High-Altitude Furnace Running at Sea-Level Manifold Pressure
An 80% AFUE, single-stage induced-draft furnace with a 90,000 BTU/h input rating is installed at 5,400 ft elevation. The rating plate states the input applies from 0 to 2,000 ft and that above 2,000 ft the unit must be derated 4% per 1,000 ft, with a corresponding reduction in manifold pressure below the 3.5 in. w.c. shown on the plate. During a tune-up the technician records manifold pressure 3.5 in. w.c., flue CO 320 ppm as measured, flue O2 5.2%, flue temperature 410°F, and a steady yellow-tipped flame with light sooting on the burner carryover slots. The burner compartment door was in place and the sample was pulled 12 in. downstream of the flue collar. The heat exchanger was inspected last season and passed, and the customer reports no CO alarms but describes the flame as "lazy" and says the furnace smells hot on first start.
1. Is the measured CO acceptable? Convert the reading to air-free CO and state the pass/fail basis you would use.
Read the worked answer
The 320 ppm number is diluted by excess air, so it cannot be compared to a limit directly. Convert with:
CO air-free = CO measured × 20.9 ÷ (20.9 − O2)
= 320 × 20.9 ÷ (20.9 − 5.2) = 320 × 20.9 ÷ 15.7 = 320 × 1.331 ≈ 426 ppm air-free
That is above the 400 ppm air-free ceiling commonly applied to the undiluted flue products of a residential furnace, so the unit fails. The reason the conversion matters here is that a 5.2% O2 reading means a lot of dilution air is present; tolerating the as-measured 320 ppm would hide a combustion problem. Note also that 5.2% O2 is on the low side of the normal 5–9% band for a naturally aspirated furnace, which points to too little excess air for the altitude. Do not sign off on the tune-up with this number. Correct the firing rate and air first, then re-sample at the same port with the door on, at steady state, before deciding whether the appliance is acceptable.
2. Calculate the correct altitude-derated input and the manifold pressure the technician should be setting. What else must be verified before trusting either number?
Read the worked answer
Derate above 2,000 ft = 4% × (5,400 − 2,000) ÷ 1,000 = 4% × 3.4 = 13.6%.
Correct input = 90,000 × (1 − 0.136) = 90,000 × 0.864 = 77,760 BTU/h (roughly 77,800).
If the plate permits derating by pressure, manifold = 3.5 × 0.864 = ≈ 3.0 in. w.c.
Two cautions. First, many manufacturers require a change to smaller (high-altitude) orifices rather than a pressure reduction, and some require both; the rating plate and installation manual govern, not a rule of thumb. If orifices are specified, install them and keep the listed pressure instead of dialing the regulator down. Second, verify the result by clocking the meter: 77,760 ÷ 1,030 BTU/ft³ ≈ 75.5 ft³/h ≈ 1.26 ft³/min, so 2 ft³ of dial should take about 1.6 minutes with all other appliances off. A clocked input higher than the derated value means the unit is still overfired and explains the low O2 and yellow tips.
3. List, in order, the corrections and checks you would make to bring CO into range, and explain why that order matters.
Read the worked answer
Order matters because each step changes the combustion air/fuel ratio that the next test depends on.
- Fix the firing rate first - derate by the approved method (orifices and/or manifold pressure) and confirm by clocking the meter. Overfiring at altitude is the most likely single cause of both the low O2 and the elevated air-free CO.
- Verify combustion air and draft - confirm the induced-draft blower wheel is clean, the pressure switch is proving, the flue is clear and correctly sized, and the burner compartment has its specified openings. A restricted flue raises CO and flue temperature together.
- Clean and adjust the burners - soot on the carryover slots points to insufficient primary air or flame impingement. Clean the slots, check the primary air shutter position against the manufacturer's setting, and confirm the flame is blue with a stable, non-lifting pattern.
- Re-test at steady state with the door on: target air-free CO well under the 400 ppm ceiling and O2 within the plate's band.
- Re-check temperature rise after the firing-rate change, since a lower input changes the rise across the heat exchanger. Document every reading before and after; if air-free CO stays high after the firing rate and air are correct, treat the heat exchanger and flue as suspect and red-tag the appliance.