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# What does derating gas boilers for high altitude do to the DOE rating?

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Member Posts: 15
I am a homeowner looking at new gas boilers and I am confused about the ratings.

I am trying to understand the high altitude adjustments for gas boilers and what it means for the Input (MBH) and DOE Heating Capacity (MBH). The "standard" adjustment that I see is to reduce the input rate 4% per 1000 ft above sea level.

For example, if the input rating is 100 MBH and the DOE heating capacity is 80 MBH (80% efficient) and the boiler is installed at an elevation of 9000 ft then the derating process should adjust the input to 64 MBH, i.e. 9000 ft = 100 – (100 * (9 * .04)) = 64. Is this correct?

Once the derating occurs what is the DOE heating capacity? Is the unit still operating at 80% efficiency and the DOE heating capacity is now 80% of 64 or 51.2?

I was told that derating the boiler for high elevation is just a matter of adjusting the gas/air mixture so the fuel burns efficiently and that the DOE heating capacity does not change. This doesn't make any sense to me given the calculations above.

Please enlighten me on this issue. Thanks.

• Member Posts: 7,356
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Altitude derating is different for natural draft versus induced draft designs. On top of that, some induced draft designs derate less than others, depending on the particular fan used and the control capabilities.

DOE numbers on mod/cons are usually quoted at 10% less than the firing rate, but the real output capacity depends on the range of supply water temps delivered. To get the AHRI rating for mod/con, divide the DOE output by 0.85. We size them using the DOE output and find that even that is conservative.
• Member Posts: 15
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Sorry I was not clear about which gas boilers. I am looking at the conventional cast iron boilers approximately 85% efficient . I want to supply my baseboards with 180° water. Again, when derating the INPUT MBH at high altitude what is the effect on the DOE Heating Capacity?

• Member Posts: 15
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Thank you SWEI, Tim and Zman for answering my question.

SWEI: I have done a heat loss calculation and for my small house I need about 22000 BTUH on a design day (-5°F to 70°F). I am comfortable keeping my house around 65°F. Also on very cold days I can run my 25000 BTU fireplace. I believe I should be looking for a unit that will give me 25000 to 30000 BTUH for two reasons. First, when I have guests they like it warmer. (This is how I keep from having guests!) Second, with more BTUs I could heat my loft if I wanted to.

Regarding your statement "the real output capacity depends on the range of supply water temps delivered": I will use an aquastat to adjust the water temperature for the coldest winter days to mild spring/fall days. I have high efficiency baseboard units so I will get enough heat running them at about 140° for the mild days and 180° or more for the coldest winter days. My baseboard heating system contains approximately 4 gallons of water and it is capable of delivering 23000 BTUH at 180° water temperature.

Zman: What formula are you using to get to 54.4 in my example of a 100 MBH boiler that is 80% efficient? Good idea about the combustion analysis. I will make sure the contractor does it.

I am looking at conventional boilers and currently the Burnham ES23 (using propane) seems to be the winner. For 9000 ft the Input rating goes from 70 to 44.8, [ 70– (70*(9*.04)) = 44.8 ] and the "DOE Heating Capacity" goes from 59 to 37.8. That is still larger than what I need.

Does anyone disagree that I should be derating the "DOE Heating Capacity" in the same manner as the "Input" rating? Or disagree with my formula?

Thank you all for your help.
• Member Posts: 1
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I understand the capacity of a gas fired cast iron boiler is derated by 4%/1000’ elevation. Does the efficiency also degrade at altitude? We have a 60 year old cast iron boiler rated at about 2.25 MBTU/hr at 5000’ elevation rated at 80% efficiency at sea level.
• Member Posts: 23,287
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I understand the capacity of a gas fired cast iron boiler is derated by 4%/1000’ elevation. Does the efficiency also degrade at altitude? We have a 60 year old cast iron boiler rated at about 2.25 MBTU/hr at 5000’ elevation rated at 80% efficiency at sea level.

Efficiency should change very little -- provided the burners are properly adjusted.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 7,837
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This might be off topic, but a thought came to mind after reading this thread.

I always work at sea level, so I never needed to consider derating. But I have always been told that you should not intentionally derate a boiler that is oversized. For example if an oil fired boiler is rated to burn 1.00 GPH firing rate, and you want to lower operating costs, you should never fire it any lower than say, 0.85 GPH, because of this reason or that reason. The same argument is made for gas burners (orifice sizes) not to be reduced in order to make an oversized boiler more in line with the heat loss.

Now I see that a 100,000 BTU boiler at 9000 feet above sea level is actually a 65,000 BTU boiler, that almost seams to defy all my training. That 9000' derating is more that 15% maximum that has been drilled into me for the last 45+ years in the trade. Does the thinner air at that altitude remove any of the factors that make a 45% derating on a particular piece of cast iron somehow different?

Just throwing that out there for discussion?

Should I start a new discussion so not to hi-jack this one?
Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
• Member Posts: 23,287
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Difficult child, @EdTheHeaterMan . Let me contemplate this a bit... The reason for not derating too far in a given boiler at a given elevation has to do with flame pattern and combustion product circulation in the boiler... but now I'll have to think about how that changes with density altitude, vs. how the fuel/air ratio changes...

oh dear.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 7,837
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@Jamie Hall I do what I can for the cause.

I remember being the second to last to be seated in the class room. right in front of Barry Zegland. (alpha order). But the teachers liked me so much that by the 4 week in class, I was seated in the front row center. Best seat in the house. I was special.
Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
• Member Posts: 22,143
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Two things at western high altitude, less O2 available and the gas tends to have a lower calorific value as they blend under high load conditions. This can require re-adjusting, if they drop from 1100 to 900 for example.

But water boils at a lower temperature at altitude for you steamers
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 23,287
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To your speculation, @EdTheHeaterMan . Basing my thinking on no o9bservations of burners at all -- but on aircraft. The flame pattern and air flow pattern in the fire chamber and boiler should be almost completely independent of density altitude, but should be dependent instead on velocity, which won't change much for a given induction fan setting or draught differential for atmospheric. So if we go backwards on setting up a burner by starting out with the fan opening or draught, and adjust the fuel flow for best combustion (rather than the other way around) the boiler should be operating properly (this is how, by the way, aircraft piston engines are operated at altitude -- you adjust the mixture control (which is really fuel flow) to get best power -- and -- surprise, surprise -- best power occurs at a considerably leaner setting than is used at sea level). Now your power output -- BTUh -- will be quite a bit less, depending on density altitude -- but that isn't really a downfiring.

Not a totally wild guess... but some vaguely informed musing.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 7,837
edited March 13
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To your speculation, @EdTheHeaterMan . Basing my thinking on no o9bservations of burners at all -- but on aircraft. The flame pattern and air flow pattern in the fire chamber and boiler should be almost completely independent of density altitude, but should be dependent instead on velocity, which won't change much for a given induction fan setting or draught differential for atmospheric. So if we go backwards on setting up a burner by starting out with the fan opening or draught, and adjust the fuel flow for best combustion (rather than the other way around) the boiler should be operating properly (this is how, by the way, aircraft piston engines are operated at altitude -- you adjust the mixture control (which is really fuel flow) to get best power -- and -- surprise, surprise -- best power occurs at a considerably leaner setting than is used at sea level). Now your power output -- BTUh -- will be quite a bit less, depending on density altitude -- but that isn't really a downfiring.

Not a totally wild guess... but some vaguely informed musing.

Thanks Grandpa! I like that you have experience in all sorts of stuff, from olden times, us kids will never have.
Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics