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Effect of stack vent, range hood vent, etc. on efficiency

Kevin Hoene | Posted in Green Building Techniques on

Hello,
We are building a new home in central IL. If you recall, we have an EPDM roof with 1:12 pitch and 10″ rigid foam above the roof deck. The vertical walls are insulted all the way to the roof leaving conditioned space in the attic. The exterior will have 2″ rigid foam and EIFS.

My question is about how stack vents and other vents affect the efficiency of a home. I realize stack vents are necessary, but I hate seeing 3 pipes run through my roof after spending a lot of $ on rigid foam above the roof. Are stack vents equipped with a damper or have some way of preventing outside air from entering our home?

Also, ductwork is being ran out the back of the house for our range hood. Same questions about maximizing efficiency for this vent.

I have read about tight homes creating situations of negative pressure. Should an HRV/ERV be installed to prevent this issue? If so, is the HRV/ERV ducted out through a wall in the conditioned attic space? We have not drywalled anything yet, but I don’t want to create a hole in our roof for a vent.

Thanks for all your help Martin and others!
Kevin

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Kevin,
    I wouldn't worry too much about plumbing vents. These pipes are not connected to the indoor air of your home -- only to the municipal sewer system or your septic system -- so they do not contribute to air leakage (as long as the perimeter of each pipe is properly sealed).

    Here is a link to a Q&A thread on another site that discusses joining multiple vent pipes in an attic before creating a single vent to leave the building:
    http://www.plbg.com/forum/read.php?1,157250,157250

    According to a source on that site, "Under the UPC, a 2 inch [vent] can run a horzontal maximum of 40 feet and 3 inch [vent] can go 70.67 feet. There is a note, though, that says, 'When vents are increased 1 pipe size for their entire length, the maximum length limitations specified in this table do not apply.' What will apply isn't stated."

    Check with a plumber and your local building department before you connect a lot of vents in your attic with horizontal pipe -- you want to be sure that you comply with your local code.

    Some Passivhaus builders go to great lengths to avoid installing a vented range hood. Again, you need to make sure that your local building department agrees with your plan. The standard Passivhaus solution is to install a recirculating range hood fan that draws air through a charcoal filter before returning the air to the kitchen, along with a ceiling grille installed in the kitchen (far from the range) that is connected to an exhaust duct of the home's HRV system.

    Whether this approach makes sense in your case depends on (a) your local code, and (b) your airtightness goals.

    Q. "I have read about tight homes creating situations of negative pressure. Should an HRV/ERV be installed to prevent this issue?"

    A. Every tight home needs a mechanical ventilation system. What type of ventilation system are you planning? Here is a link to an article on the topic: Designing a Good Ventilation System.

    As long as you aren't installing any atmospherically vented appliances inside your home -- and you shouldn't be doing that -- occasional depressurization shouldn't cause any problems. What you want are sealed-combustion appliances.

  2. Stephen Sheehy | | #2

    We installed a recirculating hood above our induction cooktop. It works fine. When we are cooking something that's likely to smoke or generate a lot of water vapor, we just hit the HRV boost.

  3. John Clark | | #3

    My 2 cents. Re-circulating range hoods will always be a compromise. Even those using a charcoal filter media have average performance.

    Your goal is to use the smallest fan possible and there are three things you need to make that possible. #1 Exhaust pipe properly sized (don't go small) with straight runs to the outside with no more than two 90 degree bends. #2 The hood must be no less than 6" wider than your range. #3 Switch away from gas to an induction range.

    Choosing an induction range over gas is probably the most important because you don't have to account for the heat generated by the gas burners.

    Typical fan sizing for induction is 100cfm / 10 inch in width of your range. Gas is total BTU of the range divided by 100.

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