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Community and Q&A

Exhausting 3 bathrooms with/ without an HRV

Sal_123 | Posted in Energy Efficiency and Durability on

I have 3 bathrooms on the 2nd floor, they two farthest are quite a distance away from each other, about 35′- 40′, with the other being kind of in the middle. Home is in  Zone 5. Each has its own Panasonic Whisperwarm bathroom fan, a 110 CFM.  To minimize penetrations, my plan is to run the three 4″ ducts from the fans to a central location in the attic, where I will plug them into a large box, a plenum. Then have one exhaust duct running vertical, straight up, through the roof.


Is this a feasible plan despite some long duct runs?

Do I need to account for 3 additive 4″ ducts when I choose the vertical exhaust duct? The probability of all 3 fans being on at the same time is low. Want to avoid a 5” or 6” roof penetrating pipe.

I appreciate the three 4″ inputs to the plenum coming from the bathroom will need to be insulated given the moist, warm air coming from the bathrooms and the fact the ducts run through the attic (it is well insulated but not conditioned). How do I account for possible moisture formation inside the ducts? Should I pitch the ducts to the plenum? Will this create a water accumulation issue inside the plenum? I assume condensation accumulation will just dry and since its an exhaust not an issue?



Since I am considering an HRV to vent the same bathrooms, with an HRV in the mix, could I plug the plenum into the HRV? Thus, when the Panasonic fans are “off”, the HRV will draw air from the bathroom via the fans. When the fans are “on”, the HRV, (whether it is one or off – assuming no power dampers) will get positive pressure flow from the same ducts. The HRV should function in this set up. The HRV should also be able to deal with any condensation that accumulates in the ducts. Does this sound right?  Am I missing something? Should the fan’s on/off switch be wired to the HRV to secure HRV is “on” when any of the three fans are on?


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  1. this_page_left_blank | | #1

    Kind of confused. Where is the proposed location of the HRV in this setup, and where is the plenum? Because the HRV has to be within the thermal envelope. By your description, it sounds like the plenum is in the attic, and presumably the HRV would be fed by the plenum.

    You say the attic is well insulated, but not conditioned. Can you describe this attic? Seems odd that you'd insulate an unconditioned attic. If it's not part of the conditioned space, what is it being insulated against?

    It seems to me the whole idea of feeding three bathroom fans into a plenum is rife with problems. In order to make sure one bathroom fan doesn't blow into another, you need one way valves. In the event that two or more fans are running, there is going to be at least some amount of them fighting against each other. Balancing is going to be a challenge. The potential benefits for this system seem very small compared to the problems.

  2. Sal_123 | | #2

    Hey Trevor,
    I want to place the HRV in the attic. The attic is not your typical attic. Due to engineering issues to support the roof tiles, it is a myriad of intersecting planes that serve to reinforce the structure. No large open spaces, many pockets or irregular size and shape. That said, it is insulated, was gonna carve out a location for the HRV, set up 2" XPS walls and install a register in the floor of the HRV's space (ceiling of a hallway)to allow passive heating/cooling. The plenum and HRV would reside inside the space.
    I appreciate your points, and not being an HVAC specialist am looking for advice. The 4" ducts coming from the bathroom ceiling fans could be dampered to prevent/minimize retrograde air flow. Agree balancing will be a challenge. Alternatively, should each bathroom should have 2 ducts? One designated for the Whisperfans (with separate venting through the roof) and another parallel duct for the HRV uptake?

  3. joshdurston | | #3

    You can't use a series fan arrangement and achieve balanced controlled ventilation with any residential HRV or ERV that I'm aware of. You could consider removing the bathroom fans, since the HRV will perform the same function (with a lower CFM rate though). If you choose to keep the Bathroom fans with the HRV, they should be exhausted thru dedicated ductwork separate from the HRV system.

    The HRV should draw from the bathrooms thru dedicated ductwork (this ductwork could potentially be shared between multiple baths). You can install a control in the bathrooms that sets the HRV to high for 20-30minutes, most people would find this is adequate bathroom venting.

    Not sure about the attic install for the HRV, you risk losing some the heat recovery by placing it in a unconditioned space. The HRV typically has drains for interior condensation.

  4. Expert Member
    BILL WICHERS | | #4

    You can have long duct runs on exhaust fans, but the long run will add back pressure resulting in reduced airflow. Be sure to use rigid ducting to minimize the effects, flex duct is much worse. Bathroom fans are not usually intended to produce much pressure so the reduction in the capacity of each fan may be a lot.

    To prevent condensation on the outside of the ducts in the attic you need to insulate them. You can get insulation made for this purpose.

    I’m not sure you could ever “balance” the fans. Usually airflow balancing is to equalize supplied air volume between multiple vents fed from the same blower source, you’re trying to balance multiple blowers into one vent which is the opposite. I think the only thing you could do would be to ensure that your common plenum has the same total cross sectional area as at least two of the 4” ducts combined (assuming no more than 2 of the fans is likely to be running at any given time), and ensure that none of the 4” ducts face each other in the plenum. You don’t want any 4” duct to “blow down” one of the others while operating. If your plenum is large enough, and the 4” duct entrances to the plenum are all properly oriented, you may be able to Make the system work. You’ll still need dampers on each fan so that outside air doesn’t flow back into the house when the fans aren’t operating. I don’t think you’d be able to share a common damper on the plenum since it may be too large for any one exhaust fan to open.



  5. Sal_123 | | #5

    Thanks Guys for the input!
    Plan to keep the two ducting systems separate.
    I try to keep all ducting rigid as much as possible.
    I appreciate your comments
    - Sal

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