for increasing the size of the ductwork is to add an additional 25 percent of volume for every two zones on the system. For example, a two-zone system should be increased 25 percent; a four-zone system should be increased 50 percent; and so on.

3. Establish the location of the dampers and thermostats.   During the pre-wire stage, run two-conductor thermostat wire to the damper locations and five to seven conductor thermostat wire to the thermostat locations. If you are using remote temperature sensors with the system, that wiring must be installed as well.

Zoning Hydronic Systems

Hydronic systems are generally zoned by default, but if you wish to be able to turn the heat on or off in different areas of a home, control valves must be installed in the hydronic piping in the same way that dampers are installed in the air ducts of a forced air system.

Hydronic valves (see Figure 29-8) are motorized to open and close according to the commands of the master zone controller. They open and close the flow of water through the hydronic system.

Figure 29-8: A hydronic motorized value

Photo courtesy of Invensys Building Systems, PLC.

Locating a Thermostat

For the best results, a thermostat should be located on an inside wall in an area that is frequently occupied. The thermostat should be at least 18 inches from any outside walls and at least 5 feet above the floor. It should also be placed in an area with freely circulating average temperature air. A thermostat should not be located in any of the following areas:

• On an outside wall

• In direct sunlight or near any heat produced by any closely placed appliances

• Near or in line with a heating or air conditioner vent, a stairwell, or an outside door

• Near any device that produces electrical interference

  Note

  Some thermostats are “power-stealing” units. This type of thermostat takes power from the “W” (see Table 29-1 later in the chapter) heat connection running from the HVAC system. If multiple zone control units are also being installed, don’t connect a power stealing unit or an older style mechanical or anticipator type thermostat to the controller. Neither of these thermostats will work properly with an HVAC control unit.

Wiring a Thermostat

The wiring used to connect a thermostat to the HVAC systems must be no less than 20-gauge wire, and for wire runs longer than 100 feet, 18-gauge wire is highly recommended. Thermostat wiring is different from other wire, such as Cat 3 or 5, audio/video, or speaker wiring. Thermostat wiring has two to seven conductors (see Figure 29-9) of solid core wiring with the color -coding for a seven-wire cable consisting of white, red, green, blue, yellow, brown, and orange. Most HVAC central unit controls, which the thermostat wire connects to, use from five to seven conductors. However, in new construction situations, installing seven-conductor thermostat wire adds to the future proofing of the house.

Figure 29-9: Examples of two- and five-conductor thermostat wiring

Standard HVAC Thermostat   Typically, the HVAC technicians install the wiring for a central-system thermostat at the time the HVAC systems are installed. The wiring for the thermostat normally is either two- or four-conductor wiring to support either 24V or 120V systems, depending on the needs of the HVAC system and the thermostat planned into the HVAC system. If the plan is for the home system technician to install a standard thermostat, you will need to follow the thermostat manufacturer’s wiring diagram. Figure 29-10 shows an example of a wiring diagram for a typical nonautomated, or noncommunicating, thermostat.

Figure 29-10: A sample wiring diagram for a standard thermostat

When connecting the thermostat wire to the thermostat terminals (see Figure 29-11), strip about ¼-inch of the insulation at the end of each conductor. On most standard thermostats, the terminals are marked with either stickers or embossed letters in the thermostat’s plastic that indicate which wire color should be connected to which terminal. Wrap the exposed end of the conductor wire clockwise around the terminal screw. The manufacturer’s wiring diagram and documentation should also have these details.

Figure 29-11: Installing the wiring on a standard thermostat device

Remote Controlled Communicating Thermostat   It is more likely that if the system installation plans for a home include the home technology technicians installing a thermostat, the thermostat is one that can be remotely controlled by some form of remote control unit and is able to communicate with a central home system control unit either with radio frequency (RF) or PLC signals.

The basic wiring for a remotely controlled thermostat is typically very similar to that of a standard thermostat. However, additional installation steps are usually required to accommodate its communications functions.

If the thermostat uses RF signals to communicate to a receiver module located near the HVAC equipment or a home system controller, follow the manufacturer’s guidelines for range and placement in a room. For example, most manufacturers recommend that an RF thermostat be place at least five feet from the floor and not behind a door or other large object in a room.

If the thermostat uses PLC signals to communicate to a home system control unit, at least two devices must be installed and possibly three. This includes one or more wall display units and a control module that serves as an interface between the display units and either the HVAC system (see Figure 29-12) or the home system control unit, and possibly a separate communications module for remote control access. Figure 29-13 shows the modules of a three-part PLC thermostat system.

Figure 29-12: An illustration of the general installation of a PLC thermostat

Figure 29-13: The components of a PLC remote control thermostat system, with the controller (left), thermostat (center), and the PLC module (right)