What is Zoning and Zone Control?
Zoning and Zone Control of Heating, Ventilating and Air Conditioning (HVAC) Systems are two ways of saying individual temperature control from one central HVAC System. Typically with almost all forced air systems there is only one thermostat to control the heating and cooling. Once that thermostat calls there is virtually no way to control the temperature in each room of the house except manually closing off the outlets in each room. This manual method is time consuming and can cause harm to the HVAC Unit as closing off too many outlets can reduce the airflow in the HVAC severely shortening the life of the furnace, air conditioner or heat pump.
True zoning is a professionally installed zone control system consisting of a damper and thermostat for each room or zone of the house and wired into a central control panel that sequences each thermostat’s call with the zone dampers in the ducts and the HVAC Unit. In some instances a by-pass damper or duct maybe required to relieve excess air when smaller zone(s) are the only ones calling.
Zoning can simply be related to having a light switch in every room of the house. You wouldn’t install just one light switch to turn on and off all of the lights in the house….Would you? The same is true for heating and cooling. One thermostat turning on the heating or cooling for the entire house, when you only need a little in one room or zone is extremely wasteful. Also, why heat or cool rooms that may already be comfortable and air is needed in other portions of the house.
WHY DO YOU NEED ZONING?
If you are only living in or occupying one room or zone of your house, why heat or cool the entire house? Why you need Zoning is for the many benefits zoning provides.
ENHANCED COMFORT – One thermostat centrally located in a hallway, dining room or even the living room cannot properly control the temperature in any other area than the area nearest the thermostat. If you have a multi-level home it is very hard to control the temperature on the second floor when the thermostat is on the first floor.
There are many factors that affect the indoor temperature in your home. Outdoor conditions such as solar gain, wind chill, shading; building design such as large glass areas, cathedral ceilings, multi- levels, below grade rooms, sprawling ranch designs; and internal factors such as fireplaces, heat from lights, cooking, appliances, etc., all affect the temperature throughout the home.
Take for example the kitchen, the most widely variable temperature room in the house. In winter with cooking not much heating is needed, however in the summertime a lot of cooling is needed. Also rooms with fireplaces, and if there thermostat is located here, while a fire is burning the rest of the home can be very cold.
Zoning solves these problems by allowing de-centralized control and allowing each zones thermostat to make the demand instead of what’s good for one has to be good for everyone.
Zoning improves the overall comfort by allowing the zone thermostats to react to changes in temperature in each zone. This prevents over heating or cooling some rooms while others rooms are under heated or cooled. Zoning provides perfect comfort, where and when you want.
ENERGY EFFICIENCY – A zoning system delivers increased energy efficiency on any HVAC System. Zoning allows you to set back thermostats in zones not being used and prevents zones from be over heated or cooled while other zones are not yet comfortable.
However now add zoning and think of how often that zone is being used and how much energy can be saved by turning off the heating and cooling to those zone when they are not being used. Or how much energy you’re wasting because they are too hot or cold while others are yet to be comfortable. Zoning savings have been achieved by many users of zoning systems. Actual results by homeowners have reported as much as 20% to 30% savings. These results in tests can be greater when combined with automatic setback thermostats achieving multiple setbacks per day.
Savings of zoning systems over single zone thermostat systems are well documented and even show a reduction in the number of cycles of the furnace and air conditioner. Reducing the cycles of any piece of equipment can extend its service life.
Zoning is not a new concept. Other forms of zoning have been used with hydronic heating systems for over 50 years, using zone valves or circulating pumps as their form of damper. In large commercial HVAC systems VAV (Variable Air Volume) systems are a more sophisticated form of zoning. Both hydronic and VAV zoning have proven similar 20% to 30% savings over single zone systems.
CONVENIENCE – A zoning system allows you to set the temperature in the room or zone you’re in and not have to go to another area of the home to change the temperature. If you’re in the bedroom at night ready to go to bed, you can adjust the temperature you want right there without having to go to the living room or some other area. You can then rest assured that you are only conditioning your bedroom zone and not all other areas of the home. You also don’t have to guess at what temperature it has to be in the living room in order for you to be comfortable in the bedroom or go around closing off outlets to make sure more air gets pumped into the bedroom.
HOW DOES ZONING WORK?
Zoning is a simple product and concept. As you have no doubt realized by now that zoning provides the ability to only condition those rooms that need heating or cooling and does not allow conditioned air into those zones not requiring it.
Zoning does this through a series of components. The first being motorized dampers that open and close based on the demands of the zone thermostats. These dampers insert into the ducts or can be installed at the air outlet for each room or zone. Multiple dampers can be controlled together for a single zone if multiple ducts serve a single room or zone. Manufactures a series of damper controls to control every type of duct and outlet.
The next key components are the zone thermostats. Systems use any standard heating and cooling thermostat. In existing homes the existing thermostat can used as a zone thermostat. Zoning Systems are compatible with all standard 4 wire and heat pump thermostats. As each zone is divided, each zone uses a thermostat to control the heating, cooling and fan operation for its individual zone.
