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Reusing existing vents

user-2310254 | Posted in Green Building Techniques on

I have installed three Panasonic Whisper Green bath fans on the top level of my townhouse. All three fans require at least four-inch vent pipes. The old fans were all three-inch slinky pipes.

I plan to use four-inch smooth pipe with the new fans but would like to reuse the openings that already exist in roof. Would it be acceptable to gang two of the runs together with a Y and exit the roof with a single piece of pipe? I was considering this approach and transitioning to a six-inch pipe for the final leg of the exhaust run.

I also have a question about repurposing a six-inch double-wall run of pipe that used to be connected to a gas furnace in the attic. (I have foamed the attic and converted the HVAC to all-electric.) I would like to use this pipe as a fresh air supply. Would that be allowable, or should I replace it with single-wall pipe?

Also… I wanted to reroute the end of the double-walled fresh air pipe and terminate it somewhere in the top floor living space. The HVAC systems and/or a bath fan would then be used to pull in fresh air. Alternatively, I have considered routing the fresh air pipe to a nearby return that runs to a Daikin ducted head installed in the attic. This approach would probably be easier to implement, and I think Daikin supports a ventilation-only mode.

Last bit of information, I have located a SensorPush wireless humidity and temperature sensor in the attic to monitor conditions. If the humidity levels start to elevate, I will probably install a dehumidifier and ventilator or try using a vapor diffusion port to keep conditions in a safe range.

So any advice?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Steve,
    Q. "I plan to use four-inch smooth pipe with the new fans but would like to reuse the openings that already exist in roof. Would it be acceptable to gang two of the runs together with a Y and exit the roof with a single piece of pipe? I was considering this approach and transitioning to a six-inch pipe for the final leg of the exhaust run."

    A. I usually recommend against this approach. It's best if each bathroom exhaust fan has its own exterior termination. Here's why: When one of the two fans is operating, it pressurizes the exhaust duct of the fan that isn't operating, pushing some exhaust air past the backdraft damper and into the bathroom where the fan is off.

    Q. "I also have a question about repurposing a six-inch double-wall run of pipe that used to be connected to a gas furnace in the attic. (I have foamed the attic and converted the HVAC to all-electric.) I would like to use this pipe as a fresh air supply. Would that be allowable, or should I replace it with single-wall pipe?"

    A. I'm not sure what you mean by using this pipe "as a fresh air supply." Do you mean for an HRV? In general, it's a bad idea to pull fresh air for a ventilation from a roof, because many types of roofing offgas VOCs in hot weather. There's an added concern: a gas furnace flue may have years of accumulated condensate on the interior of the flue, and that is nasty stuff that you don't want to breathe.

    Reading further into your post, I realize that you don't have an HRV -- you want to use this "fresh air supply" as (basically) a big hole in your thermal envelope -- what many people call a "passive fresh air inlet." I'm not in favor of this method of ventilation, because you'll be increasing the air leakage rate through your home's thermal envelope. It's an especially bad idea to pull this outdoor air from your roof.

    Q. "If the [attic] humidity levels start to elevate, I will probably install a dehumidifier and ventilator."

    A. Is this a vented unconditioned attic or an unvented conditioned attic? In general, powered attic ventilators are a bad idea. For more information, see "Fans in the Attic: Do They Help or Do They Hurt?"

  2. user-2310254 | | #2

    Hi Martin,

    On Q1, your response confirms my suspicion about the downside of combining vent lines. Thanks.

    On Q2, I do not have an ERV or HRV at this point. And the VOC issue hadn't even occurred to me. I understand that the passive fresh air inlet approach is not ideal and will consider other options.

    On Q3, The attic is now sealed. The SensorPush sensor indicates it has been staying between 43% and 45% humidity, but I will continue to monitor conditions. BTW. This is without an conditioned supply air. The HVAC contractor is scheduled for a return visit so he can add a 40 CFM supply.

    Thanks for your input. I will re-read the GBA's article on ventilation strategies. The townhouse may surprising tight (2ACH50) before I isolated the garage and foamed the attic. By my calculation, it needs 43 CFM of fresh air. So maybe a couple of Lunos e2 will address this issue.

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