Venting problems for condensing boiler

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Beads

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Some of you may have read or responded to an earlier post regarding my father's change from a 55 YO oil boiler to a new gas boiler. Thanks for the responses. I was talking to a contractor yesterday who told me that they can not vent the flue gas out a plastic vent inserted into the existing chimney. He told me that his best installation experts told him that especially with the chimney on the exterior of the structure, condensation would be a problem. He said that they could run supply and vent out the chimney if there was room, but vent only would not work.

It does not make a bit of sense to me. How can a warm tube in a chimney cause condensation problems? Note that venting laterally out the side of the house will have aesthetic problems. The only locations available are near entries visible from the front of the house.
 

Tom Sawyer

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Because by the time the flue gasses travel the length of the chimney they will cool down enough to condense. When venting any appliance, the manufacturers instructions must be followed. There isn't a single manufacturer that allows what you want to do. Use a concentric vent thought the wall and you will only have a single penetration.
 

Beads

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Thanks,

I thought that condensation was expected in the flue and that is why it needs to be sloped back to the boiler.
 

Jadnashua

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There is a distance limitation on how far and how you can run the vents on this type of boiler. As long as you can keep it within those limitations, and the calculation for length allowed gets shortened quickly if you need elbows to make it work, it might work. You have to read the installation instructions carefully.

The inlet air supply might end up cooling the chimney enough to cause condensation on the outside of the pipe, and that could affect the life of the flue liner. Prior burning deposits and moisture are not a good combination...it tends to produce acids that can eat up masonry materials. They work normally because of the hot exhaust air keeps it relatively dry.
 

Beads

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I think that all of the installation manuals that I've read have a combined length limitation of 100 feet. A 90 degree elbow counts as 6 feet and a 45 as 3 feet. I've not seen any prohibition on a warm plastic exhaust/flue pipe going up an existing chimney alone. I don't understand how that can produce condensation between new plastic flue pipe and the old chimney. I guess I will have to ask the installer how that might happen.

A concentric vent might not serve much purpose. There is snow and going straight out through the (band, box?) joist may not put a concentric vent hight enough.
 

Jadnashua

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A closed combustion system is pulling the (cold) ambient air into the burner...that pipe will get cold and could create condensation on the outside of it. The hot, humid air from combustion has a path for condensation to be taken care of (drain, trap, and often a pump). The incoming pipe, on the outside, nothing exists to prevent it. A concentric pipe might keep the outside warm enough to prevent condensation. Each unit will differ in the max length, and required ID of the pipe used.
 

Tom Sawyer

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Ok, here's the deal. On really cold days like tonight I have often seen the vent pipe close up due to frost accumulating and freezing near the top of the pipe.
 

Dana

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Condensing boilers have extremely low exhaust gas temps, and while ideally you'd have all of that condensation happening inside the boiler on the surface of the heat exchanger, a good fraction will be happening in the flue. You get substantially more flue condensation in cooler weather, when the outdoor reset raises the water temps, resulting in less condensation inside the boiler, and the duty cycles are long. The shorter the length of exhaust venting is outside of conditioned space, the less likely it is that you'll run into problems related to condensation.

If the vertical run isn't so tall that it exceeds the length limits for the boiler, it's possible to run the venting up parallel to one of the plumbing stacks, possibly sharing the chase, daylighting above the roof. In high snow regions it has to stand up pretty high, since a big snow dump from a nor'easter could cover it.

In my area ( MA ) you're not allowed to vent a fossil-burner with an efficiency over 83% into a masonry chimney unless there is a properly sized properly rated stainless steel liner, insulated between the liner and masonry with blown rock wool. Even if that solution works for your equipment, it's a real cost adder- probably more than the cost of building/planting some visual screening for a side-vented solution.

Side vent it, and plant some shrubbery or build something to hide it if you can't stand the appearance. Make sure that it's above the historical snow-depth, or sufficiently sheltered from snow by roof overhangs (even if you have to build a mini-shed roof over it) to keep it from being blocked (a known hazard usually addressed by local codes, but not always.) A side-vented solution will have higher exit-temps than a long run up through the roof or a stainless lined chimney, and less of a frost-choking hazard too.
 

Beads

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Thanks, that makes sense. I either heard it wrong from the guy I was talking with, or he heard it wrong. I just did not see how a warm pipe would cause a condensation problem between the warm pipe and the masonry chase.
 
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