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Combustion question

drhvacdrhvac Member Posts: 189
Approximate readings before and after gas pressure was adjusted. What's better?

Gas pressure 3.6
Stack 295
Co2 6.34
O2 9.2
Co 9ppm

Gas pressure 4.25
Stack 325
Co2 7.4
O2 8
Co 10ppm

This is an 80% gas furnace. Isn't it a better burn to try and get the Co2 up closer to 7.5 - 8 as long as the co, stack, etc is at exceptable numbers? I was concerned about the gas pressure being to high, but if all the other readings are good, seems fine to me?

Thoughts, thank you.


  • IronmanIronman Member Posts: 5,514
    No, don't set it above the manufacturer's spec, which is usually 3.5" WC. Setting it higher will over-fire the furnace and crack the heat exchanger.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • drhvacdrhvac Member Posts: 189
    How's that if all the other numbers are good? Stack temp is only 325, and there was a 55 temp rise.
  • captaincocaptainco Member Posts: 442
    You did the correct thing. You brought the furnace up to its actual rating versus its factory de-rated design. The orifices are small and raising gas pressure is easier than finding the correct orifices which most manufacturers state in their instructions. I am surprised your O2 only dropped 1.4% with the gas increased 3/4". That equals about a 10% increase in gas. Flue temperature range for that furnace is 302 to 402 degrees so you brought up to where it belonged. To explain LHV and HHV might take to long but natural gas is only 90% sensible heat. So it takes 110 cu ft of gas to have a real 100,000 btu input.
  • njtommynjtommy Member Posts: 1,105
    I would leave the gas valve set for 3.5" as per manufacturer and call it a day.
  • IronmanIronman Member Posts: 5,514
    drhvac said:

    How's that if all the other numbers are good? Stack temp is only 325, and there was a 55 temp rise.

    Aren't you forgetting something in you equation?
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • njtommynjtommy Member Posts: 1,105
    @Ironman he definitely is. Fan speed is a big part of the equation. Also you must look to see if it's labled a warm air furnace along with the ratings for temperature rise acrossed the furnace.

    If you return temp is 65 and the supply is 120. I see no problems with that at all.
  • drhvacdrhvac Member Posts: 189
    Captainco, the numbers I gave you were estimated, here are the exact numbers after I raised the gas pressure and made the adjustments. I'm always looking to get the o2 and Co2 as close as possible as long as the co, stack temp, etc is right.

    Stack 320
    Co2 7.34
    O2 7.8
    Co 9
    Co af 14
    Draft .025
  • captaincocaptainco Member Posts: 442
    There are five basic elements of heat transfer:
    Line of Sight
    Residence Time
    Specific Heat Capacity of the Heat Exchanger.

    Increasing gas pressure improves the first four. The specific heat capacity is affected by soot or scale.

    To adjust a piece of equipment to produce its rated btu output is the right thing to do. Because most heat exchangers are 20,000 btus each and there is no orifice made that delivers 20,000 btus @ 3.5", the only choice is to adjust the gas pressure. The other choice is to return the equipment to the manufacturer and ask for one that is the btus they say it is.

    If you bought a 3 Ton A/C unit and it had a 2-1/2 ton compressor in it I don't think you would be happy. But we buy furnaces all the time that are manufactured with smaller orifices than they are rated. Is this a scam or do they think we are to dumb to notice?

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