The zone thermostats and dampers are wired into a central control panel. This panel requires a separate 24 Volt transformer to power the panel, dampers and thermostats. The panel then also connects to the thermostat connections on the HVAC Unit. Instead of using one central thermostat, the MasterZone control panel allows the unit to be controlled by multiple thermostats.
As each thermostat calls, be it for heating or cooling, the panel takes the first call from any zone. If it is heating it will keep open the damper to the calling zone, close the dampers to satisfied zones not calling for heating, activates the furnace or heat pump and begin supplying air only to that zone. If during this call other zones call for heating those zone dampers would open and heated air would be supplied to those zones as well. Once all heating calls are satisfied the panel will shut off the furnace or heat pump. The panel will enter a purge mode to direct the excess heated air to the last zone(s) calling. Once this purge mode is over the panel will see what other calls exist. During that time other heating or cooling calls can be made and the panel with then enter the next calls, operate the appropriate equipment and cycle the dampers open to only those zones calling and close the dampers to the zones that are satisfied.
In some instances a separate by-pass damper is installed to relieve any excess air from zones that are open and maybe too small to handle the full capacity of the blower. This air is typically by-pass into the return air duct or into a common area such as a hallway. When air is by-passed into the return air duct capacity controls for both the heating and cooling are also used to prevent overheating or overcooling in the unit.
HOW DO I ZONE A SYSTEM?
When zoning any system one must look at a practical cost effective number of zones for the home or office building. Most homes are typically two to four zones. Offices can almost be any number of zones depending upon the size of the building.
Most commonly a home is at least 2 zones, those being the living room, kitchen on one zone and the bedrooms, bathrooms on the second zone. Many other ways of zoning are by levels here each floor is a zone, or by occupancy and use or by exposures. As the home gets larger the need for zoning increases and typically many newer homes may have a zone for the formal living room and dining room, a second for the back kitchen and family room, and a third for the upstairs bedrooms. Possible others would be to split the master and guest bedroom and the kid’s bedrooms. Another maybe even still be the basement recreation room. The possibilities are many and it all comes down to the comfort level and convenience one want to achieve, keeping in mind the cost associated with adding each zone. Any homeowner looking for a new home or looking to upgrade the existing HVAC should at minimum consider at least two zones between the living zone and bedroom zones.
Zoning any system requires a little thought and sometimes imagination and creativity. Every new home has the ability to be zoned, since zoning can be designed into the construction. Existing homes adding zoning can take some creativity and imagination in order to adjust to the existing ductwork. Depending upon the duct layout in-line dampers may not be able to be used and motorized registers or diffusers can be used to control the outlets. However one must always consider the cost of going with several motorized registers or diffusers versus the possibility of modifying the ductwork in order to use one in-line damper. Wring is the other obstacle, especially in retrofit systems. However with the coming of wireless thermostats this makes installing zoning that much easier.
ZONING SYSTEM DESIGN
Zoning any forced air system is easy once you know a few of the basic rules. The main consideration is to maintain a constant amount of air flow (CFM) through the HVAC Unit. This needs to occur when only one zone is open and if the zones are of varying size, when the smallest zone is open. The other consideration is not to oversize a duct system too much in order to maintain adequate velocity and airflow when all zones are open and may be calling for conditioning on those design temperature days.
The design of the duct system for today’s zoning is an important factor to a comfortable and efficient zoning system. The number of zones, along with their size, often determine the best type of design.
There are scenarios for zoning. The first, which is typically on new installations of where the duct work can be designed for zoning would be to oversize the ducts for each zone in order to get more air to the zone when it may be the only one calling. The scenario for all others would be to use a by-pass damper to relieve the excess air pressure in the duct system when a minority number of zones are calling.
In new installations where ducts are being added it is recommended to size each zone duct the same and to size the duct for approximately 2/3 of the total HVAC System CFM. This is practical on systems with 2 or 3 zones and when all zones are approximately equal in size. This is NOT practical in an installation where 80% of conditioned area is one zone and 20% is the other zone. The reason for each zone duct being the same size is that any zone could be the only zone calling and therefore that zone must handle the CFM of the HVAC Unit. When the duct is sized for 2/3 of the total CFM the smaller size does restrict the airflow and forces the air at a higher pressure and velocity, however it does not increase the air typically over that static pressure rating of the blower motor, usually 0.5”W.C. This also keeps the air velocity from being noticeably noisy. Below is a quick guide to determine the minimum equivalent size of a zone duct for each size HVAC Unit.
