Furnace altitude derate calculator

How the derated input is calculated

A gas furnace is rated at the input on its rating plate for installations up to 2,000 ft. Above that, the manufacturer's installation instructions take a fixed percent off the input for each 1,000 ft of elevation, counted from sea level. This page applies the rule you pick to the rating plate input, then works out the output at an AFUE and the seconds a gas meter test dial takes per revolution at the derated input, so a clocked meter can be compared with the target. The percent is the manufacturer's and the page never guesses it from a model name.

Input reduction

reduction % = rule % per 1,000 ft x elevation / 1,000 (only above 2,000 ft, otherwise 0)NFPA 54 section 11.1.2 says the input rating of appliances above 2,000 ft is reduced by one of three methods: 4 percent for each 1,000 ft above sea level, what the authority having jurisdiction permits, or the manufacturer's instructions. Carrier's 820SA text gives the same 4 percent rule and then sets the input by its Table 9. The 2,000 ft is the threshold and sea level is the base. At 5,000 ft the 4 percent rule takes 20 percent off. Counting from 2,000 ft would give 12 percent.

Derated input

derated input = rating plate input x (100 - reduction %) / 100100,000 BTU/h with 20 percent off is 80,000 BTU/h, the figure in the ACHR News distributor example. In the Carrier 59SC5A instructions the rating plate input is the rate for installations up to 2,000 ft.

Output

output = derated input x AFUEAt 80 AFUE the 80,000 BTU/h derated input gives 64,000 BTU/h, and at 95 AFUE it gives 76,000 BTU/h, both as the ACHR News example prints them. AFUE is a seasonal figure, so the result is an estimate. Use the output printed at the derated input when the spec sheet gives one.

Carrier Table 19 (2 percent rule)

multiplier = (100 - 2 x elevation / 1,000) / 100Carrier prints one multiplier for each 1,000 ft band, based on the midpoint altitude of the band. The 2 percent rule at 2,500 ft gives 0.95 and at 9,500 ft gives 0.81, the first and last multipliers in the table. This page uses the elevation you type, so at 4,900 ft it shows 0.902 beside the table's 0.91.

Meter seconds per revolution

seconds = 3600 x dial size x heating value / derated inputAt 80,000 BTU/h and 1,000 BTU per cubic foot, the 1 cu ft dial takes 45.0 seconds per revolution and the 2 cu ft dial takes 90.0 seconds. Carrier asks for an input within plus or minus 2 percent of the rating plate input, or as adjusted for altitude, which is 44.2 to 45.9 seconds on the 1 cu ft dial. Clock the meter with the other gas appliances off.

Rule choices

4 percent, 2 percent, other percent, or no derate up to a listed elevationNFPA 54, Carrier's 820SA, the Coleman manual and the Rheem high altitude kit instructions give 4 percent per 1,000 ft, and the 820SA turns it into a Table 9 by band. The Carrier 59SC5A and 59SC2E give 2 percent in the US, and ACHR News gives 2 percent for the Rheem R97V and R98V. ACHR News also reports that Goodman furnaces can be installed to 7,000 ft in the US without modification, which is the listed elevation choice. For any other furnace, pick the other percent and type it from the manual.

Worked examples from published furnace data

The default case is the distributor example in ACHR News: 100,000 BTU/h at 5,000 ft on the 4 percent rule. That takes 20 percent off, which leaves 80,000 BTU/h. At 80 AFUE the output is about 64,000 BTU/h, and at 95 AFUE about 76,000 BTU/h, as the article prints them. The Rheem R97V rule of 2 percent takes 10 percent off at 5,000 ft. On a 1 cu ft dial at 1,000 BTU per cubic foot, 80,000 BTU/h is 45.0 seconds per revolution, and the Carrier plus or minus 2 percent window is 44.2 to 45.9 seconds.

