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Community and Q&A

Warming passive make-up air for exhaust only ventilation

Oak_Orchard | Posted in General Questions on

I need advice on where to locate and how to condition (or at least protect occupants from the cold flow) incoming make up air. I need to bring the rigid vent pipe on the exterior (unconditioned crawl space) under the floor and enter through my 12” insulated floor baysup. There is no suitable wall or soffit entry point for the intake. I can install more than one intake if necessary.

I have 3 draws: (1) an electric clothes dryer, (2) a single 80 -120 CFM bathroom exhaust fan and (3) a 120 CFM range hood; plus a small wood stove. The Prestige boiler is direct vented.

Is an 8” diameter intake sufficient or too much? (the space is 1000 sq’; 14” cathedral ceilings; Lake Ontario, NY; caulked-to-death envelope; airtight drywall; HD cell insulation; 2 occupants)

Do I need more than one passive, make up air, intake vent?

How do I warm the incoming air?

In my situation, the air cannot be dropped on or behind a radiator and I do not want to use an independent or dedicated heater for this purpose.

Would it be good to empty the air into a closet – a laundry closet for example – or perhaps to run rigid pipe or a plywood chase upwards (from the fresh air intake at the floor level) towards the ceiling and allow the air to mix as it spills out ? I could use through the wall vents to allow a feed of fresh air into adjacent rooms. I assume an interior vertical stack like this would increase the fresh air inflow rate and add to air exchange even when appliances and fans are not running.

Do need to use a 24 timer to turn on the bathroom fan for routine cycles of exhaust-only ventilation in the heating season?

By the way, I spoke with the Aldes tech and he says passive vents are not sufficient to overcome stack effect or supply sufficent volumn of air for the range, cloths dryer and bath fan to work to their efficiency ratings. He advise power venting the make up air and linking the supply side fan electronically to the other exhaust fans. I am inclined towards the passive air supply / exhaust only solution.

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  1. GBA Editor
    Martin Holladay | | #1

    Oak Orchard,
    You don't mention whether or not you have performed a blower-door test, so we don't know how tight your home's envelope is.

    If it is Passivhaus-tight (0.6 ach50), then you probably shouldn't have a wood stove.

    If it is leakier than a Passivhaus, then you may not need any makeup air -- at least not for the clothes dryer, the bath exhaust fan, and the small range hood. That leaves your wood stove.

    Option one: live in your house for a while without a makeup air duct, and see how it operates. If you don't have any backdrafting problems with your wood stove, you're all set.

    If your wood stove back drafts, you should consider bringing in ducted makeup air to a location near the wood stove. I would include a damper to allow you to shut off the duct when it isn't needed.

  2. Oak_Orchard | | #2

    "If you do install an exhaust-only ventilation system, don’t bother installing passive fresh air inlets in the walls. Fresh air will find its way into the home through random cracks." MH, GBA Advisor

    "Every owner-builder thinks he has the tightest envelope in town." Oak Orchard River

    Not a Passive House, I just like the odds of super insulating, back-foaming and caulking everything, including interior wall penetrations. To avoid concrete, I floated the structure on cantilevered posts. Was hoping for a blower door test by insulation contractor but they want a fortune for the whole job … so that will be another DIY phase in this project.

    I would rather install the make-up air vent (or vents) now while under construction for obvious reasons. It's that control neurosis of mine …. I would rather control the make-up air intake location rather than rely on infiltration. Easier to close off these vents later than install later.

    If the building is not tight enough to warrant passive vents, a back-draft damper would not even open in the winter.

    Where should the opening of a passive make-up air vent be oriented? Leeward (negative pressure) or windward (positive pressure) side of the house?

    According to this site and others, make-up air for the wood stove is the lowest priority (compared to the other air-exhausting appliances) and probably not needed at all.

