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Community and Q&A

ERV/HRV for Dehumidifying a Bathroom

bdhome | Posted in General Questions on

My “HVAC guy” said that an ERV/HRV did not provide enough exhaust to effectively dehumidify bathrooms in a reasonable time—-even with a boost mode. I had previously gotten a quote from Zehnder and followed up with them to ask if they could put in additional exhaust ducts to the bathroom (and supply ducts somewhere else) to counter this and they said the might put in one more but they wouldn’t really recommend much more because “it would be too much.” Well… which is it?

My master bath is roughly 18’x 11’ (= 198ft2). I read that bathroom exhaust fans need to be about 1CFM per ft2. So do I need a 200cfm exhaust system? The ERV for the entire house is only 240cfm. Zehnder initially quoted me 2 tubes to the bathroom for 24cm and said they could put in a third for 36cfm but they wouldn’t recommend more. This seems too little. I know they have a boost mode, but a boost mode doesn’t boost the system that much.

Appreciate any insight. I’d love to not penetrate my envelope with bath fans and save the money as well.

Thank you.

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Replies

  1. Drew Baden | | #1

    Wouldn’t it depend on how hot and how many baths and showers you’re taking? I love hot showers so I’m adding an additional exhaust but I’m having issue finding a makeup air device. Broan also told me that my ERV is not meant for removing humidity.

    1. bdhome | | #7

      My understanding is that an ERV will remove a small ammount of the humidity compared to an HRV. An ERV doesn’t remove humidity as much as it redistributes it throughout the rest of the home.

  2. GBA Editor
    Martin Holladay | | #2
    1. bdhome | | #8

      Martin,

      Thank you. This is perfect. Unfortunately, the conclusion seems to be what I feared: “it depends.” There doesn’t seem to be a simple, agreed-upon answer.

  3. Scott Tenney | | #3

    We have just an ERV for bathroom exhaust and it works fine. But we live in Utah where the air is really dry and things tend to dry out quickly. If we lived in east TX where we grew up (where its pretty humid), I'm not sure just an ERV would be sufficient.

    1. bdhome | | #9

      We live in Minnesota. I worry about the shoulder seasons when it can get pretty humid. We also have several kids and will likely have long, hot showers.

  4. Trevor Lambert | | #4

    My bathroom is bigger than yours, and only have an ERV. I don't even use the boost 90% of the time for showers and it's fine. That's just 20-30cfm, depending on time of day and how many people are home. Just opening the bathroom door afterwards does most of the work.

  5. Charlie Sullivan | | #5

    CFM per sq. foot doesn't make a lot of sense. The same shower produces the same amount of moisture in a 50 sq ft bathroom as it does in a 500 sq. ft bathroom.

    I recommend a tighter enclosure for a shower than is common. That increases comfort in the shower because the stall warms up quickly, and it decreases the amount of moisture going into the air in the rest of the room. This New Zealand product is a great solution:

    https://showerdome.co.nz/

    But if you don't live in New Zealand you need a different solution, which can vary according to the style of shower enclosure. The most important thing is to actually have a door or curtain rather than the fully open enclosure that are popular these days and are uncomfortably cold as well as putting out more moisture than you want.

    I think that with a door, the Zehnder boost is good enough, and if you have a lowered waterproof ceiling or a lid in addition, you won't even need the boost.

    One of the advantages of the ERV/HRV is that it will continue exhausting the space at a low level. It takes a while for the water on the surfaces in the shower stall to evaporate, and so with a conventional exhaust fan, the humidity can go back up after the exhaust fan turns off.

    1. bdhome | | #10

      I hadn’t considered it, but our shower will have a floor-to-ceiling enclosure. That should help to retain a lot of the moisture within the shower space which is where the exhaust fans would primarily be located. This would keep the rest of the bathroom from over-humidifying (is that a word?). Obviously the shower door is not air-tight but it would still make a significant difference in preventing the rest of the bathroom from filling up with fog and humidity, correct?

  6. Cramer Silkworth | | #6

    Where is the Zehnder extract diffuser relative to the shower and the door? I've designed/spec'd at least a hundred Zehnder ERV systems and I think I've had only one complaint about the performance in 9 years.

    I always try to get the diffuser over, or right in front of, the shower (or highest point if the ceiling is pitched), so it can catch the steam as quickly as possible, and also try to get it across from the door so it's (kinda) pulling air across the whole room. In a master bath with bigger shower or tub & shower I will usually design for three 3" flex ducts to the larger diffuser box, but many have been just two.

    1. bdhome | | #12

      Our Zehnder quote had 2 ducts and the rep said they could do a third if I wanted but she didn’t feel it was necessary. If it’s that big of a concern per the HVAC guy, why not just install the 3rd duct—or even an additional 4th? We’d need additional return ducts somewhere else but the only issue I see with that is cost, right? Even then it’s a lot less than adding an exhaust fan. Is there a downside to having one ore more ducts than needed in a room? Would it create a draft when running throughout the day?

      I was just surprised when I got completely contradictory advice from the Zehnder rep and the HVAC consultant.

      Thank you.

  7. Jonathan Lawrence CZ 4A New Jersey | | #11

    I have Zehnder exhausts in my 6x10 bathrooms and they are located very close to the shower. With the system in boost mode, there will be a some steam in the air and a little fog on the mirror after a 5 minute shower. According to the commissioning report, boost mode in these bathrooms provides about 35cfm with 2 tubes. My master bath, which is a little bigger than yours, has 4 tubes, 2 for the shower area and 2 for the toilet. My default ERV setting is low which gives me 23cfm total in the master bath (it is 60CFM on boost). No noticeable steam or fog in there after a shower with the unit on low.

    Depending on where you live, the biggest issue you may run into with and ERV/HRV is summertime humidity. If I were to do it again, I would have placed in an inline humidifier on one of my 2nd floor ducted heat pumps. I had about 4 weeks this summer with 2nd floor humidity that required me to run a stand alone dehumidifier - high 50's without, high 40's with. First floor always stayed at 50 or below, which makes sense as the moist air rises.

    1. bdhome | | #13

      Jonathan, thank you. This helps.

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