Carrier's Table 19 gives one multiplier per band, based on the midpoint altitude of the band. The table below puts the printed values next to the 2 percent rule at the middle of each band. All 9 agree, which is how the formula was checked. Between midpoints the two differ. At 4,900 ft the table prints 0.91 for the 4,001 to 5,000 ft band and the formula gives 0.902.

Eight cases below run through the same functions the form uses, all on a 100,000 BTU/h plate with the heating value at 1,000 BTU per cubic foot. The 5,280 ft rows show the two rules side by side, 21.12 percent off on the 4 percent rule and 10.56 percent on the 2 percent rule, which leaves 78,880 and 89,440 BTU/h. Carrier's table puts that elevation in the 5,001 to 6,000 ft band at 0.89, or 89,000 BTU/h. The 1,800 ft row takes nothing off, and the last two rows land on the table's first and last multipliers above 2,000 ft.

Standards and data sources

• NFPA 54 section 11.1.2, High Altitude, as reprinted in the Uniform Mechanical Code 2018 Appendix B 101.2 - input ratings apply up to 2,000 ft. Above that they are reduced by one of three methods: 4 percent for each 1,000 ft above sea level, as the authority having jurisdiction permits, or by the manufacturer's installation instructions. Section 11.1.1 prohibits firing above the nameplate rating, so the meter window on this page stops at the rating plate input.
• Carrier 820SA/821SA gas furnace, Installation, Start-Up, Operating and Service and Maintenance Instructions (II820SA-02) - page 25: the input rating for altitudes above 2,000 ft must be reduced by 4 percent for each 1,000 ft above sea level, and the input on the rating plate is multiplied by the de-rate multiplier in Table 9, from 0.90 at 2,001 to 3,000 ft to 0.62 at 9,001 to 10,000 ft. The result shows that multiplier with the 4 percent rule.
• Carrier 59SC5A condensing gas furnace, Installation, Start-up, Operating and Service and Maintenance Instructions (59SC5A-13SI) - the 2 percent rule for the US above 2,000 ft (page 62), the 5 percent rule for 2,000 ft to 4,500 ft in Canada, the advice to get the in-season gas heating value before any adjustment, and Table 19 on page 65. Tests reproduce its 9 multipliers, from 1.00 to 0.81, with the 2 percent rule at the midpoint of each band. The 59SC2E (59SC2E-03SI) prints the same table as Table 24.
• Coleman / Unitary Products gas furnace manual (Sears parts document L0611386) - the high altitude installation note: for elevation above 2,000 ft, derate the furnace input 4 percent for each 1,000 ft above sea level, done by reducing the orifice size. Its derating chart cites NFPA 54, ANSI Z223.1 and the National Fuel Gas Code.
• Rheem RXGY-F04 high altitude kit instructions - elevations above 2,000 ft require the furnace to be de-rated 4 percent per thousand feet, factory orifices are sized for a sea level natural gas heating value of 1,075 BTU per cubic foot, regional reduced heating values may remove the need to change orifices except at extreme altitudes, and a pressure switch change is required at 5,000 ft for the models listed. The orifice examples cite Table F.4 of the National Fuel Gas Code Handbook, 2002.
• ACHR News, Joanna R. Turpin, November 27, 2017, Furnaces must be modified for high altitude applications - the Rheem R97V and R98V rule of 2 percent per 1,000 ft, with 10 percent less input at 5,000 ft, Goodman furnaces installed up to 7,000 ft in the US without modification, and a distributor example of 100,000 BTU/h at 5,000 ft that gives about 80,000, 64,000 and 76,000 BTU/h. Tests reproduce those figures.

