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Venting bathrooms: how much drop is too much?

iKoRmNG4CY | Posted in Energy Efficiency and Durability on

For a new home construction (ranch-style with full basement) in mid-Michigan, I am trying to avoid ceiling penetrations as much as possible. Where penetrations are unavoidable, I’d like to keep the potential for air-leakage as low as possible.

If I vent the main-floor bathroom vent fans to the nearby soffit, this provides an unintended thermal bridge and direct path for air-leakage when the fan is not being used. As an alternative, I am considering routing the exhaust duct down through an interior wall, where it would then travel through the floor truss/joist space to the rim board and exit through the exterior wall.
I am thinking the 8′ drop would make it hard for the cold outside air in winter to backfeed into the bathroom.

Would this 8′ drop in height, combined with another 5′ or 10′ of horizontal run, be too much for an energy-efficient fan to overcome when exhausting hot humid air?

If this drop is not too much to overcome, then I am considering an in-line fan positioned remotely in the floor truss space to make it easier to service and/or replace from the basement. Something like Panasonic’s FV-10NLF1 looks appropriate. Is this a workable idea?

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  1. kevin_in_denver | | #1

    The 8' drop might be too much. There will be negative pressure on any hole below the middle of the house. The closer the hole is to the middle, the less pressure. If you manage to put the hole right at the neutral pressure point for the house, that would minimize flow by natural convection.

    The Panasonic literature should guide you. You might have to upsize the duct, but it should explain by how much.

  2. 3R3RyaN4Uo | | #2

    You could try one of these to control the airflow.,Product.asp

  3. GBA Editor
    Martin Holladay | | #3

    B Snyder,
    You need to calculate the equivalent duct length. Here's the page that explains it:

  4. toymaker | | #4

    I don't like ceiling penetrations either, so I did a similar thing in my new Wisconsin home. Two bathrooms, each with a 6 inch vent high on the wall exiting downward thru the basement wall, about 10 foot drop and 30 foot run altogether. I fed both into a Panasonic 240 CFM inline fan. That keeps most of the fan noise in the basement. I have yet to move in so I will let you know how it works. My expectation is that the Panasonic has plenty of power. Each bathroom has an on/off switch, plus I have a timer that I may replace with a humidistat someday. I bought the fan from Great quick service and very low prices.

  5. jklingel | | #5

    240 cfm? That may suck the water right out of the shower before it hits you. Kidding. What are you planning for make-up air? I believe I've read here or elsewhere that about 100 cfm is the max for even range hoods. You may want to research that a bit, unless you already have an I am all wet.

  6. toymaker | | #6

    I bought one of those 96% efficient, 3 stage modulating forced air furnaces. It includes a 6 inch fresh air inlet into the return plenum. I had a difficult time finding a small enough furnace and considered many options. The furnace will fire at 40,000 80,000 or 120,000 BTUs and it has a DC variable speed blower. I doubt it will ever go above 40,000 btus. I even popped for a touch panel control. About $7000 in high tech. I paid less than half that for my first new car. We will see how often it crashes and I have to call the service fellow. I am going to try the first winter without an HRV and carefully monitor the indoor humidity and adjust the timer to maintain sensible levels. I'm a bit of a geek and will enjoy the process. I admit I will probably have to buy an HRV eventually. My installer even included high/low returns so I can remove the warm air near the ceiling in Summer and the cool air at floor level in Winter. He says most folks never remember or understand the benefit of switching on the equinoxes. I did not include the AC option. So I may circulate cooler basement air upstairs on warm nights. That may work a few times before the insulated basement warms up in August.

  7. jklingel | | #7

    JLinck: Same problem w/ small boilers. I am looking into using a 50 gal water tank as a buffer zone, heating it w/ an available, overly large boiler. We do that for domestic water, so why not for slab water? Seems reasonable to me. Let us know how that 240 cfm fan works.

  8. GBA Editor
    Martin Holladay | | #8

    John Linck,
    You wrote, "It includes a 6 inch fresh air inlet into the return plenum." That means that you have a whole-house mechanical ventilation system. You have chosen to install a central-fan-integrated supply ventilation system.

    Whatever you do, don't install an HRV (unless, of course, you disable your existing mechanical ventilation system). You certainly don't want TWO mechanical ventilation systems. As it is, you may already be overventilating. Does your ventilation system include a motorized damper and a control to monitor furnace run time? I hope so.

