Altitude Beyond Combustion

Why this matters

Most techs who move to a mountain town learn the burner derate in the first week, because the gas appliance manuals say so in bold. What nobody hands them is the list of everything else the thinner air changes: the airflow arithmetic that now reads low, the motor that runs hot at its nameplate load, the hydronic loop that flashes to steam at a temperature that would be fine at the coast, the refrigerant gauge that reads a couple of psi off a sea-level chart. None of those are faults, and every one of them can be mistaken for one. Burner derating is covered in its own card (see related: How Altitude Changes Combustion); this one covers the rest.

The one fact underneath everything

Air at altitude is less dense and at lower pressure. In the standard atmosphere, which is what equipment ratings and textbook constants assume:

Real weather moves both by a few percent day to day, and temperature changes density too (hot air is thinner at any elevation). For field arithmetic, the standard values at your elevation are close enough, and the error from ignoring altitude altogether is much larger than the error from ignoring weather.

Everything below follows from those two columns: less mass in every cubic foot of air, and less atmospheric pressure pushing on every open surface.

Fans move the same volume and less air

A fan or blower at a fixed speed moves about the same volume (cfm) at altitude, but each cubic foot carries less mass. By the fan laws at constant speed, the pressure the fan develops and the power it draws both scale with density. Three consequences:

• Duct static pressure reads lower for the same system, about 86 percent of the sea-level figure at 5,000 ft. A static reading that looks healthy against a sea-level design chart is not the same airflow condition the chart describes.
• Blower motor amps drop a little, which can be read as a lightly loaded or slipping motor when it is just thinner air.
• Heat carried per cfm drops, and that changes the standard airflow constants.

The sensible heat constant 1.08 in Btu/h = 1.08 x cfm x temperature difference assumes standard sea-level air at 0.075 lb per cubic foot. At 5,000 ft multiply it by the density ratio: 1.08 times 0.86 is about 0.93. The enthalpy constant 4.5 and the latent constant 0.68 scale the same way. Use the sea-level constant at altitude and every airflow you back out of a temperature rise is wrong by the same 14 percent.

A worked example: the airflow that was fine

A tech at a house at 5,000 ft is checking an air handler with a 10 kW electric heat strip, because the customer says the upstairs never warms up. The tech uses the temperature-rise method to estimate airflow. The blower has been set to deliver 1,000 cfm, which a later traverse confirms.

Measured power. With the heat strip running, the tech reads 240 V and 41.7 A at the heater circuit. That reading is taken on energized conductors: it is work for a qualified person under 29 CFR 1910.333 with NFPA 70E (current edition) work practices, a meter rated for the circuit, the clamp around one conductor, and no other part of the energized cabinet disturbed. 240 times 41.7 is 10,008 W, about 10.0 kW, which is 34,120 Btu/h at 3,412 Btu/h per kW.

Measured rise. Return air 68 F, supply air 104.6 F, a 36.6 F rise.

Backing out airflow with the sea-level constant. 34,120 divided by (1.08 times 36.6) is 34,120 divided by 39.5, about 863 cfm. That looks 14 percent short of the 1,000 cfm the blower was set for. A tech who stops here condemns the ductwork, re-checks the filter, or bumps the blower to a higher tap.

Backing it out with the altitude constant. 34,120 divided by (0.93 times 36.6) is 34,120 divided by 34.0, about 1,002 cfm. The airflow is right where it was set. The 863 was an artifact of the constant.

Checking it the other way. At sea level, 1,000 cfm across 34,120 Btu/h would produce 34,120 divided by 1,080, a 31.6 F rise. At 5,000 ft the same airflow and heat produce 34,120 divided by 930, about a 36.7 F rise, within a tenth of a degree of what the tech measured. The thinner air runs about 5 F hotter across the same strip, which matters if the equipment's listed temperature-rise range has a ceiling near the operating point: a rise inside the range at sea level can sit at its top edge at altitude.

Where the upstairs problem actually was. With total airflow ruled correct, the tech moved on to distribution and found an upstairs trunk damper half closed from a summer balancing. The altitude constant did not fix the complaint. It stopped the tech from fixing the wrong thing first.

