Altitude and Deration

When derating gas appliances for altitude (6,000 to 11,000 feet) I have been under the understanding that all you need to do is follow the manufactures recommendations. Usually atmospheric appliances are are 4% per 1,000 feet and appliances with forced combustion are 2% per 1,000 feet.I have observed the amount of time needed to raise the temp of a known quantity of water a certain amount and concluded that this deration method works quite well.



In Colorado the Natural gas supplied has a lower BTU content. Normally gas has somewhere around 100,000 BTU/CCF . Ours is claimed to be somewhere in the 750,000 to 800,000 range. The reason given for this is that it is makes appliances run more efficiently with minimal altitude adjustments. I have been under the impression that this lower BTU content does not have a negative impact on the appliances output (more than the altitude alone) as you just need a slightly richer fuel mixture to get the same amount of gas to the flame.



Recently I have heard some people claim that you just derate for the lower BTU content and not for the altitude. This makes zero sense to me as we still have the issue of lower atmospheric pressure thus less air to burn.

I have also heard folks say you should derate for both altitude and the gas.



Does anyone have any thoughts on this?



Carl