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What if …

wolley | Posted in General Questions on

I am studying the articles on GBA regarding home ventilation.
At this point it seems that supply only ventilation is right for me.
I will be getting a new HVAC system next month. Lennox SLP98 furnace, Lennox air conditioning and central dehumidifying unit. Dealer installed. Then do-it-myself adding a fan cycler and fresh air intake.
My son’s response to my plan was: “what if … you put in one 2″ pipe to bring in fresh air continuously to the return side of the HVAC system?”
I don’t know enough about home ventilation to answer him. What if …

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  1. heidner | | #1

    Location makes a big difference. A two inch pipe into the return air duct will be quite cold and energy in efficient if your house is located in Alaska, Vermont, Maine, (northern states). It might be reasonable in a dry south winter climate. But you didn't say where you live, the zone, etc.

    There are designs available for fresh air intake into the return ducts - with dampers that are activated by the furnace. .

    You do realize that by adding the pipe yourself after the dealer has done the calculations for for manuals J, D and S that you've just altered the system such that you may have paid for equipment and installation that is no longer correctly sized and may not function as expected.

  2. GBA Editor
    Martin Holladay | | #2

    You are describing (kind of) a well-known type of ventilation system called a "central-fan-integrated supply ventilation system."

    There are two important components you failed to mention, however: this type of system needs a motorized damper in the fresh air intake duct, as well as a Fan Cycler control to prevent underventilation and overventilation.

    For more information on this type of system, see Designing a Good Ventilation System.

  3. wolley | | #3

    Thanks for the info men!
    I am located in south east Missouri ... about 100 miles due south of St Louis. We have a 5 to 6 month air conditioning/dehumidifying season. For the past week outdoor temperatures have been mid 80's with humidity at 70% to 80%. My indoor humidity level right now is 65%, air temp. 73 degrees.
    In my post I said fan cycler and fresh air inlet which was short for fan cycler control and motorized damper.
    That said ... I know nothing about the "calculations for for manuals J, D and S" but will gladly educate myself ... can you point me to where I can find this information?
    Many Thanks for your informative responses!

  4. heidner | | #4

    Manuals J, D, and S are ACCA publications. Easiest place to find them is either at a public library or buy online. Hand calculations can be EXTREMELY tedious. Most HVAC designers/installers use software that is licensed (and often expensive) to design the system. In some states major changes to the HVAC system requires recalculation of at least Manual S (sizing), duct testing, and perhaps Manual J.

    If you are already having an HVAC company do most of the work -- why not ask if they can just add in the integrated ventilation system (economizer) as part of the design. It would then be sized correctly and if there any changes that need to be made to the controls on the HVAC the contractor would make them.

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