Tech Set 3: HVAC: Forced Air System

This Tech Set shows you how to improve the energy efficiency, durability, and performance of the forced air system through proper design and installation procedures.

The key is to bring the mechanical contractor into the house's design process to ensure both the easy installation of ductwork in conditioned space and the proper sizing and design of the HVAC equipment and duct placement.

1. Design ducts in conditioned space using Manual D 2. Properly size equipment using ACCA's Manual J and Manual S 3. Locate condensing unit for optimum airflow 4. Incorporate ducts into the home design and use dedicated chases 5. Use trim-able open web floor trusses 6. Specify ENERGY STAR qualified equipment

7. Specify energy-efficient controlled mechanical ventilation 8. Install an ENERGY STAR qualified programmable thermostat 9. Seal plenum and ducts with UL 181 approved sealant 10. Mask HVAC outlets during construction 11. Install a filter rated for occupant's needs

HVAC Design

The key to improving your HVAC is a shift in the design process of your home. By bringing the mechanical contractor into the house's design process (or doing the Manual D yourself) you can design a more easily installed and energy-efficient HVAC system.

Steps

1. Install HVAC equipment and ducts in conditioned space. Design ducts to ensure proper air distribution according to Air Conditioning Contractors Association's (ACCA) Manual D, rather than often inaccurate and inefficient rules of thumb. Properly sized ducts can reduce initial costs and operating costs. 2. Properly size and select the HVAC system using ACCA's Manual J and S - the industry standard for residential load calculation required by most building codes around the country - to ensure the system achieves maximum efficiency through properly sized equipment. Oversized air conditioners are not only more expensive to purchase and operate, but also do not remove as much water from the air as a properly sized system. Also, place the plenum in a central location to minimize duct lengths - this lowers material and operation costs.

3. Locate the outside condensing unit to ensure that the exterior walls or other features do not block airflow. Also, keep the unit out of direct sunlight.

4. Reserve space for ducts in the home design. Dedicate chases within the floor plan for efficient, conditioned air supply, rather than installing ducts around the structural frame. This shortens the duct runs and speeds duct installation. Include air returns for each room (except bathrooms) or install transfer grilles between rooms to promote adequate airflow.

5. Install trim-able open web trusses or floor structural components that leave room for duct installation within the floor assembly. This allows you to run ductwork and other utilities without cutting structural members. HVAC Specifications

Specifying ENERGY STAR qualified equipment will further increase the energy-efficiency of your HVAC by roughly 20%. In properly air sealed homes (where the natural ACH is less than 0.35) it is also important to install controlled mechanical ventilation.

Steps

6. Specify energy-efficient equipment that meets or exceeds ENERGY STAR qualifications. ENERGY STAR qualified central air conditioners are at least 25% more efficient than other models, and ENERGY STAR qualified heat pumps are 20% more efficient than their counterparts. 7. Provide controlled mechanical ventilation using energy-efficient methods. These can range from adding a simple and inexpensive exhaust-only fan to installing more expensive and efficient heat recovery ventilation equipment, which transfers a significant amount of energy from the exhaust air to the supply air. Examine a case study that compares three controlled ventilation systems in otherwise identical homes. Read the final report (PDF Format). 8. Install an ENERGY STAR qualified programmable thermostat. Programmable thermostats automatically adjust the temperature of your home when you are asleep or away, which can save you roughly $100 per year.

HVAC Installation

HVAC contractors may miss important steps if they are rushed through a job or rely on rules of thumb. For this reason, it is important for builders to personally check to make sure that the plenum and ductwork are properly protected during construction and thoroughly sealed using code approved products.

Steps

9. Seal all ducts and plenum junctions with UL 181 rated foil tape, or apply mastic, the only sealant approved by the ACCA. Improperly sealed ducts are all too common, so personally inspect them to ensure that the actual ducts are sealed properly (and not just the duct insulation). This is especially important if the ducts are in unconditioned attics or crawlspaces.

10. Mask HVAC outlets during construction. Sealing off the duct supplies and returns during construction keeps dust, dirt, and construction debris from entering the duct run. This is especially important if the supply is in the floor. It is also important to vacuum ducts, boots, and grilles before turning on the central system because large amounts of dust or any debris can damage the mechanical equipment.

11. Install a filter rated for the home's needs. A proper filter keeps unhealthy particulates out of the air. To determine which filters are appropriate, follow the mechanical equipment manufacturer's instructions.

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