The Effect of Altitude on a Furnace

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Homes constructed at an altitude greater than 2,500 feet need to accommodate the lower air density for heating. Thinner air -- or air that's less dense -- has less heat-carrying capacity and requires different furnace equipment for efficiency. Heating units in Denver, for example, need adjustments that aren't necessary for furnaces and peripheral equipment in New Orleans -- and that’s not just because of the warmer Louisiana climate. It’s because New Orleans is at sea level and Denver is “mile high.”

Heating Equipment

  • A furnace's heating capacity depends on the density and oxygen content of the air, along with the pressure drop across the metered area. Altitude affects all of these components. Thin air carries less heat, so you may lose about 4 percent furnace capacity for every 1,000 feet of altitude, which works out to 20 percent loss at 5,000 feet. This is an approximation, used for estimating equipment needs. Whether you choose liquid propane (LP) gas, natural gas or an electric furnace, adjusting and revising your equipment for high altitude is essential. The size of your furnace only partly depends on the size of the area you want to heat. The altitude affects the furnace size as well.

What Altitude Affects

  • In addition to airflow calculations, fans and ductwork sizes change with the higher altitude. Pumps, coolers, motors and electronic equipment react differently in high altitudes. Motors cool less efficiently at higher altitudes and need some adjustments, or derating. Ratings on the front of the furnace are for sea level; derating or lowering the rating is necessary for high altitudes. The ambient temperature, the altitude and the humidity all affect furnace equipment. You don't have to figure this out completely by yourself, as the Air Conditioning Contractors of America publishes installation standards and specifications for furnaces (see the Reference section). Many contractors offer free furnace sizing and estimates as well.

Example

  • If your heating unit produces 35,000 British thermal units (Btu) at sea level and you move it to an altitude of 3,000 feet above sea level, you can estimate a loss of 12 percent, or 4 percent for each 1,000 feet. A 4,200-Btu reduction allows your unit to work at a 30,800 Btu rate. Thin air carries less heat for warmth, and less cooling at the high altitude causes the motor to overheat easily. Much like making a cake from a mix at a high altitude, you’ll have to look for specific instructions from the furnace manufacturer. The manufacturer’s engineers consider the temperature, humidity and altitude to gauge the furnace, fans and ductwork for the region. Manufacturers have high-altitude instructions specific to each design and style of furnace. Make certain your contractor applies the manufacturer’s suggestions and calculations for the equipment you choose.

High-Altitude Conversion Equipment

  • Conversion kits are available for LP gas and natural gas furnaces. Most furnaces need high-altitude pressure switches if they're used at high altitudes. An electric hanging furnace especially suited for high-altitude use has an automatic reset to de-energize the unit if it overheats. Factory-authorized and trained installers working in high altitudes have furnaces, conversion kits and solutions for the geographical region.

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References

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