  3. Expert Member
    ARMANDO COBO | | #3

    I’m always against exhaust only ventilation because it creates negative pressures in the house. It means that pollutants and VOCs are not allowed to be removed for a good Indoor Air Quality, and if you live in an area where Radon is present, you could have a recipe for trouble. I design all homes with supply only or balanced ventilation.

  4. Oak_Orchard | | #4


    That would mean using a supply side fan/blowing air into the house. So which is a better design: a negative pressure or positive pressure ventilation method in a tight house?

    Exhaust-only mechanically driven air will, if the openings are provided on the other side of the house, winter or summer, clean it and cool it faster and therefore cheaper than the other way around.

    Pressurizing the house mechanically with supply-side only ventilation will build up more pollutants, humidity and condensation (change the dew point, mess up your attic or cathedral ceiling insulation) in the envelope and probably make the occupants' experience less pleasant.

    You can do the experiment yourself.

    If you want to pull more air through a house with a fan, put the fan in the window sill blowing to the outside. Open a window (set to a smaller area than the opening where the fan is set) in the opposite side of the room or building. Observe the flows and pressures.

    Then reverse this arrangement: turn the fan so it blows in with your same second window opening at the other side.

    Observe that the rate and volume of air flow with the exhaust-only (i.e. blowing out with the fan) set up is greater than the rate and flow of the supply-only set up.

    To offset the positive pressure disadvantages of supply-only ventilation, you will need to increase the area of the passive opening on your supply-side approach (i.e. to relieve the pressure and air volumes). That means more heating and cooling costs, less comfort due to higher internal pressures and more humidity drive into the envelope.


  5. davidmeiland | | #5

    Armando, how would exhaust-only ventilation prevent pollutants and VOCs from being removed?

  6. Expert Member
    ARMANDO COBO | | #6

    I look at it differently. Apparently you guys do not have a problem with a balanced system (my preferred way), which has an HRV or ERV, right? Well, on a supply only you can bring air from the outside into a plenum passing thru a good filter (MERV 9+) and a tempering system, although not as common. The positive pressure in the house will prevent any outside pollutants not to get sucked inside by the negative pressure, and for indoor pollutants to escape thru the kitchen exhaust, dryer vent, any holes in the building enclosure, and exterior doors when used. We've used this system a lot in the SW to prevent the desert sand from coming inside. It works great in a moderate climate; in a cold climate you may need to install a tempering system.

  7. davidmeiland | | #7

    Armando, by outdoor pollutants, I assume you mean anything that would get filtered by your filter. Other stuff will simply get moved in via the ventilation system as it would due to negative pressure.

    At a conference yesterday, a presenter on ventilation stated that a house under negative pressure acts like a MERV 6 filter, which I found amusing and somewhat logical.

    In my own house, very economical exhaust-only seems to work fine. I've done plenty of humidity datalogging to confirm that moisture is at acceptable levels. I know where the incoming air sources are, they are significantly intentional. The attic and crawl are impeccably clean and dry. My cost of heating is low, so a HRV would take decades to pay back in energy use terms. Rural air near the ocean is clean, so I don't worry about what's floating around in it. I may still install a HRV at some point because filtration would probably be a benefit (we have a dog, for instance).

    That's my own house, which I operate. Any new construction that I do at this point is going to have a HRV, because I think that will be a better fit for the average non-technical owner.

  8. Expert Member

    Armando, that's an ingenious solution to keeping sand and other undesirable outdoor pollutants out of houses in your region. In more severe climates I would imagine more of a problem than the need to temper the air would be that the positive pressure increases the exfiltration of moist warm interior air into the wall and roof structure with all the attendant difficulties that can cause.

  9. Expert Member
    ARMANDO COBO | | #9

    You make a good point, Malcom, if one would allow high humidity in the house, but with usual leakage of 1ACH50, I don’t worry on using the walls and roof systems as a filter, and mechanical ventilation should take care of any interior moisture; and yes, it is easier in the SW. With supply only ventilation in colder climates I’ve used tempered systems; and in hot humid climates, I have not used supply only, just the balanced ventilation method with an ERV.

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