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Flue gas condensation for modcon

rhl
rhl Member Posts: 97
edited February 2021 in Gas Heating
I have a WM Aquabalance ModCon Series 1. It's pretty cold for here, 24 deg F.

My wife pointed me to a pile of ice forming below the exhaust of the modcon.

The boiler is on outdoor reset and is currently operating at 140 degrees supply with 120 degree return.

Therefore I expected the flue gases to condense inside the heat exchanger. I guess its possible for _some_ water to come out, but this seems like a lot.

I just cleaned out the sediment trap, and its dripping a bit now, but the condensate drain has no water coming out, and inspecting the drain line it seems somewhat dry.

Any thoughts on what could be going on?

Comments

  • kcopp
    kcopp Member Posts: 4,091
    Pretty normal. If you see a plume out of the exhaust that's water vapor. You are bound to get a bit of frozen condensate ...especially at -24.
  • rhl
    rhl Member Posts: 97
    Sorry, not -24 but "approximately 24 degrees"
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 13,136
    Completely normal at the -24F degrees. You going to have to keep an eye on that flue exhaust and keep it clear. Most of the condensate is supposed to drain back down the flue pipe toward the boiler and drain out the boiler drain. However when the exhaust gas cools enough more moisture will condense out of it. And at -24 it's really condensing.

    Check to make sure the flue pipe is low on the boiler end.

    Possibly insulating the flue pipe will help

  • rhl
    rhl Member Posts: 97
    Its 24 degrees, not -24 degrees,I used the symbol ~ or tilde to denote approximate, not negative.
  • kcopp
    kcopp Member Posts: 4,091
    Is this a two pipe set up (vent and intake ) or the concentric combined set up?
  • rhl
    rhl Member Posts: 97
    Two pipe setup. 
  • HVACNUT
    HVACNUT Member Posts: 4,864
    edited February 2021
    Totally normal. Just make sure it doesn't dam. Assuming it's a horizontal flue, a tee with the run on the vertical and the exhaust in the bull will solve it. As is, the condensate at at the end of the exhaust isn't dropping off, but trailing back on the underside of the pipe. It needs a positive drop off.
  • rhl
    rhl Member Posts: 97
    does this mean the flue gases aren't condensing inside the heat exchanger? If the return temps were lower would this happen less? any temp at which it would stop?
  • kcopp
    kcopp Member Posts: 4,091
    edited February 2021
    It happens in the HX and the flue most all the time. Unless you are running really high temps.
    How long a run is this vent?

  • rhl
    rhl Member Posts: 97
    the vent is not too tall, maybe at most 2ft vertical and a foot horizontal.
  • Jamie Hall
    Jamie Hall Member Posts: 20,462
    The flue gasses are condensing in the boiler -- up to a point. Then they keep on condensing as they cool further. Some of that condensate will be in the exhaust and will drip back in -- but some of it will still be condensing when it hits that 24 degree air.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • PerryHolzman
    PerryHolzman Member Posts: 234
    Totally normal. I get quite the large icicle off of my exhaust vent from my 2006 installed Vitodens 200 during the winter. even 90F exhaust from your boiler will have humidity in it, which will condense and freeze in cold enough weather.