The Scope That Missed the Makeup Air
You are writing a scope for a 40-unit mid-rise with individually vented gas furnaces in closets, a continuously running 80 CFM bathroom exhaust fan in each unit, and a central corridor served by a single rooftop makeup-air unit that has been disconnected for years. The building was tightened two years ago with new windows and door sweeps. Since then, tenants report backdrafting odors, condensation on closet walls, and headaches. A combustion safety test on three units shows spillage at the furnace draft hood within 60 seconds of fan operation. The owner wants a scope that "just fixes the ventilation" and has a budget cap that will not cover full unit-by-unit HVAC replacement.
1. Prioritize the scope. What must be done first, and what can wait, given the budget cap?
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Combustion safety comes first and is non-negotiable. Spillage within 60 seconds of fan operation means the units are depressurized enough to reverse the draft hood, which is a carbon monoxide risk. The immediate scope is to test every unit, not a sample, and to correct the pressure problem at the source.
The first-tier measures are: restore or replace the corridor makeup-air unit so the corridor is not a negative-pressure path into units; verify each unit's exhaust fan flow and replace oversized or continuously running fans with properly sized, intermittent or demand-controlled ventilation; and seal the closet pressure boundary only after confirming adequate combustion air. Where a furnace cannot be made safe by pressure correction, the scope must include a direct-vent or sealed-combustion replacement or a powered combustion-air solution, even if it consumes budget.
The second tier, deferrable if budget is short: closet insulation and air sealing, corridor envelope work, and fan upgrades beyond what safety requires. The owner's framing ("just fix the ventilation") is directionally right but incomplete; ventilation without pressure management can worsen spillage if the new system is unbalanced.
2. Write the acceptance criteria you would put in the scope so the contractor cannot declare the job done prematurely.
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Acceptance criteria should be measurable and tied to the failure mode. Proposed criteria:
- Worst-case depressurization test in each unit with all exhaust fans, dryers, and the corridor system operating: closet pressure relative to outdoors must not exceed the appliance manufacturer's limit, and in no case should the draft hood spill.
- Spillage test at the draft hood: no sustained spillage beyond 60 seconds under worst-case conditions, verified on every unit.
- Combustion analysis: CO in the flue gas within the appliance rating, and ambient CO in the conditioned space below the program threshold (commonly 9 ppm) during and after the test.
- Corridor pressure relative to units: the corridor must not be more negative than the units it serves; document the measured differential.
- Exhaust flow verification: measured CFM within 10% of design for each fan replaced or retained.
- Documentation: a unit-by-unit test log with before/after readings, not a summary statement.
These criteria make the contractor responsible for the pressure relationship, not just for installing equipment.