Does altitude affect AFUE? or just the input/output BTUs?

I searched and searched and searched yesterday for an answer to the question "Does altitude affect AFUE?" but did not come up with any definitive answers. Hopefully this discussion will answer that question more easily for folks in the future.

Derating combustion appliances typically applies the equation of about 2% per 1000 ft above sea level (from most manufacturer's specifications). So here in Santa Fe (at 7000 ft ASL), the derate would be 14%. Applying this to a 100 MBTUH input capacity appliance, the "natural" derating would make the appliance's input capacity actually 86 MBTUH. In all of the documentation I could find I could not determine if this derating ever affected the advertised AFUE or SSE rating, only the actual input/output in direct relation to the AFUE.

So my first question is, after an appliance has been naturally derated for altitude, can it be assumed to still function at the manufacturer's AFUE?

And then my second question is, assuming the above is true, and from what I have read here (, see appendix E), it would seem that Steady State Efficiency (and consequently, AFUE) actually improves with altitude. I know, right? Does anyone know at which rate the SSE (or AFUE) might improve with each 1000 ft increment above sea level?

Tags: AFUE, BTUH, Efficiency, SSE, State, Steady, altitude, derate, input

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Do I hear crickets? Nobody has any response to this? Was it something I said? :p...

Hi Rod, I'm certainly not an expert on this topic, but wanted to let you know someone is following to see if an answer pops up.  And yes I hear the crickets as well.

The paper in the link was a bit too long for me to read it all so I'll take your word for SEE and AFUE both showing some improvement with altitude.  Even though the total BTUs being produced decreases, it seems logical that the combustion process can be adjusted for the lower air pressure.  If a furnace were tuned up at sea level and then installed at 7,000 feet I would expect it to perform poorly.  But re-tuned, it should be close to "as manufactured"" in performance.  A little better or a little worse I can't guess.

Interesting.  Since we have air time I'll ask you another altitude question.  If you transport a window from sea level to 7,000' do you allow the pressure to equalize during the trip and then seal the vent?


Hey Bud, thanks for interrupting the sound of crickets! ;)

In reading that paper, just jump right to the appendix E as that paper has an immense amount of information. I generally read the whole thing and it took me a good hour and a half to assimilate. Good info there though, if you've got time.

To answer your question (or rather, to pose yet another one), do windows need to equalize to the outside pressure? I think all that will happen is that the windows will bulge outwards a bit from the pressure difference, but I don't see how it would affect R-value or functionality any. Do you?

So... is anyone else going to jump on this topic?

I'm not positive that I have an answer for this as well but I am interested in the topic and would guess that the answer lie in the ASHRAE Standard 103 Method of Testing for Annual Fuel Utilization Efficiency of Residential Central Furnaces and Boilers. I have not read this Standard and do not have it in my library of ASHRAE Standards but I suspect that there maybe some information in that. Anyone familiar with this Standard?

So.... seriously? No one can answer this question about altitude affecting AFUE? Sheesh! Alrighty then, seems I'm going to have to bust out a can of triple dog dare for all you HVAC experts to jump on. Or can it be that folks really don't know? Seems unlikely...

First, it is the wrong question. AFUE is a comparative consumer number whereby the layman is presumably assisted in choosing an appliance by its advertised efficiency. 

If we are addressing condensing appliances, all others being a waste of time and money, then we assume sealed combustion, direct vent and a negative pressure gas valve. Unless the appliance has a Lamda control it will have to be tuned to the application i.e. available gas pressure and to your question the gas/air ratio. 

The significance of combustion efficiency at altitudes above sea level are nothing compared to temperature differential and heat coefficient mentioned in Appendix E.

When designing systems in Sante Fe we rarely see a furnace but spec. many condensing boilers derated for output but enhanced by low operating temperatures giving us steady state combustion efficiency well into to upper 90's. 

Well, I don't really think I asked the wrong question, "Does altitude affect AFUE?" Because that's what I wanted to know. That's the number I use in many calculations, to present to the layman (my customers) accordingly.

About 50% of the homes I audit in the Albuquerque/Santa Fe area have furnaces, not boilers. And of those furnaces, about 90% are 80% AFUE, non-condensing units, 40% of which run on propane. The diversity here runs the gamut. My question stems from often seeing 80% efficiency units (boilers or furnaces) without any evidence of altitude adjustments, whether in the form of orifice sizing, pressure regulating, or via Lamda controls. I just don't see any labels indicating so. So I have to assume that the unit has not been adjusted for altitude, thus my need to know if the efficiency is affected. Does that make sense?

I think that my question has been answered offline, with much more of my research coming to the conclusion that altitude does not "significantly" affect efficiency. And at this altitude (7000 ft), it's about a wash, so, no need to adjust the AFUE.

Thanks for the input Morgan, and welcome to this forum!


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