After my previous post on gas pressure, I received the following comment:
Great blog, It’s packed with good info!
We are at 4500 foot altitude that can increase to 6500 in just a few miles.
You might want to touch on the effect high altitude has on the gas / air mixture to run correctly.
Being born and raised in the Great Midwest, I guess we don’t think about things like “altitude” causing problems. I appreciate when someone makes suggestions like this because it helps all of us learn. So, in response to the comment, here is a little info for those readers who are in the “higher altitude regions”.
Furnaces are usually constructed at the factory for natural gas-fired operation at 0 – 7,999 feet (0 – 2,438 m) above sea level. The manifold pressure must be changed in order to maintain proper and safe operation when the furnace is installed in a location where the
altitude is greater than 7,999 feet (2,438 m) above sea level. This will also vary with the BTU content of the gas. Here is a table that should help with these settings. As Always — be sure to check with your individual manufacturer for their specific recommendations for proper manifold pressures.
Here is an example of a manufacturer’s charts for elevation corrections of manifold pressure/BTU content of gas:
As you can see, based on the BTU Heating Value (BTU/cu ft) of the gas and altitude changes, so doe the required manifold pressure to provide proper firing rate. It is very dramatic with propane.
The other thing to remember, since we are on the subject of high altitude applications, is AIR FLOW. The air flow rates listed in the manufacturer’s standard blower performance tables are based on standard air at sea level. As the altitude or temperature increases, the density of air decreases. In order to use the manufacturer’s indoor blower tables for high altitude applications, certain corrections are necessary.
A centrifugal fan is a “constant volume” device. This means that, if the rpm remains constant, the CFM delivered is the same regardless of the density of the air. However, since the air at high altitude is less dense, less static pressure will be generated and less power will be required than a similar application at sea level.
Hope this is helpful information for those of you in the higher elevation areas.
Thanks to the reader who suggested providing this information so we all are better informed — it is appreciated