100,000 BTU Furnace at 0.9 in. w.c. Static and 74°F Rise
A 100,000 BTU/h, 80% AFUE single-stage upflow furnace sits in a closet with a louvered door. Its rating plate lists a 35–65°F temperature rise and 0.5 in. w.c. maximum external static. You measure 0.90 in. w.c. total external static, 3.5 in. w.c. manifold pressure, 66°F return air, 140°F supply air, and flue gas at 220 ppm CO with 10.5% O2. The filter is a new 16x25x1 pleat, the blower is on medium-low, and the burners show a steady blue flame with no rollout. The homeowner says the furnace has always been 'loud' and that it cycles on the limit switch on cold nights.
1. Calculate the airflow this furnace is actually delivering and judge it against the rating plate. What is the most likely cause?
Read the worked answer
Temperature rise = 140 − 66 = 74°F - outside the 35–65°F range and above the maximum, which is what trips the limit.
Airflow from the sensible-heat relationship, using output = 100,000 × 0.80 = 80,000 BTU/h:
- cfm = 80,000 ÷ (1.08 × 74) ≈ 1,000 cfm
A 100,000 BTU/h 80% furnace is normally set up for roughly 1,200–1,400 cfm, so the blower is delivering well under nominal while running at 0.90 in. w.c. against a 0.50 in. w.c. rating. On the fan curve, higher static means lower cfm - the measured airflow confirms it.
Most likely cause: excessive system resistance, not a bad blower. The closet installation, the small 1-inch pleat filter, an undersized or crushed return, or a tight evaporator coil are the usual contributors. Next step is to split the static: read supply and return separately and read the drop across the filter and coil. Whichever component shows a drop above its published value is the restriction.
Sequence: correct the duct/filter restriction, then move the blower tap only if the rise is still out of range, and recheck the rise after every change. Left as-is, the over-temperature condition shortens heat exchanger life and drives CO up.
2. Compute the air-free CO from the reading given, and state whether the unit can be left running while you correct the airflow.
Read the worked answer
Air-free CO corrects the measured value to zero excess air:
- Air-free CO = measured CO × [20.9 ÷ (20.9 − O2)]
- = 220 × [20.9 ÷ (20.9 − 10.5)] = 220 × 2.01 ≈ 442 ppm air-free
The commonly applied flue limit for this class of appliance is on the order of 400 ppm air-free, so this unit is at or above the threshold - not acceptable.
Judgment:
- The high O2 with elevated CO points to flame impingement/cooling from poor airflow across the heat exchanger, plus probably marginal combustion air in the closet. Fixing airflow and confirming the louvered door and combustion air openings are clear is the first corrective action.
- Do not leave the appliance operating while it is over the limit. Shut it down, tag it, and inform the homeowner in writing.
- After airflow correction, re-run combustion analysis with the burner compartment closed and the closet door in place. If air-free CO is still over the limit, or draft at the vent connector is marginal, the unit stays off until the heat exchanger and venting are inspected and the fault is repaired.
- Record before/after CO, O2, CO2, draft, and temperature rise on the invoice.
3. The installer proposes simply moving the blower to high speed and calling the job complete. Evaluate that plan.
Read the worked answer
It is a partial fix at best and may create new problems.
- Moving to high speed may pull the rise back into the 35–65°F band, but the system is already at 0.90 in. w.c. against a 0.50 in. w.c. rating. Higher speed on a restricted duct system typically raises static further, increases blower amp draw toward or past the motor's rated load, and increases noise and velocity in the branches.
- Duct velocity is the real constraint: if the trunk and returns cannot move 1,200 cfm at a reasonable static, the blower cannot manufacture the airflow. Fix the return and filter restriction first - a larger media filter or a bigger return grille often recovers more cfm than a speed change.
- Do not 'fix' the rise by derating the burners with drilled orifices or by lowering manifold pressure below the rating plate. That changes the input and voids the listing.
Acceptable plan: measure and correct the restriction, set the tap that produces the rated rise at the corrected static, verify manifold pressure at 3.5 in. w.c. with a manometer, and re-verify CO, draft, and rise with the closet closed.