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Thread: Fresh-Air.

  1. #1
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    Fresh-Air.

    How pedantic are you guys/gals with Fresh-Air intake on AC Systems. Is it mandatory by regs or standards in your parts.

    Here in OZ it is a requirement to drag in Freshair.
    The primary function of the design engineer is to make things difficult for the fabricator and impossible for the serviceman.

  2. #2
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    Fresh air requirements are based on a number of factors . Classification of occupancy is a main factor in determining FA

    requirements. This then determines if you base the FA on occupants or square footage. Next you consider if the space has

    exhaust fans ,and also consider air changes per hour . So to answer your question for AC fresh air is required and there are

    regulations for its application. These are set out by ASHRAE.

  3. #3
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    Our contractor fear fresh air because of the inability to control humidity. What do you do to control %RH?
    I suggest supplemental dehumidification.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  4. #4
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    In commercial, Mechanical engineers will size the equipment and duct work with outside air requirements in their calculations.

    residential 99% dont

  5. #5
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    Measuring indoor air quality ie CO2 and goes a long way in reducing operating costs in relation to fresh air. As teddy bear

    mentioned pre conditioning the air helps in this process. It all depends on your local. For example teddy bears concern

    regarding humidity are completely different to say somebody who lives in Arizona.I live on the wet coast so concerns about

    temperature and humidity and fresh are totally different.

  6. #6
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    Why afraid of fresh air at units?
    No fresh air and building could be negative. Then unfiltered fresh air comes through the doors when opened. Better to go through unit.

  7. #7
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    I have a couple of 30,000 cfm AHU's. Minimum OA is a tad over 50%... We're basically exhausting all of the cool or warm return air outside. So when you do the math, it can be quite expensive to run large amounts of OA.

    And again, as TeddyBear mentioned, controlling humidity in the summer is extremely difficult. On paper, the engineers designed it perfectly....in the real world, not enough reheat capabilities within the AHU and no VAV reheats.

  8. #8
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    Everything looks great on paper!

    I don't quite follow. The oa should bring fresh air in. A power exhaust would send it out. Not being a smart ass just trying to understand. A heat recovery wheel would help that situation.

  9. #9
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    I love fresh air, when i can use it. But when its VERY hot or VERY cold, I will shut down my oa to help with the unit. The only problem with that (like i said before) engineers size the units with oa. when you shut down oa, the fan will draw in air from where it can get it, around the curb, through the doors and in most cases its unfiltered. So in some cases closing oa will help Humidity and temps but in others it wont. If you draw in and its not pulled over the coil, you can make things worse with higher humidity and temps.

    I have been trying to get new build outs not using economizers on RTU and having a separate HRV's instead to meet oa requirements but its a battle to change the norm.

    I love to purge a building in the morning, smells like victory

  10. #10
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    I can see a 5-10% on Commercial,but you 'figure in' more and unless your in a 'temperate zone' is a very heavy beast of burden,imo,costing way more than the lack of.
    So,how do you 'compromise' a bldg or resi in Houston,TX? Or,anyplace South?
    Yeah,I get the early a.m.,fresh air gig,but come 9:00a.m,all 'fresh' is a loss. from May till Nov.
    Yeah,I know....Blah,Blah,Blah.
    No matter how long you have been doing this,
    Go back and reread the Basic's.You WILL Learn something.
    Why is it called,an Act of GOD when IT has Nothing to do with Him?
    Will of the Devil would be more appropriate IMO.Just Saying.
    PSALMS, 18 & 25.
    I am Tired of High Efficiency Propaganda.

  11. #11
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    The problem with depending on the heat/cool equipment for ventilation is for fresh air to be introduced the fan needs to run. Except for larger commercial equipment most don't run a constant fan.

    I know there are some work arounds for this but usually $$ Then there are people complaining about being uncomfortable.
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

  12. #12
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    The State of Arkansas has adopted the International Mechanical Code. So by law there must be ventilation air in non-residential buildings.
    Could be a dedicated OA system. More usually the HVAC systems are intended to bring it in. The local engineers are good about following
    Code but contractors and technicians seem to only care if ventilation air is specified on plans. Inspectors don't seem to care much.
    I see new systems that don't have ventilation air pass inspection, and there is no other ventilation air in the building. And many residential
    thermostats are installed in non-residential buildings.

    According to the International Mechanical Code:
    “Mechanical ventilation systems shall be provided with manual or automatic controls that will operate such systems whenever the spaces are occupied.
    Air-conditioning systems that supply required ventilation air shall be provided with controls designed to automatically maintain the required outdoor air
    supply rate during occupancy.”

    The only way I see to satisfy "designed to automatically maintain... during occupancy" is with a programmable fan function
    (On during Occupied, Auto during Unoccupied). If the thermostat doesn't have a programmable fan you would have to depend on the occupant
    keeping the Fan in the On position, which is not an automatic control. Or jumper the controls to make the fan run all the time.

  13. #13
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    Inspectors don't seem to care much.
    As for 'Inspected' jobs,
    Guess you haven't ran into 'Richard' lately?
    Staties may be 'different,but most Major munisip's are BIG on this game.
    No matter how long you have been doing this,
    Go back and reread the Basic's.You WILL Learn something.
    Why is it called,an Act of GOD when IT has Nothing to do with Him?
    Will of the Devil would be more appropriate IMO.Just Saying.
    PSALMS, 18 & 25.
    I am Tired of High Efficiency Propaganda.

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