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Ventilation

user-6323668 | Posted in Mechanicals on

I am building a net zero house, 2000 SF, 2 stories in Mystic CT. It has 2 full baths, heating and cooling by 2 mini splits, ventilation with an HRV, range hood in kitchen (not sure if it will have makeup air capability). Most of what I have read says to exhaust from bathrooms, laundry room and maybe the kitchen. Supply to bedrooms and living room.

Q1 – Will the ventilation system blowing air into the bedrooms prevent conditioned air from getting into the bedrooms?

Q2 – Can I use the HRV high mode in lieu of bath fans and can it be programmed to run automatically for a set time after the shower is used?

Q3 – what are the pros and cons of having (or not having) a makeup air unit for the kitchen hood fan?

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Replies

  1. STEPHEN SHEEHY | | #1

    Courtney: I haven't noticed any problems with the HRV preventing circulation of conditioned air into rooms without a minisplit. Remember, the HRV circulates a relatively small amount of air.
    My HRV has a boost switch on the bathroom wall. It has three options, 10, 30 or 60 minutes. It works well and we've not missed bathroom fans at all.
    Whether you need makeup air for the hood depends on how powerful the fan is, how airtight the house is and whether you have a wood stove or ither appliance that might backdraft.

  2. Jon_R | | #2

    An HRV supply duct to a room might be 10 CFM. An open door is more like 300 CFM. A bathroom with a shower might require 70 CFM to prevent condensation on the walls/ceiling.

  3. GBA Editor
    Martin Holladay | | #3

    Courtney,
    Q. "Will the ventilation system blowing air into the bedrooms prevent conditioned air from getting into the bedrooms?"

    A. No. Ventilation air flow rates are quite low compared to space heating air flow rates or cooling air flow rates.

    Q. "Can I use the HRV high mode in lieu of bath fans?"

    A. Yes. For more information on this issue, see this GBA article: Does a Home with an HRV Also Need Bath Fans?

    Q. "Can it be programmed to run automatically for a set time after the shower is used?"

    A. In most cases, yes -- but if you have any doubts, talk to the HRV manufacturer.

    Q. "What are the pros and cons of having (or not having) a makeup air unit for the kitchen hood fan?"

    A. If you have a range hood fan that is rated at 400 cfm or more, building codes mandate the installation of a makeup air system. For more information on this topic, see Makeup Air for Range Hoods.

  4. user-6323668 | | #4

    Martin,
    In answer to my question "Will the ventilation system blowing air into the bedrooms prevent conditioned air from getting into the bedrooms?"
    You said "No. Ventilation air flow rates are quite low compared to space heating air flow rates or cooling air flow rates."
    Would this still be true if the mini-split is quite a long way from the master bedroom?

  5. GBA Editor
    Martin Holladay | | #5

    Courtney,
    During the winter, ventilation air introduced by your HRV will be entering your home at a temperature that is lower than the indoor temperature. This is an unavoidable fact. In most cases, however, the introduction of this ventilation air doesn't cause comfort complaints, as long as the system is well-designed.

    If you have an HRV that is (for example) introducing 70 cfm into your house, and the air is being directed to 3 bedrooms and the living room, then each supply register will be introducing about 18 cfm of air. That's not much.

    The designer of the ventilation system needs to locate the supply registers with comfort in mind. You don't want the air to blow across the pillows of the bed. In many cases, the fresh air is introduced into a closet in the bedroom, and the closet doors are louvered or undercut.

    If these common-sense rules are followed, comfort complaints are rare. That said, you can still end up with a cold bedroom when you depend on a minisplit that isn't located in the bedroom -- an issue that has been discussed on about 500 GBA pages. The solution to that issue is to leave the bedroom door open during the day, and limit the area of the bedroom windows.

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