High-Altitude Conversion on a Natural Gas Furnace
A residential natural gas furnace rated 80,000 BTU/h input at sea level is installed at 5,200 ft elevation. The installer reused the sea-level orifice and left manifold pressure at 3.5 in. w.c. After startup, the flame is long with yellow tips, and soot appears on the supply registers. Combustion analysis at steady state shows O2 3.4%, CO2 10.6%, CO 24 ppm, and stack temperature 465°F. Supply air is 122°F with 70°F return. The technician proposes raising manifold pressure to 4.2 in. w.c. to 'stabilize' the flame and restore heat output. The manufacturer's altitude kit lists a 4% input derate per 1,000 ft above sea level and a specific orifice for 5,000-6,000 ft. The customer reports the furnace short-cycles on the high limit on cold mornings.
1. Calculate the altitude-derated input and state whether the existing orifice and manifold pressure are appropriate. Show your work.
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The altitude derate is 4% per 1,000 ft, so at 5,200 ft the reduction is 5.2 x 4 = 20.8%. Target input = 80,000 x (1 - 0.208) = 80,000 x 0.792 = 63,360 BTU/h. The existing sea-level orifice is not appropriate because it is sized to pass sea-level gas input; at altitude the furnace must be re-orificed per the kit and the manifold pressure set to the kit's specified value. Raising pressure to 4.2 in. w.c. increases fuel flow and moves the unit further from the derated target. After installing the correct orifice, clock the meter to confirm roughly 63,000 BTU/h input, then verify manifold pressure, draft, and combustion. If the altitude kit is unavailable, do not leave the furnace overfired; contact the manufacturer or replace the appliance with a properly derated unit.
2. Explain what the O2, CO2, CO, and stack readings indicate, and why raising manifold pressure would be the wrong response.
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O2 at 3.4% is low and CO2 at 10.6% is high for natural gas, showing too little excess air for the fuel being burned. CO at 24 ppm and a 465°F stack temperature support overfiring and incomplete combustion; the yellow flame tips and soot are visible symptoms. Raising manifold pressure adds even more gas, lowers excess air further, and drives CO and soot higher. It can also worsen high-limit cycling because the heat input rises while airflow is unchanged. The correct direction is to reduce fuel input to the altitude rating, then re-test. A proper post-correction test should show O2 and CO2 within the manufacturer's range, CO below its limit, stable flame, acceptable stack temperature, and a temperature rise inside the rating plate range.
3. Outline the correction and verification steps, including what to do if the altitude kit is unavailable.
Read the worked answer
Shut off the gas, install the specified altitude orifice, and set manifold pressure exactly per the kit. Reassemble, leak-test, and start the furnace. Clock the meter to confirm input near 63,000 BTU/h, then measure steady-state combustion: O2, CO2, CO, and stack temperature. Verify draft and check for spillage or rollout. Measure return and supply temperatures and calculate temperature rise; confirm it is within the rating plate range and that the high limit no longer cycles. Check CO in the occupied space and at the registers. If the kit is unavailable, do not simply lower manifold pressure unless the manufacturer specifically permits that method; red-tag the overfired condition, arrange safe temporary heat, and obtain the correct conversion parts before returning the furnace to service.