  9. toymaker | | #9

    If I install an HRV it will replace the 240 cfm fan, not add to it. I also will monitor the system and swap in a smaller fan if warranted. The fresh air damper is passive, not motorized. Little air should be drawn in the fresh air intake unless it is removed some other place, like the bath exhaust fan. We will see. My furnace includes controls for fan speed, etc. But, I wonder if I will need to remove it from the ventilation loop eventually. No doubt it will need some tinkering over the first heating season. I will probably experiment with a humidistat too. If I do go with an HRV I will close the furnace's fresh air intake. While I do enjoy the geek aspects of all this, I am a believer in KISS principles. So separate systems may win out. One reason for the basement is to allow access to all the systems to allow such tweaking. BTW. the house will be very well insulated and air sealed. I am particularly a nut about thermal bridges. I want to control the the heat flow and ventilation and not let the wind and cold do it for me.

  10. GBA Editor
    Martin Holladay | | #10

    John Linck,
    Since you have a central-fan-integrated supply ventilation system, your bathroom fans aren't use for mechanical ventilation -- just for removing odors and excess moisture. I think 240 cfm is too much, but others will disagree. Just be sure you don't run the fan very often. because the fan carries a big energy penalty.

    Since the bath fans are rarely used -- after all, they aren't part of your home's mechanical ventilation system -- it wouldn't make any sense to replace them with an HRV. An HRV is expensive, and it only makes sense if it represents your home's mechanical ventilation system. But you've already chosen and installed a different system.

    You should definitely install a motorized damper and a FanCycler control on your fresh air intake. On the coldest days of the year, your furnace will be running for most of the day, depressurizing the air intake duct and overventilating your house. That's a HUGE energy penalty. You need to install a real control, with a real damper, and have it commissioned so that it ventilates at the ASHRAE 62.2 rate -- and no more. Otherwise you'll be surprised at your heating bills.

  11. dickrussell | | #11

    For homes without an HRV, I wonder how the Heartland Dryer Vent would work as a replacement for a typical wall cap for a bathroom fan. The thing connects to a 4" duct, so unless you have a fan that sucks the water out of the showerhead, along with towels and small kids, the size and volume might be ok. The only problem I can think of is turning off the fan too soon. If the humidity condensed at the cold exit parts, then froze, that light plastic shuttle might conceivably wind up frozen in place. That's pure speculation. Any opinions?

    I know the mfg doesn't make a 6" or larger version of that dryer vent, for use with range hoods with larger cfm capacities than a clothes dryer. I called and asked them.

  12. toymaker | | #12


    I am cheered that you think my bath fan will be off most of the time. I had expected the fresh air intake in the return plenum would draw very little in a well sealed home, mechanical damper or no, so I thought running the bath fan would be needed to remove excess humidity. That's why I had planned a programable timer control in addition to the regular switches. In my pre-forced air furnace thinking I expected to include a fresh air intake somewhere and figuring out where to dump the cold air was difficult. Dumping it into the return plenum solved the distribution problems and avoids any cold drafts. Maybe I can control a mechanical fresh air damper with a humidistat and open it only when humidity gets too high or to meet fresh air standards. I look forward to experimenting.

    I may go with an HRV in any case as the energy penalty of the furnace's fresh air intake may be too much. I will measure the flow and do some rough calcs. If too high I will just seal up the intake or more likely divert it to the HRV and run the HRV independently of the furnace.

    Also, my bath ventilation system has a 6 inch rigid steel duct with 5 - 90 degree els and 30 foot of run. Even with the Panasonic's qualities I expect to get less than 240 CFM.

    I don't have enough knowledge or faith in furnace installers to expect my systems will be perfect from day one if ever. Some tweaking will no doubt be needed and I look forward to the fun of that. Bottom line I will not live in an energy sucking, stuffy home whose humidity levels lead to decay at worst and stained window sills at least. Returning to the original post, I have one 3 inch plumbing vent as my only ceiling/roof penetration. I considered alternatives but the inspector was firm.

    As someone once said, "There are more ways of killing a cat than choking it with butter".

    Thanks for your help, really enjoy the site.

    john the toymaker

  13. bdrfab | | #13

    John, I'm not sure where exactly you are, but may I suggest going to and see if there is a HVAC contractor in your area? You may have better luck finding someone who understands what you're trying to to do and is able to do it.

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