Adjusting Gas-Burning Equipment for High Altitudes
HVAC system design at higher elevations can be a little tricky. in this post I'll discuss some adjustments you'll need to make when your project is in Utah, Idaho, New Mexico, Montana, or other Rocky Mountain states where the air is less dense.
First off, fuel burning equipment, such as furnaces and water heaters, will not work properly at elevations higher than about 2,000 feet without a high altitude conversion kit.
The general guidance is to reduce orifice size by 4% for every 1,000 feet of altitude. Here is some language from a Carrier Weathermaker catalog.
High altitudes have less oxygen, which affects the fuel/air mixture in heat exchangers. In order to maintain a proper fuel/air mixture, heat exchangers operating in altitudes above 2000 ft require different orifices. To select the correct burner orifices or determine the heat capacity for a high altitude application, use either the selection software, or the unit’s service manual.
The new gas orifices in this field-installed kit make the necessary adjustment for high altitude applications. They restore the optimal fuel to air mixture and maintain healthy combustion on altitudes above 2000 ft.
If you're new to higher elevations, or if you're not familiar with these equipment modifications, talk to your vendor. If he / she serves customers in high elevations, he / she will know what you're talking about.
What this deration means is, if an 80% furnace is rated for 100,000 btuh of input and 80,000 btuh of output at sea level and you take it to 4,000 feet, instead of 80,000 btuhs, you'll get 16% less heating, which is 67,000 btuhs. That's quite a bit less. If you go to 7,000 feet, for example in Albuquerque, New Mexico, you'll have 28% less capacity.
Click here for a link to an altitude correction chart.