Gas furnace lives in a 3x6 closet off the hallway.

The failure is negative pressure in the closet causing combustion spillage due to the return air setup.

Here's what's happening:

1. Improper return air path: The contractor created a return grille in the closet door and used the furnace's open filter slot as the return air intake. This means the furnace is pulling return air directly from the small 3x6 closet space.

1. Closet depressurization: When the furnace blower runs, it's pulling air from the closet faster than it can be replaced through the door grille. This creates negative pressure in the closet.

1. Combustion air interference: The B-vent furnace needs adequate combustion air. The negative pressure in the closet is fighting the natural draft of the B-vent, causing combustion gases (including CO) to spill back into the closet instead of venting properly up the stack.

1. CO increase when door closes: Closing the door further restricts air exchange, worsening the negative pressure and increasing spillage.

Code violation: This violates IRC G2427.5 (combustion air requirements) and G2427.6 (venting of Type B vents). The furnace needs either:

• Direct connection to a properly sized return air duct from the hallway
• Adequate permanent openings for combustion air (minimum 2x10" openings within 12" of floor and ceiling)

Fix: Remove the return air connection from the closet, install a proper return air duct from the hallway to the furnace, and ensure adequate combustion air per code.

Failure: The closet is the return plenum, creating negative pressure that starves the 80% furnace of combustion air and backdrafts the B-vent.

Why CO spikes when the door closes: With the door open, the blower pulls return air from the hallway. When closed, the blower pulls only from the sealed closet. Since the furnace has no dedicated combustion air ducts (standard for 80% in conditioned space), it must pull combustion/dilution air from that same closet. The blower (typically 1,200+ CFM) vastly overpowers the natural draft of the B-vent (~100-200 CFM). The closet depressurizes to -0.05 to -0.15" w.c., reversing flow up the vent. Flue gases (200 ppm CO) spill into the closet, get sucked into the open filter slot, and recirculate through the heat exchanger—raising CO further.

Code Violations (IRC/IFGC):

1. M1602.2 / IFGC 304.1: Return air shall not be taken from a closet containing a fuel-burning appliance.
2. IFGC 304.5 / IRC G2407.5: 80% furnaces require 50 cu. ft. volume per 1,000 BTU/hr or dedicated combustion air ducts. A 3x6x8 closet = 144 cu. ft. A 100k BTU unit needs 5,000 cu. ft. Volume is grossly insufficient.
3. IFGC 503.10 / IRC G2427.10: Venting system must operate under positive/neutral pressure. Negative closet pressure violates the vent's listing.

The Fix (Requires Licensed HVAC Pro):

1. Seal the closet door grille permanently.
2. Install a ducted return pulling from the hallway/central space (min. 1 sq. in. per CFM, typically 14x20 or 16x25 grille + duct).
3. Provide combustion air: Two permanent openings (high/low) to attic/crawlspace (1 sq. in. per 4k BTU/hr each) OR direct-vent conversion kit OR replace with 90%+ sealed combustion unit.
4. Verify: Post-repair, clock meter, check temp rise, and re-analyze flue. CO must be <100 ppm air-free (AHRI Guideline X).

Do not run the furnace until fixed. This is an immediate life-safety hazard (CO poisoning).