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gas pressure

drhvac Member Posts: 190
Did a service on a Weil MClain boiler CGA-25-PIDN 52,000 btu's. As soon as I got there I noticed the flue had a lot of signs of condensation on it. When I checked the gas pressure it was right at 3.5", but the Oxygen was high at 11.2, and the C02 was low at 5.39 and stack temp was low at 280. I had to increase the pressure to almost 4.6 to get the O2 down, and the CO2 up, It went to 9.7 and 6.23. The stack temp went to 325f. Evrything is closer to where it should be when the pressure is higher, but is it exceptable to leave it this high? Obviously it looks like the flue gases are reaching its dew point when the pressure is at 3.5" and it is probably raining inside of th flue. The CO wasn't effected by the changes in gas pressure. It was between 2 - 4 ppm AF. Thank you.


  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    Clock the meter?

    Did you clock the gas meter to see at what rate you are firing? Generally it is not a good idea to increase the manifold pressure higher than the usual 3.5" on an atmospheric boiler. You may now be overfiring the boiler at 4.6"wc..

    The low flue gas temperature may be caused by too low water return temperatures, not underfiring. Atmospheric burners generally run at high excess air and low CO2 so your combustion numbers may have been correct as you found them.

    Check the gas consumption by clocking the meter to insure you are not firing above the rated 52K BTU input.
  • drhvac
    drhvac Member Posts: 190
    water temp.

    The water temp started at about 100f and was up to 175f during the test. The temp. of the water rising didn't really effect the readings on the meter. I wanted to clock the meter, but it was a new meter with digital displays, no dials. Any idea's on how to clock that?
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    I've seen this before...

    and it wasn't related to the firing rate. It was related to the fixed flow draft hood causing a cooling/dilution effect on the flue gasses, thereby causing them to rain.

    The fix was to install a Field barometric vent draft control damper along with required roll out switch, then block off the existing relief vent hood.

    This maintains a perfect draft across the combustion zone, eliminates excess dilution air from cooling down the rising flue gas, and makes everything connected work as it was designed to work.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • nugs
    nugs Member Posts: 77
    gas pressure

    What temperature is it outside there.
  • drhvac
    drhvac Member Posts: 190
    warm today

    It was 60 degrees. This boiler has a vent damper and there is a water heater that is also connected to flue.
  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    Probe location

    Where are you placing the combustion analyzer probe when making the measurements? It seems you may be getting some of the dilution air mixing with the flue gas, reducing its temperature. 325F is still low for an atmospheric boiler. Usually in the 400F to 450F range is typical. The CO2 and O2 readings also seem like they may be diluted with additional air.

    Are you getting the probe right down into the top of the heat exchanger area, where it will not be exposed to the dilution air from the draft hood?
  • drhvac
    drhvac Member Posts: 190

    The probe was put right above the boiler below the draft hood so no dilution air was altering the readings.
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