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HVAC – Zoning and zone control

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HVAC – 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.

A professionally installed zone control system consists of a damper and with the best smart thermostat programs for each room or zone of a house or building, with these being wired into a central control panel that sequences each thermostat’s call with the zone dampers in the ducts. In some cases, a bypass damper or duct may be required to relieve excess air when smaller zones are the only ones calling.

While there are upfront costs for equipment, from an operational standpoint, zoning offers an efficient use of energy for comfort heating and cooling. In non-zoned structures with varied use patterns, a single point of control (thermostat) will result in non-used areas being conditioned, making them just as comfortable as spaces that are in active use. This, unfortunately, is not a good use of resources. Zoning provides decentralized control and allows a thermostat in each zone to set the demand for the zoned area, rather than subjecting all areas to the concept of “what’s good for one has to be good for everyone.”

Savings on energy bills are well documented when comparing zoned systems to single-zone buildings. But the savings picture does not end at the utility bill. Since the HVAC equipment is not being called on to constantly top up heating or cooling systems, it often shows a reduction in the number of cycles for the furnace and air conditioner, and reducing the cycles of any piece of equipment can extend its service life and reduce maintenance costs.

The basic rules of system design
The main consideration when planning out a zoning project is the need to maintain a constant amount of air flow (CFM) through the HVAC unit. Whether only one zone is
open, or if several zones are calling for heating or cooling, the airflow will have to be within the operating parameters of the equipment.

In new installations it is recommended to use the same size for each zone duct, and to size the duct for approximately two-thirds of the total HVAC system CFM since it must be able to handle the CFM of the HVAC unit. Keep in mind, however, that this is practical on systems with two or three zones where all zones are approximately equal in size. This is not practical in an installation where 80 per cent of the conditioned area is in one zone.

When the duct is sized for two-thirds of the total airflow, the air will reach a higher pressure and velocity; however, it does not typically increase beyond the static pressure rating of the blower motor. This also keeps the air velocity from being noticeably noisy.

Systems over five tons are typically commercial and would use a bypass damper to relieve the excess air pressure when the majority of zones shut down. This rule also applies for complex residential systems with more than four zones. Whatever air cannot be directed into the zone must then be bypassed.

The components of zoning
In a zoned system, motorized dampers open and close based on the demands of the particular thermostat controlling the zone calling for heating or cooling. These dampers can be inserted 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.

Thermostats used in the zoning systems can range from rather basic to complex. In fact, retrofits in an existing home can often incorporate the existing thermostat into the zoning project. As each zone is divided, an additional thermostat will be necessary 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.

In some instances, a separate bypass damper is installed to relieve any excess air from zones that are open but are too small to handle the full capacity of the blower. This air is typically bypassed into the return air duct or into a common area, such as a hallway. When air is bypassed into the return air duct, capacity controls for both heating and cooling are used to prevent overheating or overcooling in the unit.

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 be almost any number of zones depending upon the size of the building.

Offices and other commercial spaces
Zoning for commercial office buildings makes a lot of sense, as every person has their own idea of comfort, and it is rare that all areas of a facility will follow similar use schedules.

Conference rooms often go unused for hours, and individual offices may be empty when sales people are on the road, or when employees are on vacation. These can all be controlled individually with individual thermostats.

Commercial systems are basically just bigger residential systems with more capacity. Duct design for these will almost always include a bypass system, especially those over three zones. These systems tend to be easier to retrofit, however, as drop ceilings can often be used as a common return and are a great place to bypass the air.

Commercial zoning systems of three zones or more will need a bypass, and it is always important that the bypass damper be located as far away from the blower as possible. Barometric bypass dampers can be used on equipment up to 7.5 tons (3,000 CFM). Systems beyond 10 tons (4,000 CFM) should use a motorized bypass and a static pressure control.

Residential zoning
Homes are typically split into fewer zones than commercial spaces, with the simplest systems having only two zones, one incorporating the living room and kitchen, and the other covering off the bedrooms and bathrooms, but zoning options are limited only by the imagination and budget of the client.

New homes are the easiest to zone, since the system can be designed into the building plans. Working with existing homes 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. Where this is a problem, motorized registers or diffusers can be used to control the outlets.

Did you know?
The manual method of zoning by closing off register grilles in rooms often requires constant adjustment and can actually cause harm to the HVAC equipment, since closing off too many outlets can reduce the airflow in the system, severely shortening the life of the furnace, air conditioner or heat pump. A proper zoning system can automatically balance the temperatures and be designed to improve the overall comfort and efficiency of the HVAC System.

Richard Foster is the president of Zonefirst, a manufacturer of zoning products for the HVAC marketplace. He can be reached at rfoster@zonefirst.com.