Limitations of this calculator

• Your furnace decides the rule. The page does not know the model, and the installation instructions for the exact model set the percent, the threshold and the method. Make the choice on the form from the manual.
• Orifice drill size and manifold pressure are not computed. They come from the manufacturer's tables for the model, the gas heating value and the specific gravity of the gas. Carrier says its manifold pressure tables already adjust for both altitude and heating value, with no further derate on top.
• A step sits at 2,000 ft. Just above it, the 4 percent rule gives about 8 percent off and the 2 percent rule about 4 percent, which matches the first band of Carrier's Table 19. Nothing in the sources ramps the reduction in from 2,000 ft. Carrier's 820SA manual also prints a multiplier table by band that can differ from the straight 4 percent rule by a point or two either way, and the table governs where a manual prints it.
• Natural gas only. Propane has its own orifice and pressure tables, and the heating value range on the form is for natural gas.
• Canada follows different rules. Carrier prints a flat 5 percent for 2,000 ft to 4,500 ft there, and the Coleman manual lowers the manifold pressure over that range. Use the manufacturer's Canadian instructions and the installation code for a job there.
• NFPA 54 section 11.1.2 was read as the Uniform Mechanical Code Appendix B reprints it, not in an NFPA edition. It allows three methods above 2,000 ft: 4 percent per 1,000 ft above sea level, what the authority having jurisdiction permits, or the manufacturer's installation instructions. Check the edition your jurisdiction adopted.
• Elevation means the site's own. A house on a slope can sit hundreds of feet above or below the elevation a weather report lists for the nearest airport, and the reduction changes with each 1,000 ft.
• Two-stage and modulating furnaces have more than one input rate. Enter the high fire input from the rating plate, and read the manufacturer's instructions for how the lower rates are set.
• High altitude kits and pressure switches are not selected. The Rheem RXGY-F04 instructions require a pressure switch change at 5,000 ft for the models they list, and other makes differ.
• This is a planning estimate, not an installation procedure. A licensed contractor, the manufacturer's instructions and the authority having jurisdiction decide the installation, and the page makes no claim of code compliance.

Common altitude derate mistakes

1. Counting the 1,000 ft steps from 2,000 ft. Both Carrier and Coleman count from sea level once the site is above 2,000 ft. At 5,000 ft the 4 percent rule takes 20 percent off, which leaves 80,000 BTU/h from a 100,000 BTU/h plate. Counting from 2,000 ft takes 12 percent off and leaves 88,000 BTU/h, 10 percent too much input.
2. Using the wrong percent for the furnace. The 4 percent and 2 percent rules are both in print, for different furnaces. At 5,000 ft, 100,000 BTU/h becomes 80,000 BTU/h on the 4 percent rule and 90,000 BTU/h on the 2 percent rule. Applying the 2 percent rule to a 4 percent furnace leaves 12.5 percent too much input. Read the manual for the model.
3. Taking a second derate from a manifold pressure table. Carrier says its manifold pressure tables are as measured at altitude and cover both altitude and heating value, and not to apply an additional derate factor to them. Derate once, by the method the manual gives. A derated input worked out here is a target to check, not a figure to subtract again.
4. Changing orifices before asking about the heating value. Carrier says to get the expected in-season gas heating value from the gas supplier before any adjustment for capacity or altitude, because the heating content at altitude may already provide the reduction. Rheem sizes its factory orifices for a sea level heating value of 1,075 BTU per cubic foot and says regional reduced heating values may remove the need to change them except at extreme altitudes.
5. Clocking against the sea level input. A derated furnace that fires at its sea level rating clocks fast. For a 100,000 BTU/h plate the sea level target is 36.0 seconds per revolution on a 1 cu ft dial at 1,000 BTU per cubic foot, while the 20 percent derated target is 45.0 seconds. A meter that shows 36.0 seconds means the furnace is firing 25 percent above its derated input.
6. Reading Table 19 as exact at every elevation. Carrier's multipliers are based on the midpoint altitude of each 1,000 ft band. At 4,900 ft the 4,001 to 5,000 ft band prints 0.91, while the 2 percent rule at 4,900 ft gives 0.902. The table is the figure to follow for a furnace that prints it. Use the formula to see how far the table sits from your elevation.

Common questions about furnace altitude derate

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