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Tech Set 3: 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.

A Install HVAC equipment and ducts in conditioned space
B Properly size and select the HVAC system
C Locate the outside condensing unit to ensure that the exterior walls or other features do not block airflow.
D Reserve space for ducts in the home design.
E Install trimmable open web trusses or floor structural components that leave room for duct installation within the floor assembly.
F Specify energy-efficient equipment that meets or exceeds ENERGY STAR qualifications.
G Provide controlled mechanical ventilation using energy-efficient methods.
H Install an ENERGY STAR qualified programmable thermostat
I Seal all ducts and plenum junctions with UL 181 rated foil tape, or apply mastic, the only sealant approved by the ACCA.
J Mask HVAC outlets during construction.
K Install a filter rated for the home's needs.

Goal of the HVAC Package for New Construction

The goal of this Tech Set is to decrease the cost of construction, while improving the maintenance and energy efficiency of housing by promoting the effective systems integration of the conditioned air delivery network into the structural framework.

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Tech Set Details

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 trimmable 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.

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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

  1. 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.
  2. 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.
  3. 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.

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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

  1. 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.
  2. 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.
  3. 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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Implementing the Tech Set

In this section, we examine in detail how to best implement the recommendations from this Tech Set.

Design Process

The IRC requires that the heating and cooling equipment be sized based on building loads calculated in accordance with ACCA Manual J® or other approved calculating methodologies. ACCA's Manual J is now published as Edition 8 (MJ8TM) and four Addenda to this Edition are currently available. Each Addendum covers design features of MJ8TM that can be undermined by making assumptions about the facts of construction. Using old editions of the Manual and failure to input actual design conditions could result in a design that is inefficient and uncomfortable-meaning unhappy customers and costly call backs.

Typically, Manual J calculations are performed by the HVAC subcontractor, and submitted with the filing for a mechanical permit while the house is under construction. However, the design process should include the HVAC subcontractor at the design stage. Larger ducts can be accommodated in the floor plan and energy performance details of the structure can be adequately communicated to the HVAC designer. This will ensure that the system is designed as efficiently as possible, and will likely decrease the time it takes to install the ducts and reduce the equipment size-both of which will lower costs.

Develop a User's Guide

We recommend that builders provide equipment specs and a User's Guide for homeowner that includes a regular maintenance schedule for equipment tune-up and efficiency calibration. This guide should also include an isometric of duct layout for the owner's efficient use of register grilles to balance airflow within the home.

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Code Issues

The trimmable open web trusses will require an ICC-ES evaluation report and local official acceptance. Most currently have both, but be sure before you install them.

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Mainstreaming these Recomendations

Although all of the technologies and techniques suggested by this Tech Set are allowed by code, barriers that limit the effective integration of the conditioned air delivery network into the structural framework exist. The following steps are recommended to encourage the building industry to readily adopt the recommendations from this Tech Set:

  • Develop a Design Guide

    A design guide for designers and builders with details like trayed ceilings that house return duct(s), decorative columns as supply ducts, minimized duct lengths, a centrally-located air handling unit, and other details that work to keep the system within the conditioned envelope needs to be developed.

  • Manufacturers

    Manufacturers should develop training programs that assist the technical integration of systems, controls, and installation of the HVAC equipment and ductwork. A definitive and thorough equipment-commissioning plan should be detailed and followed by the installer.

    Manufacturers should also develop details in a User's Guide that facilitates consumer comprehension of system features and energy conserving patterns. This will enhance the longevity of the HVAC equipment and lower utility bills which leads to satisfied customers.

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