Common COMcheck Mistakes and How to Avoid ThemCOMcheck is a widely used software tool developed by the U.S. Department of Energy to demonstrate compliance with energy codes for commercial buildings. When used correctly, it streamlines permit approval and helps ensure buildings meet minimum energy-efficiency standards. However, many users—especially those new to energy-code compliance—make recurring mistakes that can delay approvals, cause rework, or result in noncompliant designs. This article covers the most common COMcheck mistakes, explains why they matter, and provides practical steps to avoid them.
1. Choosing the Wrong Code or Jurisdiction
Why it matters
- COMcheck supports multiple model codes (IECC, ASHRAE 90.1) and state or local amendments. Selecting the wrong code or jurisdiction leads to incorrect compliance paths and failed submittals.
Common mistakes
- Picking a default national code instead of the specific state or local code.
- Ignoring local amendments or using an older/incorrect code year.
- Confusing prescriptive compliance with trade-off (UA or EAp) options for a jurisdiction that only allows prescriptive paths.
How to avoid it
- Verify the required code year and any local amendments with the building department before starting.
- In COMcheck, explicitly select the correct code and jurisdiction from the drop-down menus.
- If the jurisdiction has unique rules (e.g., mandatory measures), confirm which compliance paths are accepted.
2. Inaccurate Building Type, Area, or Orientation Inputs
Why it matters
- COMcheck’s calculations depend on accurate geometry and building classifications. Incorrect inputs skew results for envelope, fenestration, lighting, and HVAC compliance.
Common mistakes
- Entering gross floor area instead of conditioned floor area.
- Mislabeling building use (e.g., office vs. retail) which affects internal loads and occupancy-based lighting allowances.
- Incorrect building orientation leading to wrong exterior wall and window solar exposure assumptions.
How to avoid it
- Use conditioned floor area (area served by heating/cooling systems) for inputs.
- Match building use to the closest category in the code and document any mixed-use areas separately.
- Verify building orientation with site plans; if uncertain, confirm with the project architect.
3. Mischaracterizing Envelope Components
Why it matters
- Compliance checks for walls, roofs, floors, windows, and doors depend on accurate U-factors, R-values, SHGC, and areas. Errors here often cause failures.
Common mistakes
- Using default or assumed U-values without matching to specified assemblies.
- Forgetting to include thermal breaks, insulation continuity, or framing fractions.
- Misreporting window types (fixed vs operable) or shading devices.
How to avoid it
- Use manufacturer data or detailed assembly calculations for U-factors and SHGCs.
- Account for framing, thermal bridging, and cavity vs. continuous insulation explicitly.
- Model glazing and shading exactly as planned (including overhangs, fins, or external shading).
4. Incorrect Lighting Inputs and Controls
Why it matters
- Lighting power densities (LPD), control strategies, and space-by-space inputs affect compliance strongly—especially with tightened lighting requirements in recent codes.
Common mistakes
- Providing whole-building LPDs when space-by-space method is required or more favorable.
- Omitting automatic daylight controls, occupancy sensors, or exterior lighting controls.
- Using outdated fixture efficacies or not accounting for integral controls.
How to avoid it
- Choose the space-by-space method when it gives better results and be precise with space definitions.
- Include all required controls and document them (daylight dimming, multi-level switching, occupancy sensors).
- Use manufacturer data for fixture wattages and ensure any lumen maintenance or LED drive reductions are justified.
5. Overlooking HVAC System Details and Efficiency Ratings
Why it matters
- HVAC performance (SEER, EER, HSPF, AFUE, fan power, economizers, etc.) drives the energy trade-offs in COMcheck. Missing or incorrect equipment performance causes large errors.
Common mistakes
- Entering nominal equipment efficiencies rather than installed, certified ratings.
- Forgetting to include system-level controls such as economizers, demand control ventilation, or energy recovery.
- Incorrectly modeling central plant systems or mixed HVAC types for multi-zone buildings.
How to avoid it
- Use certified equipment ratings from manufacturer cut sheets or AHRI/ETL listings.
- Include all system controls and sequencing, and be consistent with how ventilation is counted (outdoor air rates).
- Break mixed systems into separate COMcheck subsystems or use the approach your jurisdiction accepts.
