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  1. #1
    Join Date
    Mar 2007
    Posts
    10

    Confused Fresh Air Intake

    I run my furnace fan all the time and my filters (3M Filtrete 1000) generally are saturated after 3-4 weeks. It seems strange to me that they pick up so much dust, considering that we never open our windows (primarily due to allergies). We also keep our house (6 years old, the ducts have been cleaned) pretty spotless, so I'm not sure where all the dust is coming from. We run the fan all the time for two reasons: it lowers the level of dust in the house and it keeps temperatures more even throughout our 2-story (plus basement) house. Running an AirAdvice report didn't identify any high levels of dust, just low levels except when lots of activity is happening in rooms.

    I'm having an Aprilaire 5000 air filter installed in my home and while I'm at it, I'm considering putting in a fresh air intake (something like the Scuttle 216).

    I'd like your thoughts on fresh air intakes. The reason I'm considering this is I have very slight drafts around double hung windows (through the contact felt on the sides of the windows) and I thought that perhaps the intake will create enough negative pressure to prevent some of the air flow around windows and doors, thereby preventing some dust/drafts from entering my home. Also, I'm sure getting fresh air continually added to the HVAC probably is a good thing.

    My concerns:

    (1) Is a fresh air intake more trouble than its worth, especially near Chicago, with its hot summers and cold winters? Will I introduce significant workload to my AC and heating when it has to work overtime to heat/cool a steady flow of outside air?

    (2) If I run my fan all the time, how should I have the fresh air intake set up? Normally I believe it opens when the fan turns on and closes when the fan turns off. I'm not sure I need the intake open all the time, so I thought perhaps a timer (with an override for very hot or very cold days) would be best.

    (3) What diameter intake would you consider adding?

    Any thoughts regarding fresh air intakes would be appreciated.
    Last edited by rrz; 03-15-2007 at 08:27 AM.

  2. #2
    Join Date
    Mar 2004
    Location
    SE Michigan
    Posts
    17,995
    Im surprised that your home does not have a fresh air intake being only 6 years old. With tight construction, it is code that you have one here in MI.

    The fresh air intake will tie into the return side of the system.

    Most of the dust in your home are skin cells ( up to 70 % ). We loose 10 million cells per day.
    Behind the ostensible government sits enthroned an invisible government owing no allegiance and acknowledging no responsibility to the people.

    Theodore Roosevelt

  3. #3
    Join Date
    Mar 2007
    Posts
    10
    I don't believe I currently have a fresh air intake unless it's up on the roof...wouldn't it normally be a short run of ductwork from your return near your furnace to the nearest wall (which in my case would be just above ground level outside because my furnace is in the basement)?

    So if homes in MI require an intake due to code, does it present a problem to have the intake open all the time when you run your furnace fan, given the cold winter temperatures?

    Skin cells.....ewww.
    Last edited by rrz; 03-15-2007 at 09:07 AM.

  4. #4
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478
    Quote Originally Posted by rrz View Post
    I don't believe I currently have a fresh air intake unless it's up on the roof...wouldn't it normally be a short run of ductwork from your return near your furnace to the nearest wall?

    Skin cells.....ewww.
    Sometimes it is piped into the platform the unit sits on into the attic space. (If its in a closet.)
    You ought to see the bugs that are eating those cells.

  5. #5
    Join Date
    Mar 2004
    Location
    SE Michigan
    Posts
    17,995
    Most residential fresh air intakes are either 4 or 6 in. in diameter. They consist of an adjustable barometric damper that is connected to an insulated pipe which ties into the return air, They are usually mounted on the sidewall of the home and function when the indoor blower cycles.

    You may want to have a service technician take a look the next time you have maintanance done to your system.

    If your going to run the fan continuously, you might want to look into a variable speed set up. This can save you money in electric bills.
    Behind the ostensible government sits enthroned an invisible government owing no allegiance and acknowledging no responsibility to the people.

    Theodore Roosevelt

  6. #6
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    Quote Originally Posted by rrz View Post
    My concerns:

    (1) Is a fresh air intake more trouble than its worth, especially near Chicago, with its hot summers and cold winters? Will I introduce significant workload to my AC and heating when it has to work overtime to heat/cool a steady flow of outside air?
    Your heating and cooling equipment would need to be sized with fresh air intake in mind for both heating and cooling design days in order to avoid the indoor temperature from straying too far from a desired temperature. In summer, introducing fresh outdoor air imposes both a heat and a moisture load on the cooling coil. In winter, fresh air introduction not only is cold, but dry.

    Since you run your fan continuously, you have a concern with loading the house with moisture in summer (in addition to adding a sensible heat load between a/c run times) when the a/c is not cooling, and in winter, cold, dry air would be continually introduced. This is where a damper would be used in the fresh air intake to avoid those scenarios. Another choice would be to go with an ERV or HRV unit, depending on which works best for your area. These units will give you continuous ventilation but minimize moisture loading or cold air introduction factors.

