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I am seeking advice on a potential modification to my furnace setup. Currently, the exhaust PVC pipe outside my house is situated too close to my windows and AC, prompting me to consider extending it vertically up to my roof. However, I am concerned about potential issues arising from condensation during winter months, particularly in British Columbia where temperatures can drop to approximately -10°C. Given my limited expertise in HVAC systems, I am unsure whether this modification could lead to water accumulation and potential damage to my furnace. I would greatly appreciate any insights or recommendations regarding the feasibility and potential risks associated with this proposed modification. Thank you.

To add more details here is my current setup and what I am planning to do

enter image description here

  • Red: Is my external wall
  • Yellow: Roof
  • Blue: is my existing PVC Pipe
  • Green: Is what I am planning to add

enter image description here

Here is the measurement

  • Green Box: The pipe length is 75 inches
  • Red Box: 31 Inches
  • Blue Box:12 inches
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    you should be concerned about the lengh of the pipe first
    – jsotola
    Feb 19 at 18:04
  • Hi jsotola, I added a picture of the measurement
    – Hawk
    Feb 19 at 18:13
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    What is the brand/model # of the furnace? That will determine how long a vent pipe you can install and how high it can rise.
    – FreeMan
    Feb 19 at 18:26
  • Let me check the furnace brand and model
    – Hawk
    Feb 19 at 18:32
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    refer to the furnace installation instructions for exhaust pipe requirements then ask a question if you do not understand something
    – jsotola
    Feb 19 at 20:02

2 Answers 2

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Yes, that much length on the exterior is a freeze concern. You normally have a fairly short stub through the wall or roof which doesn't allow the steam to cool enough to freeze. An enlarged terminal pipe can help where it is a problem.

Also, the exhaust shouldn't have a hook on top. Furnace exhausts should go horizontal or vertical (upward), and intakes draw from below. Even if you don't have an intake you should follow standards. Since high-efficiency furnaces all have equipment to handle condensation, there's no need for any sort of rain cap. Just point it at the sky. A little rain won't add much to the existing condensation volume.

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  • I wouldn't put a P trap on it, but at least a 90 and a 22... and a bird screen. - No reason to run the pump every time it rains. Cite for "the exhaust shouldn't have a hook on top" ?
    – Mazura
    Feb 19 at 20:54
  • Shouldn't because virtually none in my region do, and they add restriction by virtue of elbows. It would take a heck of a lot of rain to cause the pump to run more than maybe once.
    – isherwood
    Feb 19 at 20:56
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    That's not a "p trap". It's just a 180 elbow. It's not a trap unless it's holding water.
    – isherwood
    Feb 19 at 20:57
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You haven't mentioned exactly which Goodman model you have but instructions are fairly consistent across models and time. I'm referring to instructions for a *MVC96 model. Your plan to extend the flue pipe is reasonable with some caveats.

  • The elbows pictured are too small. Use medium- or long-sweep 45 and 90 degree fittings. "A medium radius (1/4 bend DWV) elbow measures 3 1/16” minimum from the plane of one opening to the centerline of the other opening for 2” diameter pipe."
  • Condensation will occur in the pipe. Goodman advises that where the pipe is subject to near- or below-freezing conditions it should be insulated with 1/2" closed-cell foam. Condensing furnaces are designed to accommodate their condensate and a limited amount of rain/storm water entering from the flue and flowing back to the furnace.
  • Termination of the flue with a U-bend is not advised. Let the flue open horizontally or vertically, optionally with a tee termination in either case.
  • Length of the flue and number of 90 degree elbows, with two 45 degree counted as a single 90 degree, are limited. An increase in number of elbows decreases the total allowable length. The size (BTU/hr input rating) of the furnace also plays a part. If a tee termination is on the flue it must be included in the count.

Goodman provides the following table for sizing the pipe and number of elbows. Notice that for the 80,000 BTU model the width of the furnace cabinet also matters. "B" width is 18 inches; "C" width is 21 inches.

Goodman intake/flue sizing chart

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  • Great info here. I forgot about the tee as a termination. That's a good solution. However, Goodman doesn't seem to be taking -30F into consideration. That paltry insulation suggestion won't cut the mustard, so I'd want a larger pipe outside (2-1/2 or 3"). Also unmentioned is that any insulation needs to be protected from the elements. It can't just be a simple foam wrap.
    – isherwood
    Feb 22 at 20:42
  • @isherwood Yeah, one would have to choose a foam with a UV-durable jacket like the stuff they use on air conditioner lines. No matter how much insulation one uses any residual water in the pipe is still going to freeze after the furnace turns off. I think the first little bit of insulation just gives the exhaust flow a fighting chance of warming the pipe during the heat cycle so that most of the condensate, and hopefully any ice droplets from last time, can drain away before the next freeze.
    – Greg Hill
    Feb 23 at 0:34
  • Over-insulating would diminish the chance of ambient heat (afternoon sunshine) helping out. An oversize pipe has more room for containing ice buildup without impairing air flow -- but because it also has more surface area it'll take longer for the exhaust flow to bring the pipe up above freezing and might actually build up ice worse than the small pipe would. It's hard to guess where the inflection point between too small and too big might be.
    – Greg Hill
    Feb 23 at 0:37

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