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Community and Q&A

If one installs an ERV/HRV do you need to install bath and kitchen exhaust fans as well?

Bubba Shirk | Posted in Green Building Techniques on

I realize code asks for at least a kitchen exhaust fan but doesn’t exhaust fans in kitchens and baths negate the advantages of an ERV/HRV?

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  1. Riversong | | #1

    This is an endless debate.

    No, point source moisture removal fans don't "negate" the advantages of an HRV/ERV but will somewhat reduce the potential energy savings.

    But that's the wrong question. The right question is what are the purposes and functions of point-source removal and whole-house ventilation, which are two different strategies, and do they work best in isolation or as a team?

    Point source moisture evacuation is designed to do that function well: remove excess humidity at the source of generation, do it quickly and efficiently over a relatively short period of time (called intermittent ventilation).

    Whole-house ventilation serves the more generalized function of "dilution as the solution to pollution" by providing a constant or relatively constant stream of fresh outside (sometimes filtered) air. A heat or energy recovery ventilator takes this one step further and recovers a portion of the energy (sensible heat) and/or the heat of vaporization (latent heat). How much energy is recovered is a function of the efficiency of the unit, the delta-T and the delta-RH, the run time and CFM of the fans, and the static losses in the ductwork.

    Either whole-house heat recovery or exhaust-only ventilation with passive make-up air inlets can also serve the function of providing fresh air to those rooms where it is most beneficial: bedrooms and living areas.

    Most energy-efficient builders today are relying on whole-house heat-recovery ventilation to perform both tasks: source removal and generalized dilution, which also helps reduce indoor humidity when it's dryer outside than in.

    My contention is that an HRV/ERV can never be as effective in source removal as a dedicated fan, even when provided with a boost mode, which boosts the airflow from all, not just one, outlet vent. For that reason, and given the importance of removing humidity before it can disperse or cause damage, I prefer dedicated source removal fans in all bathrooms that have showers or jacuzzis and kitchen range hoods (no more than 200 CFM). The small, short-term energy price is worth the benefit.

    Additionally, since in a tight house in a cold climate, it is imperative to evacuate moisture to keep the indoor relative humidity below a safe 35%-40% level, it makes no sense to recycle moisture with an ERV. ERVs make more sense in hot/humid or mixed climates where they might somewhat reduce AC loads.

  2. GBA Editor
    Martin Holladay | | #2

    Either an HRV or an ERV can be used to provide bathroom exhaust; that use is routine. You can contact HRV or ERV manufacturers to verify that point.

    An HRV or ERV should never be connected to a range hood. Range hoods should almost always be exhausted directly outdoors. To learn more about range hood fans, see Makeup Air for Range Hoods.

  3. Riversong | | #3

    You can contact HRV or ERV manufacturers to verify that point

    Objective information rarely comes form those who profit from a product.

  4. Bubba Shirk | | #4

    Thanks for all your help!

  5. William Li | | #5

    Reading a thread in this area from a while back ( made me think of a related question.

    I have a couple of bathrooms which need venting, sitting appropriately back-to-back. Alone, each bathroom requires about 100-110 CFM based on volume. I have a choice of using a single inline fan (presumably sized at 220 CFM) with air intakes from both bathrooms and venting outside once or else using 2 independent fans of 110 CFM and venting each individually, using either a through-wall or side-venting method.

    The thread from above seems to indicate that having a single inline fan with a switch in each bathroom has usability issues (ie, someone turning the fan off while the other bathroom is still steamy). However, having 2 fans almost doubles the materials cost (there seems to be only very little cost difference between a 110 and a 220 CFM fan, inline or ceiling mount) and gives you a second hole to detail.
    So, what is the consensus about which makes more sense? In usage, either bathroom can be used at various times. Does the presence of a HRV system change the answer?
    FWIW, I'm in Vancouver Canada.

  6. Riversong | | #6


    If the purpose of a bathroom exhaust fan is to evacuate humidity during and after a shower or steamy bath (e.g. jacuzzi, which is a great example of an ungreen non-necessity, as is multiple-head showers), without unduly imbalancing the required whole-house air exchange or unduly increasing heat loss in winter or creating negative pressure in summer, then each bathroom must have a dedicated fan with its own timed controls.

    Duct terminations are, ideally, on a sidewall well below a vented soffit and away from operable windows, rather than through the roof or in a soffit. A ceiling-mounted exhaust system at the thermal boundary is also, ideally, in a soffit below the thermal/air barrier and vented laterally to and through the wall, or down and out to get it away from the soffit and prevent cold air backdrafting.

  7. Andy Ault, CLC | | #7

    Robert's right, each bath should be independently controlled.

    A single inline with multiple intake points is really intended for a single space such as a large master bath. We will use these to put one intake in the shower stall, one directly over the tub, and a third in the water closet (these are obviously pretty large bathrooms). Then we tie all three intakes into a single fan with a single exit point.

    In this type of situation, it is a definite cost savings with no compromise in function. But in separate rooms, the potential for compromised function far outweighs a cost savings argument (aka penny wise, but pound foolish...)

    And for controls for any type of bath fan, we really like the Leviton Digital Timer switches. Simple to use. Powerful enough for any size fan. Can be wired as a three way (i.e. one switch next to the shower stall and another one in the water closet). 10 & 20 min options are good for the water closet and 30 & 60 min options are good for the shower or bath tub.

  8. Riversong | | #8


    I like the Leviton push-button variable timer switch too, but I ran into a problem with them. Since I use my bath fans for whole-house ventilation, I connected a Grasslin programmable digital timer in parallel with the Leviton switch and there was some electrical feedback between the two which rendered the Grasslin inoperable. So I had to switch them out for the Grasslin programmable analog timer (the ones that rotate like a clock and have mechanical flip levers for runtimes).

  9. William Li | | #9

    Thank you all for your answers. FWIW, I boggled privately at a quote I got from one contractor who wanted to charge almost exactly double to hook up one 120 CFM inline fan to 2 bathrooms vs. the cost of hooking up the same Panasonic inline fan to just one bathroom. (quoted cost for each room was about 4x cost of the inline fan from Internet price books) It seemed like I really ought to get 2 fans out of the deal, if I was going with this guy... ;> I'm happy to pay a professional tradesperson to do work, but this seemed a bit out of whack.
    I think after reading the threads, I'll try out Mr. Riversong's detail of dropping the duct down in a chase on the inside of the bathroom (coming down through a ceiling penetration), then out through the wall, ensuring sufficient slope for drainage once past the first bend where the ceiling fan sits. Otherwise, because of my house geometry, I'd end up having to either drop down through the soffit outside or go up through the (concrete tile) roof.
    Would it be safe for me to use just one slightly bigger vent piping outside, combining the two bathroom's independently-switched fans at a Y- or T-junction inside the attic before dropping down and out the wall? Or, do I need to ensure that I have an independent duct and vent outside for each fan? The two bathrooms share a wall, and because of tiling in one of the bathrooms, I'd end up routing that bathroom's vent duct through the chase in the other bathroom anyways.
    Once again, thanks for the quick answers. This and Fine HomeBuilding are the best websites out there, even for amateurs like me.

  10. Riversong | | #10


    Unless you incorporate back-flow preventers, combining exhaust ducts is a very bad idea. And any ducting downstream of the Y would have to be sized for both fans running simultaneously. In addition, the back-flow preventers, and any additional changes in direction would add static pressure and reduce air flow.

    What you need are two completely independent systems.

  11. William Li | | #11

    Thank you. That makes sense to me.

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