    Any mechanical/forced introduction of outdoor air must be filtered.

    As coolwhip mentioned, consider converting your present blower to variable speed, since it runs continuously. You don't need to move air through the ducts at design velocities if you're just trying to circulate air with no heating or cooling going on. You'll save some bucks at the electric meter with variable speed.

    As for dust, a LOT of it comes from....us. Human skin shedding accounts for a good deal of household dust. Pets also add a heavy dust/dander load to indoor air. Infiltration from outdoors is another. Carpets shed fibers, along with furniture, clothing, curtains, etc. Carpets are also absolute magnets for catching dust, and when you walk across the thing the dust gets stirred up again. Personally we are on a plan at my house to expunge all carpeting over time and replace with solid flooring products with low VOC content.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  7. #7
    Join Date
    Jul 2006
    Location
    Southern Ca, Elkton Md
    Posts
    7,572
    The filter will Get dirty alot quicker when the fan runs 24/7. If you just ran on auto and you got 3 months out of the filter would you be happy? When you operate your fan 24/7 the operating hours is more then your run cycle times in 3 months on auto.

    And just because it says 3 months on those filters, dosen't mean they last that long. Its up to three months, so if its dirty change it. The filter can only hold so much, then you start to damage your equip, and let dirt into the system.

    I personally don't think the fresh air intake will help your filter stay cleaner longer, but it is a good idea to have one install.
    "Correct Installation is the Key"

    .1 has killed more HX then Rush Limbaugh

    What is your TESP?

  8. #8
    Join Date
    Jun 2005
    Location
    The Gray Northwest
    Posts
    661
    You say your filter is saturated after 3-4 weeks. Does that mean it's completely loaded and blocking air flow? Just because it looks dirty does not mean it needs replacing. The only way to know for sure is to have a differential gauge that shows the pressure drop accross the filter. Running your fan 24/7 will load up the filter faster, but it should last at least 3-4 months with a continuous fan. Keep in mind that in order for an electronic air cleaner to work effectively, it needs at least 4 feet of straight duct up stream of it. Do you have that much space? Also Bringing in outside air will not change the particulate acuumulation in your home. It's a good idea to have some fresh air to mix with the return but it will do nothing for your dust situation.

  9. #9
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    3M filters are too restrictive for most systems,and the clog very quickly.

    If yiour draft is coming IN from the windows ,only while the fan is running ,you likely have supply air duct leakage going ouside the conditioned space.This causes negative pressure in the home,so youget infiltration around opening in the building envelope.

    Best to seal the leakage ,and then add fresh air .

  10. #10
    Join Date
    Mar 2007
    Posts
    10
    My filters are so saturated that enough dust is being pushed through the media that it sets off my allergies.

    The variable fan idea sounds like a good one. Perhaps that can be tied to only opening the fresh air intake when the heating/cooling kicks in.

    How do ERV and HRV units fit into the equation? One of the concerns I have is that the fresh air will introduce too much humidity or dryness into my house, but I don't necessarily want to spend a fortune on ERV and HRV units either. Are ERV and HRV units really necessary for homes near Chicago?
    Last edited by rrz; 03-15-2007 at 10:47 AM.

  11. #11
    Join Date
    Nov 2006
    Location
    SC
    Posts
    930
    Some simple stuff, but you don't want to overlook the easy and obvious:
    1. Is the filter in backwards? If so, it will clog quickly. Look for the air flow arrows on the side and point them the right direction (towards the blower).
    2. Is your ductwork tight? Perhaps you are leaking air out the supply side or in to the return side. That would give you a big dust load.

  12. #12
    Join Date
    Mar 2007
    Posts
    10
    Thanks Coolwhip and Shophound, your answers were helpful.

    Overall, are fresh air intakes a good idea near Chicago? Or would you insist on an ERV or HRV?

    What precautions should you take, given the extreme temperature/humidity fluctuations (i.e., when you should keep the damper closed 100% of the time (e.g., below 20 degrees?).

    Do they create more problems for a HVAC system near Chicago than their benefit of bringing fresh air into your system?

    Again, I don't think I have a fresh air intake unless it's on the roof--have you ever heard of that location for an intake?

  13. #13
    Join Date
    Mar 2004
    Location
    SE Michigan
    Posts
    17,995
    In a tightly constructed home which you have, I would recomend that you have a way of introducing fresh air into the home. Ask your friends and neighbors to recomend a good heating and cooling contractor in your neck of the woods. They can come out and evaluate your system and go over options with you.

    Obviously, everything revolves around how much you want to spend on indoor air quality and the applications of your exhisting system.
    Behind the ostensible government sits enthroned an invisible government owing no allegiance and acknowledging no responsibility to the people.

    Theodore Roosevelt

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