6. Improper Use of Trade-Off (EA compliance) and Envelope UA Methods
Why it matters
- Trade-off (also called performance or EA compliance) lets users compensate less-efficient components with higher performance elsewhere. Incorrect trade-off entries or assumptions lead to invalid compliance margins.
Common mistakes
- Misapplying trade-offs across components that are not tradeable per the code or jurisdiction.
- Using the UA alternative without careful matching of assembly areas and boundary conditions.
- Failing to document or justify trade-offs with backup calculations.
How to avoid it
- Confirm which components are eligible for trade-offs under the selected code and jurisdiction.
- For UA methods, ensure area-weighted calculations and boundary definitions match actual construction.
- Keep clear documentation (schedules, area takeoffs, and supporting calculations) for reviewers.
7. Not Accounting for Exterior Lighting, Plug Loads, or Process Energy
Why it matters
- Some jurisdictions require or allow inclusion/exclusion of exterior lighting, plug loads, or process energy; miscounting these can create discrepancies during review.
Common mistakes
- Assuming exterior lighting is optional or included by default.
- Forgetting to include significant plug/process loads where the code requires them.
- Double-counting or excluding equipment that should be addressed in a different compliance path.
How to avoid it
- Clarify which non-building-system loads must be included with the local code official.
- Model exterior lighting separately and show control strategies.
- If processes are substantial, document how they are handled (excluded, modeled, or covered under other standards).
8. Poor Documentation and Missing Backup Files
Why it matters
- Plan reviewers expect a consistent, documented package. Missing backup calculations, equipment cut sheets, or marked-up plans cause delays.
Common mistakes
- Uploading only the COMcheck report without labeled plans, schedules, or manufacturer data.
- Failing to include notes explaining assumptions, especially for trade-offs or alternative compliance paths.
- Not keeping version control when designs change after an initial submittal.
How to avoid it
- Provide a compliance packet: COMcheck report, marked-up architectural plans (showing assemblies, orientations, and areas), HVAC schedules, and manufacturer data.
- Include a short cover letter or remarks section clarifying assumptions and code selections.
- Keep files versioned and re-run COMcheck after any design change.
9. Rounding Errors and Unit Mismatches
Why it matters
- Simple numeric mistakes—rounding, wrong units (ft vs m, Btu vs kWh), or typographical errors—can flip a pass to a fail.
Common mistakes
- Entering metric values into fields expecting imperial units or vice versa.
- Rounding areas or efficiencies too aggressively.
- Typo-ing decimal points in U-factor or wattage entries.
How to avoid it
- Verify units for each input field and be consistent with the project’s measurement system.
- Keep sufficient decimal precision for U-values, areas, and efficiencies.
- Double-check entries or have a second reviewer audit the data.
10. Failing to Re-run COMcheck After Design Changes
Why it matters
- Construction documents evolve. If COMcheck isn’t updated, the final built project may not meet the approved compliance demonstration.
Common mistakes
- Submitting a COMcheck report at permit stage and not updating it after glazing changes, HVAC swaps, or envelope revisions.
- Relying on initial margins of compliance without verifying the final design still meets them.
How to avoid it
- Re-run COMcheck whenever significant design changes occur: glazing sizes, insulation levels, HVAC equipment, lighting layouts.
- Keep the final signed/compliant COMcheck report with record drawings as part of project closeout.
Practical Checklist Before Submission
- Confirm code year and local amendments (select correct jurisdiction).
- Verify conditioned floor area and building orientation (use accurate geometry).
- Document all envelope assemblies with certified U-factors and SHGCs (no assumptions).
- Use space-by-space lighting inputs when appropriate and include required controls (daylight, occupancy sensors).
- Enter certified HVAC efficiencies and include system controls (economizers, ERV, demand control ventilation).
- Keep detailed backup: marked plans, manufacturer cut sheets, and explanation of trade-offs (complete compliance packet).
- Re-run COMcheck after any design change and version-control your reports (final report matches built conditions).
Final note
COMcheck is a powerful tool, but it’s only as reliable as the inputs and documentation behind it. Small errors—wrong code year, a mis-entered U-factor, or omitted controls—are common but avoidable. Treat the COMcheck submission as part of the construction documents: verify inputs, maintain clear backup files, and re-run the model whenever the design changes to ensure a smooth approval process.
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