80% Furnace With a 74°F Rise and Elevated CO
An 80% AFUE, 100,000 BTU/hr induced-draft furnace with a PSC blower serves a ranch house. Nameplate: manifold pressure 3.5 in. w.c., temperature rise 35–65°F. The homeowner says the house "never gets warm" and that the previous company "cranked up the gas." Steady-state readings: supply gas pressure 7.2 in. w.c., manifold pressure 4.6 in. w.c., return air 68°F, supply air 142°F, pressure drop across a brand-new 1-inch MERV 13 filter 0.58 in. w.c., total external static pressure 0.94 in. w.c., flue O2 7.0%, flue CO as-measured 180 ppm, draft −0.02 in. w.c. The blower is on the medium-low tap.
1. Compute the temperature rise and the air-free CO. What does each number tell you, and what is the significance of the two together?
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Temperature rise = 142°F − 68°F = 74°F, against a nameplate range of 35–65°F. That is above the maximum, which is a listing violation, not just an efficiency complaint - an over-temperature rise can crack a heat exchanger and is a leading cause of CO production.
Air-free CO = measured CO × 20.9 ÷ (20.9 − O2) = 180 × 20.9 ÷ 13.9 = ≈271 ppm air-free. The correction matters because a dilution-air reading hides the true concentration; on a natural-gas appliance, roughly under 100 ppm air-free is a clean burn, 100–400 ppm warrants investigation and correction, and above 400 ppm is a shutdown. At 271 ppm this unit is producing CO at a level that must be corrected before you leave.
A temperature rise can only be driven by two things: too much heat input, or too little air across the exchanger. Here both are present - manifold pressure is 4.6 in. w.c. against a 3.5 spec (roughly 15% overfired, since flow varies with the square root of pressure), and the static pressure is far above the blower's rating. The high rise and the CO are symptoms of the same two defects.
2. The filter is new. Explain the relationship between filter pressure drop, the blower curve, and temperature rise, and give your airflow remedies in order of cost and effort.
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A PSC blower delivers less air as external static pressure rises - that is the shape of its performance table, not a fault. At 0.94 in. w.c. TESP the blower is well off its rated point, so CFM falls and the same BTU/hr is absorbed by fewer pounds of air: ΔT = input ÷ (1.08 × CFM), so low CFM means high rise.
The filter alone is eating 0.58 in. w.c. A 1-inch pleated MERV 13 is the classic offender - it is doing the job it was sold to do, but through a filter slot that was never designed for it.
Remedies in order:
- Swap to a low-pressure-drop filter of the same MERV for now, or better, install a 4–5 inch media cabinet with a much larger face area; expect the drop to fall to roughly 0.10–0.20 in. w.c.
- Confirm the blower tap. Medium-low on this chassis is likely below the 400 CFM/ton the system needs; move to a tap that matches the cooling airflow (typically medium-high/high) and re-check.
- Verify the evaporator coil is clean - it was not measured separately here and a dirty coil hides behind a good filter.
- Then look at the duct system (supply and return static) if TESP is still above 0.5 in. w.c.
Correct the gas pressure as well: set manifold to 3.5 in. w.c. after the airflow fix, then re-run combustion analysis and the temperature rise together. One cannot be verified without the other.
3. The homeowner says "just leave it, we're selling the house." What do you do, and what goes in writing?
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A 74°F rise on a 65°F-max appliance plus 271 ppm air-free CO is an unsafe condition; a real-estate transaction does not change that.
Actions:
- Shut the appliance down at the gas valve and red-tag/disconnect per your company policy and the manufacturer's instructions. Do not leave it operating because the owner asked.
- Verify a CO alarm is present and working on each level and near sleeping areas; if not, say so in writing.
- Verbally explain the two measured defects (overfired manifold, excessive static pressure) and that the risk is heat-exchanger failure and CO migration into the living space.
- Leave a written tag on the unit plus an invoice/work order that records the actual readings: manifold pressure, TESP, filter drop, ΔT, O2, CO as-measured and air-free, and the customer's refusal to authorize repair. Have the refusal acknowledged and signed if possible.
- Offer a return visit after the repair to re-run combustion analysis and temperature rise.
The written record protects the occupants, and it protects you - an undocumented verbal warning is worth nothing if the house is sold and something happens.