ZONED HVAC SYSTEMS

   ZONED HVAC SYSTEMS

Larger homes often use two or more separate heating and air conditioning units for different floors or areas. Multiple systems can maintain greater comfort throughout the house while saving energy by allowing different zones of the house to be at different temperatures. The greatest savings come when a unit serving an unoccupied zone can be turned off.

Rather than install two separate systems, HVAC contractors can provide automatic zoning systems that operate with one system. The ductwork in these systems typically has a series of thermostatically controlled dampers that regulate the flow of air to each zone. Although somewhat new in residential construction, thermostats, dampers, and controls for zoning large central systems have been used for years in commercial buildings.

If your heating and air conditioning subcontractors feel that installing two or three separate HVAC units is necessary, have them also estimate the cost of a single system with damper control over the ductwork. Such a system must be carefully designed to ensure that the blower is not damaged if dampers are closed to several supply ducts. In this situation, the blower still tries to deliver the same air flow as before, but now through only a few ducts. Back pressure created against the blades of the blower may cause damage to the motor. There are three primary design options:

1. Install a manufactured system that uses a dampered bypass duct connecting the supply plenum to the return ductwork. Installing the bypass damper is the typical approach. When only one zone is open, the bypass damper, which responds automatically to changes in pressure in the duct system, will open to allow some of the supply air to take a shortcut directly back to the return, thus decreasing the overall pressure in the ductwork (Figure 7-9).

2.Create two zones and oversize the ductwork so that when the damper to one zone is closed, the blower will not suffer damage. This approach is only recommended for two zones of approximately equal heating and cooling loads.

3.Use a variable speed HVAC system with a variable speed fan for the duct system. Because variable speed systems are usually more efficient than single-speed systems, they will further increase savings.

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