High Temperature Rise in a Gas Furnace
An 80% AFUE gas furnace, 100,000 BTU/hr input and 80,000 BTU/hr output, with a blower rated at 0.5 in. w.c. external static and a nameplate temperature rise range of 35°F to 65°F. The homeowner complains of a hot house and a dusty smell. Measured: return air 68°F, supply air 137°F, return static 0.38 in. w.c., supply static 0.48 in. w.c., pressure drop across a 1-inch pleated MERV 11 filter 0.28 in. w.c., pressure drop across a clean A-coil 0.24 in. w.c. Manifold pressure 3.4 in. w.c. natural gas, steady blue flames, flue draft verified, and clocking the meter confirms very close to 100,000 BTU/hr input. The blower is on the medium-low tap. The duct system has six 6-inch branch supplies and a single 14 x 20 in. return grille. No CO reading has been taken yet.
1. Is the furnace over-fired, under-aired, or both? Walk through how you separate those causes using the numbers given.
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Temperature rise is 137 − 68 = 69°F, above the 65°F nameplate maximum. That alone tells you airflow is too low relative to firing rate, but it does not say which side is wrong.
Check firing rate first. Meter clocking confirms about 100,000 BTU/hr input, matching the nameplate, and manifold pressure at 3.4 in. w.c. is slightly under the usual 3.5 in. w.c., so the furnace is not over-fired. The problem is air.
Quantify it. Airflow ≈ output ÷ (1.08 × ΔT) = 80,000 ÷ (1.08 × 69) ≈ 1,075 cfm. For a 50°F mid-range rise the same furnace needs about 80,000 ÷ (1.08 × 50) ≈ 1,480 cfm. That is roughly 27% low. Total external static is 0.38 + 0.48 = 0.86 in. w.c., well above the 0.5 in. w.c. rating. The filter alone accounts for 0.28 in. w.c., the coil 0.24, leaving 0.34 for the duct system. The single 14 x 20 return grille is small for roughly 1,500 cfm. Diagnosis: under-aired from a combination of restrictive filter, low blower tap, and undersized return.
2. What corrections would you sequence, and how do you verify each one?
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Start with the cheapest restriction: replace the 1-inch pleated MERV 11 with a low-pressure-drop filter or, better, a 4- to 5-inch media cabinet sized for the airflow, and measure the drop across it - target roughly 0.10–0.15 in. w.c. clean. Next, move the blower to a higher tap and compare actual static against the blower performance table at the required cfm; do not simply go to high without checking that the motor is rated for it. Then address the return: a 14 x 20 grille cannot support 1,500 cfm at a reasonable face velocity, so add return capacity or enlarge the return duct.
After each change, re-measure total external static and temperature rise. The job is done when the rise is inside 35–65°F (ideally mid-range), total static is at or below the blower rating, and airflow is near 400 cfm per ton of cooling if an A-coil is present. Before leaving, take a CO reading in the flue and check draft, because a furnace that has been running hot may have a stressed heat exchanger.
3. What findings would make you red-tag the furnace instead of adjusting airflow?
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Red-tag if you find a cracked or perforated heat exchanger, flame rollout, a failed draft hood or blocked flue, measurable CO in the supply air or spillage at the draft hood, or an over-fire condition that cannot be corrected. A furnace that has run at a 69°F rise may have a heat exchanger that has been thermally stressed, so inspect it visually and, where the design allows, with a combustion analyzer and a CO test in the supply plenum.
Also red-tag if airflow cannot be brought into range without major duct redesign and the rise remains above the nameplate maximum - continued operation overheats the heat exchanger and can crack it. Document the readings, shut off the gas to the appliance, tag it, and give the homeowner a written explanation. Airflow adjustments are appropriate only when the heat exchanger, venting, and combustion are proven sound.