Many geothermal and general contractors pay a subcontractor to perform Manual J calculations when its required. Accurate heating and cooling loads are absolutely critical to properly designing a geothermal system (See 4 Steps to Designing a Geothermal System) and according to Ryan Carda creator of Loop Links, its the number one more common mistake that geothermal designers make in the design process. If you’re new to geothermal, download the geothermal survival kit to get a better understanding of how the technology works.
In many states, it is now a requirement to perform a manual J calculation to pull a permit for any HVAC job. If you are a general contractor or geothermal installer it will pay to do your own Manual J calculations because so much of the job depends upon accurate loads. Continue reading to learn the 11 tips to creating the best Manual J calculations for your job.
We asked John Walsh, a Heatspring Instructor with 40 years of experience in the HVAC industry, to offer some tips and code guidance for contractors interested in learning more about doing their own heat loss calculations.
John’s 5 Tips to Perform a Manual J Calculation:
- Verify that the correct outdoor design temperatures are used for the heating and cooling load calculations, and that they are consistent with values in Table 1A of ACCA Manual J.
- Verify that the correct indoor design temperatures are used based on IECC Section 302.1 of the 2006 and 2009 IECC.
- Verify that the building geometry and glass area match what is shown on the plans and compliance documentation. Glazing orientation is important to verify for cooling load calculations but has no effect on heat loss calculations.
- Verify that the levels of efficiency shown in the load calculations are consistent with the energy code compliance documentation. Insulation R-values, glazing U-factor, and SHGC are important to confirm.
- Verify that the make, model number, and equipment size is specified on the plans or compliance.
John’s 6 Field Inspection tips:
- Verify the make and model numbers for the heating and cooling systems installed for the building, and compare that to system specified on the building plans or documentation.
- Verify that the system has the same output capacity, and the same level of efficiency as specified in the plans (If a different system is specified).
- Require the responsible party to verify that the systems installed comply with the IECC (If a larger system, or an additional system, is installed).
- Verify that the efficiency levels of insulation and windows (U-factors and SHGC) meet or exceed the levels that are called out on the plans or documentation. Levels that are too low can cause the system to be undersized. If cooling is installed, verify that the glazing area and orientation is installed per the approved building plans.
- Verify that the energy features of the house are installed by the manufacturer’s installation instructions.
- Verify that the refrigerant charge level was tested by the installer.
John’s review of Residential Heating and Cooling Load Calculation Requirements: Code Notes [2006 and 2009 IRC, 2006 and 2009 IECC ]
Mechanical systems in residential construction are commonly oversized which increases installation costs, wastes energy, and reduces comfort and moisture control. Properly sized equipment will last longer, provide greater comfort, reduce noise, and save homeowners money. [Editors note: this is especially true for geothermal heat pump systems] Yet builders and code officials are uncertain as to how to evaluate such calculations to make sure they meet the intent of the code and the sizing methodology approved in the Air Conditioning Contractors of America (ACCA)
The 2006 and 2009 IECC require sizing calculations be performed on every home–by referencing IRC Section M1401.3. Section M1401.3 requires heating and cooling systems be sized to ACCA Manual J – Eighth Addition or other approved heating and cooling load calculations. The ACCA sizing methodology has sufficient built-in safety factors to accommodate most conditioning needs. Therefore, it is important to follow all instructions in Manual J, use precise area measurements, and specific data.Heating and cooling loads can be determined using a whole-house approach, or by performing a room-by-room load calculation. The room-by-room approach provides the information needed to determine the number of cubic feet per minute (cfm) of conditioned air needed to satisfy the heating and cooling load for the room. This information can, then, be used to determine the duct size necessary to deliver heating and cooling for the space. [ Editors note: See the 4 steps to designing geothermal to see where these steps comes into play when designing a system ]
The IECC regulates the indoor design temperature for use in performing load calculations. The IECC specifies that the maximum heating indoor temperature shall be 72°F, and the minimum cooling temperature shall be 75°F. Table IA of ACCA Manual J requires that the outdoor winter and summer design temperatures be based on the 99 percent value for winter, and 1 percent value for summer. To select the appropriate system, based on the heating and cooling load calculations, IRC Section M1401.3 requires that ACCA Manual S be used to size equipment. Excessively oversized equipment causes short-cycling, and creates unnecessary stress on the equipment. Also, larger systems require larger duct sizes, increasing the installation cost. In areas where humidity is an issue, an oversized system will degrade the humidity control. A properly sized system will run almost continuously at design conditions, and provide the proper level of dehumidification during the cooling season.
- 2006 and 2009 IRC M1401.3 Sizing Heating and cooling equipment shall be sized based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies.
- 2006 and 2009 IECC 403.6 Equipment Sizing Heating and cooling equipment shall be sized in accordance with Section M1401.3 of the International Residential Code.
- 2006 and 2009 IECC Section 302.1 Interior design conditions. The interior design temperatures used for heating and cooling load calculations shall be a maximum of 72° (22°C) for heating and a minimum of 75°F (24°C) for cooling.
- 2009 IRC Section M1401.3 Sizing Heating and cooling equipment shall be sized in accordance with ACCA Manual S based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies.
- Copyright, 2009, International Code Council , Inc. Falls Church, Virginia. 2009 International Energy Conservation Code; 2009 International Residential Code. Reproduced with permission. All rights reserved.
- Copyright, 2006, International Code Council , Inc. Falls Church, Virginia. 2006 International Energy Conservation Code; 2006 International Residential Code. Reproduced with permission. All rights reserved
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