# Thread: Question about derating gas furnace for altitude.

1. Regular Guest
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Apr 2005
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Putting a 10 ton unit in Santa Fe, New Mexico. Altitude 7,000 feet. Lennox LGA120 with high heat.

The Lennox specs have a "high altitude derate" table, and they list the gas manifold pressure. 3.6" in w.g. for 2,000-3,000 ft. 3.1" in w.g. for 7000-8000 ft.

How do I translate this into decreased output btuh?

I've been in Texas for 5 years now (lived in Salt Lake City previously, 4200 feet) and haven't derated anything since arriving. I used to just discount 4% per 1,000 feet because that was the recommendation in the literature. (Carrier's, I think.)

I haven't used a gas manifold pressure chart to derate output before, and could use some pointers.

Suggestions?

Thanks!

Jim

2. 7000 to 8000 ft will be 74% of 2000 to 3000 ft

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Q1/Q2 = sq. root of P1/P2 example:

1000000/Q2 = sq.root of 4/3.1 next

1000000/Q2 = 1.135923668/1 next

crossmultiply

1000000 = 1.135923668(Q2) next

1000000/1.135923668 = Q2 next

Q2 = 880340.8435

So if you derate a 1000000 btuh unit by dropping pressure from 4 inches w.c. to 3.1 inches w.c., you have apx. a 12% derate.

An engineer showed me this formula. Its a fast way but not perfect. The only way to be sure is to clock the meter and get btu value of gas. And factor in the correction factor for temp of gas. 60 deg. F is standard. And atmospheric pressure.
I have the complete formula if you want it. I used to work for a large boiler manufacturer in the engineering lab.

[Edited by oil lp man on 05-02-2005 at 10:55 PM]

4. You're right oil lp I was dyslexic.

Pressure varies as flow squared, not flow varies as pressure squared like I had it.

compared to 2000-3000 feet then 7000-8000 feet would have to be derated to 92.8%

Square root of (3.1/3.6)

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The whole reason for having to derate the equipment is because of low atmospheric pressure at high altitudes and because there's less oxygen in the air to get complete combustion. On propane and natural gas furnaces ...you'll soot up the unit if you don't derate. When they had lots of atmospheric burners in the old days it involved putting in smaller orifices for the burners. I remember having to solder the orifice closed and redrill a smaller hole.

[Edited by ozone drone on 05-03-2005 at 09:12 AM]

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