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Rust in one furnace, but not the other

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zepfan
zepfan Member Posts: 467
in Gas Heating

does anyone have an idea on what could be causing this amount of rust in the below pictured furnace, while there is one of the same capacity(66mbh) , age(2011) and model right next to it that there is no rust. We did not install either of these furnaces, but there were inspected by our AHJ, have they own independent vents 4” b vent, and are installed in the same combustion zone. Right next to one another.


the first I serviced either of the furnaces was today. I did a combustion test on both with all readings within an acceptable range, and the proper manifold pressure


the only thing that I can think of is that the one with the rust is installed closer to the outside wall in a below ground basement that it may be subjected to a higher humidity level when the furnace is idle in the summer? The heater without any rust has the water heater connected to its B vent and maybe with just the pilot on in the WH that is keeping the vent above dew point at all times? The furnace with the rust has its own independent b vent. I checked , and both rain caps are in place on the roof. I am kind of leaning towards a humidity issue because if there was an issue with the vent, then why would the gas pipe in the furnace be rusting as well? Any suggestions would be much appreciated. Thanks to all

IMG_6168.jpeg

Comments

  • mattmia2
    mattmia2 Member Posts: 17,466

    looks like it is condensing in the hx. is it controlled in such a way to run a long enough cycle get up to temp in a heat call? what are the supply and terturn temps under operating conditions? is it heating a space from unusually cold temps or keeping it at unusually cold temps?

    zepfanGrallert
  • Grallert
    Grallert Member Posts: 1,147

    Do both of them have the same connected load? Looks like one is condensing while firing perhaps.

    Miss Hall's School service mechanic, greenhouse manager, teacher, dog walker and designated driver

  • pecmsg
    pecmsg Member Posts: 7,281

    I need a picture from further away.

    GGrossHVACNUT
  • pedmec
    pedmec Member Posts: 1,460

    Those are fan assisted furnaces and the most likely issue that is happening is that you are condensing in the flue. Part of the problem with fan assisted furnaces is that they run close to their dew point. They are designed to run at 85% efficiency. That means a lower flue gas temperature. But the other bigger issue is there is no dilution air entering into the vent to lower the dew point of the flue gases. The unit that has no rust has dilution air entering the flue from the water heater. It is probably helping lower the dew point so you don't get any condensation.

    JimPEdTheHeaterMan

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