Direct Vent Primer - Concentric vs. Separate Pipes (PVC in my case)

mnowaczyk Posted By mnowaczyk, Jan 14, 2011 at 4:41 PM

  1. mnowaczyk

    Member 2.

    Feb 19, 2009
    I'm not sure if this is the right place for this, but I feel more comfortable here than on HVAC forums (after getting so much fireplace insert advice here).

    Replacing the oil boiler with a high efficiency gas boiler (doesn't exactly belong in the boiler room because this is not solid fuel).

    I've learned that "Direct Vent" applies only to scenarios where the intake air comes in from the outside via a pipe, isolating the combustion entirely to outside air. From experience with forced air heater installation, I believe that it may also require the intake and vent to be on the same side of the house to assure equal pressure is on both the intake and exhaust. (If on opposite sides of the house, a strong wind on the exhaust, and vacuum on the intake could result in reversed air flow in the combustion chamber.) I've also learned that the required distances from window and door openings are reduced in cases when direct venting (both intake and exhaust). I believe that in a scenario where you are using house air for combustion (possibly making the house a vacuum), and just piping the exhaust air out, you need to be 3 feet (or more, maybe 4 feet) from any window. The idea here is that the vacuumed house could suck that exhaust right back into an open window if it's not far enough away. However, when direct venting, the house should not be (or at least should be less of) a vacuum. So there's what I know about direct venting, just enough to be dangerous.

    So here's what I'm checking into now.

    We've got a new Triangle Tube boiler that requires direct venting with 3" PVC. I've used Concentric vents. I've used 2" separate vents. I believe I see the benefits of both and will most likely use a separated intake / vent in this scenario because the pipes will need to exit the house at most 6" above the ground level, or very possibly below grade, or in a below grade window well. I assume that eliminates the possibility of using a Concentric kit as they have the termination built onto the end of the concentric pipe. I think concentric kits are nice, requiring only a single hole in the house and also heating intake air while cooling exhaust. However, it doesn't seem like an option for this installation... unless we ran the pipe all the way up the old masonry chimney as opposed to through the wall. This IS an option, but I need to verify the maximum run length first, as my run might possibly be over 40 feet. (I've got a big mutha of a chimney,which extends above the ~10 foot high 4th floor attic, and this boiler will be installed in the basement (where the current boiler is).

    So what the heck an I asking?

    I've got a few questions:

    1) What are your opinions on running direct venting up a very long chimney (of course assuming the effective length, adding length with any fitting, is under the maximum lengths for the appliance?

    2) I've read about intakes icing over if they are too close to the exhaust (which is steam) especially on humid days, AND when the boiler is firing at a reduced rate. (Many boilers like this one have the ability to fire at as little as 25% of the rated BTU.) This is a concern for me, especially if I'm piping my intake or exhaust through some ground (which might be covered in snow) and piping up a few feet to be well above a snow-line. So what suggestions are available to assure a direct vent setup so close to the ground does not freeze over?

    3) With the above questions, would a chimney top vent installation be less likely to freeze over with more fresh air blowing by the direct vents? Or would a direct vent more enclosed from the elements be less likely to freeze over? 6 of one, half a dozen of the other? I'm inclined to think that more fresh air is better, with a goal of keeping moisture away from the air intake.

    4) Distance between intake and exhaust? What is ideal? It seems that the two pipes should be on the same side of the house, and ideally affected by the exact same wind pressures. However, you don't want them too close to each other. I believe the minimum distance between two pipes is 12 inches. Is there also a maximum? Maybe a distance at which the system is no longer considered a true "direct vent"?

    5) Let's confirm the distance questions above and talk about how a Concentric vent conforms. Concentric air intakes are certainly less than 12" from the exhaust. However, the exhaust is directed away from the intake and should shoot away from the house. I assume that this design compensates for the lack of a 12" distance. Additionally, concentric vents benefit from the warmth of the exhaust to possibly help assure the intake does not freeze over. Is this all correct?

    So there's five questions here, asking opinions. And if it's true that opinions are like ...... , and everybody's got one. Let's hear it! (your opinion, not your ..... ) :)

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