Motors and electronics run hotter

Motors are cooled by air moving over them, and thinner air removes less heat. NEMA MG 1 defines usual service conditions for motors as an altitude not exceeding 3,300 feet (1,000 meters). Above that, a motor running at full nameplate load runs hotter than its rating assumed, unless the ambient is lower than rated, which at elevation it often is. Manufacturers publish derating or allowable-ambient tables for the cases that matter; a condenser fan or pump motor running near nameplate in a hot equipment room at 7,000 ft is one of them.

Variable-frequency drives, inverter boards and power supplies carry their own altitude limit in their instructions, commonly around 1,000 to 2,000 meters before a derate applies. Some switching and protective devices also state a maximum altitude, because thinner air insulates less and breaks an arc less readily. When a drive faults on overtemperature at altitude with clean filters and a moderate room, read the installation manual's altitude section before replacing it.

Water boils lower

Water boils at about 212 F at sea level and loses roughly 1 F for every 500 feet of elevation, so at 5,000 ft it boils near 203 F at atmospheric pressure. That changes several trades:

• Hydronics. A closed, pressurized loop is unaffected by the boiling point at the surface, because its pressure is set by the fill valve and expansion tank. An open or low-pressure system, the top of a tall loop where pressure is lowest, or the suction side of a pump can flash or cavitate at temperatures that would be safe lower down. The atmosphere contributes about 28 feet of head at 5,000 ft against about 34 at sea level, so a pump's available net positive suction head is roughly 6 feet less before anything else changes.
• Water heaters. The relief valve is a pressure-and-temperature device and does its job regardless of elevation, but the margin between a high tank setpoint and local boiling shrinks.
• Cleaning and disinfection. Hot-water extraction cannot hold liquid water at the wand above local boiling, so a sea-level temperature target cannot be read at the tool. A disinfection step that relies on boiling has to boil longer: CDC guidance is to boil drinking water for one minute, or three minutes above 6,500 feet.

Gauges read differently

Refrigerant saturation charts are in absolute pressure underneath, and a gauge reads pressure above the local atmosphere. At 5,000 ft the atmosphere is about 2.5 psi lower than at sea level, so the same refrigerant condition reads about 2.5 psig higher on the gauge. Read against a sea-level chart, the gauge overstates the saturation temperature slightly. At typical high-side pressures the error is around a degree and lost in the noise; at low suction pressures and in vacuum it is not.

• A compound gauge's inches-of-mercury vacuum scale is referenced to sea level and cannot reach 30 inches at altitude no matter how good the pump is.
• A micron gauge reads absolute pressure and is unaffected, which is one more reason evacuation is judged on microns.

The other lists that change

• Engines. Generators, mowers and pressure washers lose power with elevation. Standby generator sizing at altitude uses the manufacturer's derate table, and small carbureted engines may need high-altitude jetting.
• Pools. UV intensity rises with altitude (WHO puts it at roughly 10 percent per 1,000 meters), so unstabilized chlorine burns off faster, and evaporation is faster in the dry air common at elevation.
• Outdoor coils. Less mass flow through a condenser coil means less heat rejected at the same fan speed and higher head pressure. Capacity falls off without anything being broken. See related: Capacity Falls Off at High Altitude: Derate vs Fault Decision Tree.
• Equipment that ships or requires a high-altitude kit: gas furnaces, boilers, water heaters and dryers (burner orifices and pressure switches; see the combustion card), and some condensing furnaces whose draft pressure switches are changed for altitude.

How to know you accounted for it

• The job notes carry the site elevation, from a map or the jurisdiction, and the constants used.
• Any airflow derived from a temperature rise or a static pressure states whether it was altitude-corrected.
• Motors, drives and equipment with an altitude limit in their documents were checked against the site elevation before being blamed.

References

• See related: How Altitude Changes Combustion; The Site-Specific Cause: Water, Power, Altitude, Humidity
• NEMA MG 1, usual service conditions for motors
• CDC drinking water guidance on boiling water for emergency disinfection
• World Health Organization, Global Solar UV Index guidance on UV and altitude
• OSHA 29 CFR 1910.333 and NFPA 70E (current edition), energized electrical measurement practices