System CFM 800 CFM 1,000 CFM 1,200 CFM 1,400 CFM 1,600 CFM 2,000 CFM
Zone Duct 12”x8”/12”Ø 14”x8”/12”Ø 16”x8”/14”Ø 18”x8”/14”Ø 20”x8”/16” Ø 22”x8”/18”Ø
Branch Ducts 5-6” Rounds 5-6” Rounds 6-6” Rounds 5-7” Rounds 5-7” Rounds 5-8” Rounds
Systems over 5 Tons typically are commercial and would use a by- pass damper to relieve the excess air pressure when the majority of zones shut down. In retrofit and systems with 4 zones or more, over sizing the ducts is not practical. In these instances a by-pass damper is used to relieve the excess air back into the return air duct or dump the air into a central area of the building, such as a hallway, where often there is a common return. In this instance try to locate the by-pass air as far away from the return air intake as possible. The key to a good zoning system is to deliver the conditioned air to the calling zone as fast and quietly as possible in order to satisfy the demand. Whatever air cannot be directed into the zone must then by by-passed. This develops the formula for calculating the size of the by-pass damper.
Total CFM – Smallest Zone CFM = By-Pass CFM Once the amount of by-pass air is known it is just common sense to size a duct adequately to handle the amount of air. A choice of both round and rectangular/square by-pass damper sizes are available.
When by-passing air into the return air duct it is imperative that capacity controls such as the AFC or Leaving Air TM Sensor be used in order to protect the HVAC equipment from freezing, overheating or pressure limits on the compressor.
MINIMUM POSITION DAMPER
Another form of by-pass is using minimum position dampers, such as the ZD and RDS dampers. Setting the larger zone dampers to a minimum position can also be a method of relieving excess air pressure. This should be used when only small amounts of air need to be by-passed as the minimum position air in some cases can cause a zone to overshoot its comfort zone. Caution must be exercised when using minimum position dampers for by-pass.
ZONING EFFICIENCY and Downsizing
The standard way of not living in your whole house all of the time and mostly occupying one zone of the home at a time proves the need for zoning. Zoning makes the use of the heating and cooling more effective by only conditioning those zones being occupied or that may need it. Therefore typically the majority of the time, even on a two zone system only one zone is typically calling.
When this occurs the furnace and air conditioner is oversized in BTU capacity when supplying only a single zone. Due to this many HVAC Installers will downsize the heating and/or cooling units based upon the use of zoning. On a two zone system this many not be recommended, however when using 3 or more zones it is practical to downsize to the next lower capacity unit.
An HVAC Unit should be sized to heat and cool the home at design temperatures (the hottest days in summer and the coldest days in winter). Realistically, how often do design conditions occur? Of course this depends upon where you live. In the milder climates downsizing is much more of a possibility than in the severe heating climates of Northern states or the southern cooling climates.
HVAC installers have been quite successful in going to the next smaller size unit when using zoning. In many cases heating and cooling units are often oversized, especially on older homes. Doing a heat loss and heat gain calculation is very important in determining the size of the heating and cooling unit. Once the loads are determined for the overall structure, the affect that zoning will have on the overall load can be determined. Seldom used zones such as basements or extra bedroom loads can be looked at as not always needing conditioning at the same time as more often used zones. In instances where 3 or more zones are used may be advantageous to downsize to the next smaller size of heating and cooling unit.
NOTE: There are times during extended periods at design temperatures where will take longer for the zones to satisfy as the heating or cooling. It is important to note that some zones would have to be adjusted in order to direct more BTUs to the more important zones.
Downsizing while also increasing efficiency is often the best way to go. When considering the alternative of using 2 separate lower efficiency units in order to zone vs. one high efficiency unit with zone damper system, the cost difference is negligible if not sometimes less. Applying higher efficiency and zoning to cooling can also increase the effectiveness of the cooling. Here again if utility rebates are offered for the higher efficiency this can further offset the added cost of zoning and possibly add other comfort options.
Typically the more zones you have the wider the diversity in the use of the zones. This factor can help in downsizing the unit. Take for example an exercise room that may only be used an hour or two a day. The family recreation room that is only used for a few hours in the evening and not when all are sleeping in the bedrooms or eating in the kitchen dining room zones.
Zoning and high efficiency equipment can increase the overall energy performance of your home and keep rising energy costs down to manageable level.
Barometric Bypass Damper
A question that comes up frequently when quoting zone control systems is, why and when do I need a bypass damper? As the individual zone dampers close, the system static pressure will tend to rise. In order to maintain constant airflow through the HVAC system, a barometric damper can be used to bypass some of the discharge air back to the system. The bypass damper should be installed in such a way as to connect the supply air duct to the return duct. Damper adjustment is done by moving the weight up and down the arm. It should be set to bypass just enough air to maintain a quiet, draft free system. Barometric bypass dampers can be used on systems with static pressure ratings up to 0.75”W.C. Bypass dampers are sized based on the cooling tonnage.
Condensing Unit Damper Size
2.5 tons or less 8” bypass
3 tons 10” bypass
4 tons 12” bypass
5 tons 12” bypass
7.5 tons 14” bypass
10 tons 14” bypass
For systems larger than 10 tons, subtract the CFM of the smallest zone from the total system CFM, then size the bypass damper to handle 100% of this difference at 2,500FPM.