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

confusedHO
confusedHO 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.

Comments

  • SWEI
    SWEI Member Posts: 7,356
    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.
  • confusedHO
    confusedHO Member Posts: 15
    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?

  • confusedHO
    confusedHO Member Posts: 15
    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.