Condensing Furnace Installation Review in a Crawlspace

The combustion numbers (8.8% CO2, 22 ppm CO, 105°F stack) are normal for a condensing furnace, so the appliance is burning cleanly. Every problem here is in the venting, condensate, and air supply:

• Slope away from the furnace. A condensing vent must drain back toward the appliance or to a listed low-point drain. Sloping toward the termination traps condensate in the pipe, which can block flue gas, trip the pressure switch intermittently, or freeze and crack at the termination.
• Termination beneath an operable window. A window 5 ft above and directly over the exhaust is a flue-gas re-entry path. Manufacturer clearances typically require several feet of separation from operable windows and doors, and the termination must respect grade and snow depth, not just sit 14 inches up.
• Exhaust/intake orientation. Fourteen inches apart may be within some manufacturer allowances, but the intake must be positioned below the exhaust and away from prevailing wind, or the flue gas gets recirculated into the burner air.
• Long horizontal condensate run with no trap. The furnace trap must be primed and the drain must not create a pressure path back into the flue. A 20-ft uninsulated run can freeze or lose its trap seal, causing flue gas to escape indoors.
• Equivalent length. Four elbows on the exhaust and three on the intake add substantial equivalent length. Many 2-inch tables cap out well before 18 ft plus four elbows; this must be checked against the manual and upsized if exceeded.
• Sealed crawlspace access. If combustion air was being drawn from the crawlspace, sealing it changes the air balance; if the crawlspace is truly vented, the condensate and vent routing still need freeze protection.

The most probable failure is the reversed vent slope plus the improper termination, acting together. The reversed slope gives condensate a place to collect; the termination under the window gives any spilled or re-entrained flue gas a direct route into the living space. In cold weather the collected condensate can also freeze at the termination and block the flue entirely.

Demonstration:

1. Slope check - level or measure the horizontal run and show the fall direction with a photograph.
2. Pressure switch and flue blockage test - run the furnace and watch pressure switch behavior and inducer current; a partially blocked flue shows up as pressure switch chatter or a lockout.
3. Re-entry test - with the furnace firing and the window open, sample CO at the window opening with a calibrated analyzer while a second person records outdoor baseline CO. A rise above baseline above the window shows re-entry.
4. Condensate observation - disconnect or view the low point after a run cycle and show standing water where the manufacturer requires drainage back to the furnace.

Any one of these results justifies shutting the appliance down and tagging it rather than waiting for a complaint.

Order matters because some corrections change others:

1. Shut down and tag until the vent and termination are corrected if flue gas re-entry or blockage is demonstrated.
2. Rework the vent layout. Recalculate equivalent length using the manufacturer's table with elbow allowances. If the 2-inch run exceeds the allowance, upsize to 3-inch or shorten the run and remove elbows.
3. Re-slope back to the furnace or install a listed low-point drain, and correct the termination so it is clear of the operable window, above grade and snow line, and oriented with the intake below the exhaust per the manual.
4. Rebuild the condensate system with the manufacturer's trap, a properly sized and insulated drain line, and freeze protection in the crawlspace.
5. Re-evaluate combustion air after the crawlspace access was sealed; restore the intended air path or document that the appliance is direct-vented and does not need it.
6. Re-test and document - combustion analysis, draft/pressure switch operation, a CO check at the window, and photographs of the corrected slope and termination, all compared to the original readings.