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Condensate in chimney

Alec_2 Member Posts: 10
Had a question based on a recent change to my chimney. The boiler (and the water heater) shares a three flue brick chimney with a fireplace. For some reason, the fireplace is routed to the other two flues. The boiler is a Weil-McLain EG-75 serving a single pipe steam system. Recently, the chimney was modified to have a bluestone slab on brick piers. Prior to that, the chimney was open to the sky.

The stone cap makes a great improvement for the draft of the fireplace, so the previous ash blow backs that we used to get (along with rain entry) seem to be a thing of the past. But when the boiler is running, the flue gas now is condensing all over the slab, to the point of dripping down the outside of the chimney and running down the roof. You can see it on the roof in the attached pic. Previously, the flue gas was visible as a white plume, but just drifted away.

The question is whether this is normal or not, and whether it poses a problem. The flue is not lined, and there is a draft hood on the boiler, so my guess is the flue gas is pretty close to condensing, and the cold stone slab is what does the final bit.

Can the draft hood be modified to reduce the amount of ambient air mixing with the hot flue gas? Or would adding a stainless liner to the flue do some good? I don't think the boiler is cracked because it doesn't seem to use much water. I do have a combustion analyzer if it's worth taking some flue gas readings.



  • kcopp
    kcopp Member Posts: 4,432
    Drop a liner in the flue....best and safest option.
  • Jack
    Jack Member Posts: 1,047
    The boiler isn't cracked. Flue gases are saturated. There is a great deal of moisture in there and your cap has given it a great place to cool and condense. Be aware that the ph of gas condensate is about 3.2. That is pretty hot. As it condenses and drips down the side of the brick chimney it is going to "eat" the mortar in the chimney, it being a lime based product (the mortar that is). A liner is always a good idea in a masonry chimney. I doubt however that it will solve your problem. As the flue gases are hitting the cap, the cold cap, given the mass and temperature of that cap those flue gases are going to continue to do what they are doing now. You can expect the outside of the chimney to turn a streaky white color from the efflorescence of the condensate/masonry combination.
    As it sits now, I don't think there is a satisfactory fix. Probably best would be to cut the slab down to cover the fireplace flue and install a SS liner that would extend above the stone cap.
  • Alec_2
    Alec_2 Member Posts: 10
    I can see how Jack's idea of cutting the cap back and extending a SS liner up above the stone slab would work, but it won't look pretty.

    What about the idea of putting a piece of thin stainless sheet metal on standoffs hanging horizontally about 1/2" below the stone cap? So there would be a low-mass deflector to keep the flue gas from hitting the cold slab as it makes the right angle turn out into the atmosphere? My assumption is that it wouldn't take long in each boiler firing cycle for the sheet metal to get above condensation temperatures.

    Also, here's a dumb-ish question, but if I do install a flue liner all the way down to the boiler, how do I connect the water heater? Right now, the ~8" boiler connection goes in one side of the chimney and the water heater goes into another. By code, can you have two gas appliances on the same liner? I'm sure there is a common solution to this problem, so be kind in your responses!

    Thanks, and Merry Christmas to all...
  • ChrisJ
    ChrisJ Member Posts: 15,700
    You can do as many as you want into one liner as long as it meets the liner manufacturer's requirements. You have to derate a certain amount for multiple appliances. You would use a tee to connect the two, or two tees. All depends on how it would work out best.

    What size liner would you use, how long (how tall) will it be and what are the input btu ratings on the appliances?

    Also, can you fit an insulated liner in it by any chance? An insulated liner would help as well.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • j a_2
    j a_2 Member Posts: 1,801
    Clearly this is something you need to be careful with. What is the flue size on both flues using gas appliance's? What is the height and what is the horizontal length...how about this picture up your configuration.... Tees may be ok but wyes are much better, in any common venting....Google the 7 times rule on gas venting...it will put u close to your sizeing
  • Gordy
    Gordy Member Posts: 9,546
    Individual SS Chimney caps would have been a better option. All though not as ornate. They do a better job of keeping varmits out of the chimney also.

    Along with the acidic condensate eating at the brick, and mortar. The moisture absorbed into that same substrate will do damage when deep freeze happens. Water proof that chimney with a saline sealer that breaths to allow moisture to escape, but repels water.