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Are HVAC Design, Air Flow Analysis, Duct Modification, and Commissioning Part of Your Home Performance Process?

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As home performance practitioners we can affect the house. We touch every part of the thermal enclosure and can verify that the thermal features of the home are installed in a manner that will allow them to operate at their rated efficiencies. We even use the blower door to verify building tightness.  As HVAC home performance practitioners, we can use what we learn about the thermal features of home to design a properly sized heating and cooling system and if possible, a properly sized distribution system.  We can also verify the performance of the system we design by properly commissioning the system after installation.  What are the elements of a good residential HVAC design and commissioning?

HVAC Design Methodology and Software

The HVAC design should be done use ACCA Manual Methodology. It should consist of ACCA Manual J Eighth Addition version 2 load calculation, ACCA Manual S equipment sizing match up and ACCA Manual D duct design. The ASHRAE 62.2 standard should be used to design ventilation for the home and it should be factored into the load calculation at the appropriate place. The load calculation should be done using an ACCA approved calculating software. Due to the mathematical complexities of doing a proper load calculation it should not be attempted using long hand none software methods. ACCA provides speed sheets that can be used alongside their Manuals to do the Manual S match up and Manual D duct design. These are not designed to take the place of software which allows for accuracy and consistency in the design process. From ACCA’s website- “ACCA Speed-Sheets are Excel spreadsheets you can use in conjunction with certain ACCA technical manuals to assist in learning these important system design concepts. Speed-Sheets must be used in conjunction with the printed manuals on which each is based and are not a replacement for full-featured ACCA-approved software.”  The two most popular approved design software are Wrightsoft (my favorite) and Elite.  There are a few others list on ACCA’s website.

The Thermal Components of the Building in the Load Calculation

For the past year or so I have been contracted to do “HVAC Design Review” for a major green building verification company. Although the loads I review are 99% new construction, the lessons learned are applicable across the board. Mis-represented building components can drastically alter a load calculation.  A load calculation is essentially an energy model of the home.  The thermal inputs can greatly sway the load. It Is very important that we accurately as possible input all the buildings thermal components as described by Glenn Hourahan ACCA’s Senior Vice President, Research & Technology , in his article last week.  Building infiltration has as big or bigger impact on the load calculation as any single thermal component. You can see the impact of using actual blower numbers vs ACCA defaults of Tight-Loose for infiltration load values in this case study that I did and wrote about a few years back.  In new construction these inputs are taken from plans and specs. I verify that what was used in the load matches what was used in the builder provided plans and spec’s.  As home performance contractors we verify these elements during our audit process or planned thermal upgrades. Unless it is broken piece of equipment it is important that we install equipment that matches the load after ALL planned thermal upgrades have been installed.

Matching the Designed System to Existing Ductwork

Once we have adjusted our design for the actual thermal elements of the home and used the ACCA Manual S selection process to select the proper equipment it is time to evaluate the duct system. The existing equipment is likely oversized or will be once we make home performance related thermal upgrades to the home. If a complete duct change out designed using ACCA Manual D  to match the newly selected heating and cooling plant is out of the question, some duct modifications may still be in order. This may include the installation of volume dampers and/or the addition of more return air. If we sealed the existing ductwork, we want to keep track of before and after static profiles.

Commissioning the System

Next year Energy Star and RESNET will introduce standards that will allow Energy Star Raters to grade the installation of HVAC systems installed in programs using Energy Star and HERS Ratings.  If a system install grades high enough points will be added to the HERS score. They have found that even in Energy Star programs where the system is being designed to criteria based on ACCA and ASHRAE standards and verified by Raters, an average of 41% still have installation defects related to improper airflow and incorrect refrigerant charge.

As HVAC home performance contractors, we are doing homeowners a great disservice if we install a system and walk away from it.  Especially if the homeowner has paid us to improve the efficiency and comfort of their home!

Part of the Energy Star commissioning verification is verifying TOTAL duct leakage is below 8 or 12 cfm/100sqft depending on the number of returns. As home performance contactors this should have been part of our audit process and if needed home performance work scope.

Once the new HVAC system is installed, we should:

  • Verify all local and whole house ventilation rates meet the ASHRAE 62.2 standard
  • Set the fan to the design airflow based on our design static pressure, using the manufacturers fan chart. Measure external static pressure to verify fan airflow.
  • Measure room to room air flow and verify it is within +/- 10% of Manual J design airflow
  • Verify blower fan watt draw is with in manufacturer specification
  • Set refrigeration charge to manufacturer specification

This information should be logged on a startup sheet and added to the system file

By including HVAC design, air flow analysis, duct modification and system commissioning as part of our home performance process we are guaranteed to deliver on our promise of comfort and efficiency. Without them we do homeowners a great disservice and greatly detract from any home performance measures we recommend or install.

Jeremy Begley

Posted In: Building Performance

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