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Apprentice Q&A · #082

What does a "hash mark" with a number inside a duct run mean on a blueprint?

2nd YearBlueprint ReadingYELLOW · Check First

This is typically an indicator for a system duct segment identifier, a CFM indicator, or a cross-reference tag linking back to a specific detailed section view drawing sheet (e.g., Tag #4 refers to Detail View on Sheet M-502).

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Plain-English answer

This is typically an indicator for a system duct segment identifier, a CFM indicator, or a cross-reference tag linking back to a specific detailed section view drawing sheet (e.g., Tag #4 refers to Detail View on Sheet M-502).

Field checklist

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Hey boss, I’m checking blueprint reading: What does a "hash mark" with a number inside a duct run mean on a blueprint? I found the likely issue and want to verify the next step before I lock it in. Do you want me to adjust it now or check the drawing/detail first?

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Use this as training guidance. The foreman, approved drawings, project specs, manufacturer installation instructions, employer safety policy, and AHJ/code requirements always control the final answer.

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