Tech Notes
High Altitude Gas-Fired Heater Applications
Published ratings for Gas-fired duct furnaces and heaters are typically for operation at sea level. These
furnaces capacities depend on the air density, oxygen content of the air and pressure drop across the gas
metering device(s). These components are all affected by altitude.
Gas-fired burners require oxygen to cleanly and safely burn gaseous fuels such as Natural or Propane
gas. This oxygen is provided by the air delivered to the burner. At higher altitudes, air has less oxygen per
unit volume, so gas firing rate must be adjusted to maintain proper combustion of the fuel. This can be
accomplished by decreasing the burner fuel orifice size to de-rate the burner(s). The current guideline
states that starting at 2000 ft. above sea level the gas input rate is reduced 4% and another 4% for each
1000 ft. above 2000 ft. elevation. (This is known as the 2/4 Rule).
The furnace’s heating capacity is also affected by the change in air density. The circulating air fan
delivers the same volume of air in cubic feet per minute (CFM) at altitude as it does at sea level.
However, due to the lower density, the air has less heat carrying capacity, resulting in a comparable
reduction in furnace capacity.
The suggested gas input de-rate therefore coincides with the furnace capacity de-rate, allowing for
straightforward sizing of the furnace based on the heating requirements at elevation.
Gas furnace capacity ratings are based on sea level operation. Selection of the proper furnace size
(capacity) must be based on standard conditions at sea level. For installations above 2,000 ft., the
furnace should have a sea level input rating large enough that it will meet the heating load after deration
for altitude.
Because the furnace gas firing rate must be reduced at the installed altitude, determine the furnace size
by input rating required at sea level for the design conditions using the following formula:
Q (Btuh) = CFM x (ρ x 0.24 x 60) x ΔT
(Eff. / 100)
Where:
CFM = Airflow rate through furnace
ρ = Density of air at design conditions (Temperature & Barometric pressure)
ΔT = Design temperature rise
Eff. = Published furnace efficiency
Note = 0.24 is the specific heat of air (Btu/lbmF)
Note: For applications with indoor return air as entering air the conventional formula may be used:
Q (Btuh) = CFM x 1.08 x ΔT
(Eff. / 100)
Select the closest available furnace size with Btuh input rating equal or greater than the calculated Btuh
input requirement. Once the required size furnace is selected, apply required de-rate for specified
installation altitude, ie. for an installation at 4000 ft. above sea level, the required de-rate will be 4 + (2 x
4%) or 12%. The furnace will need to be orificed for the new input rating.
See following example.
Example:
Design Conditions:
Entering air temperature = 10 oF
Leaving air temperature = 80 oF
Calculations:
System airflow
= 3200 cfm
Elevation
= 4000 ft.
Efficiency
= 80%
ΔT = 80 – (10) = 70 oF
ρ = .0846 lbm /cu.ft. (@ sea level and 10 oF)
Eff = 80%
Q = (3200) x (.0846 x 0.24 x 60) x 70
80/100
Q = 341,004 Btuh
Selection:
HMA350 (Rated 350,000 Btuh @ sea level)
Adjust for Altitude:
Orifice for altitude @ 4000 ft,
(4000-2000)/1000= (2.00 x 4%) + 4 % = 12% de-rate
.88 x 350,000 = 308,000 Btuh
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