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  1. #1
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    Difference between MUA / Fresh Air / Supply Air

    First time posting so hopefully doing so in the right forum.

    We are a mechanical insulation company so my question may seem irrelevant for some but in our trade, the distinction between these ducts is very critical in determining whether they need to be insulated or not. Unfortunately, engineers have gotten lazy and they often like to group different systems under one label causing confusion for us insulators.

    I need to clarify the differences between a MUA, Fresh Air and a Supply Air duct.

    Let me know if the following is correct...

    MUA: Air from outside that is partially conditioned before supplying an area (production area) or a RTU/AHU unit.
    Fresh Air: Air coming directly from outside and is not conditioned.
    Supply Air: Air that has been fully conditioned (cooling or heating).

    Again, I understand that from the point of view of an HVAC installer, their may be no difference but from an insulation standpoint, it is important.

    I ask because we have a job where there is a MUA unit outside the building but the engineer renames the duct as Supply Air once inside the building with no other units on that line. To us, that is still a MUA duct. The specs says to insulate the Supply Air only. Typically, you never insulate a SA serving its own space. A MUA is usually not insulated or only the first 3m into the building. So I need to know, regardless of how the engineer labeled it, whether it's a SA or a MUA duct.

  2. #2
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    Aug 2002
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    You using really general terms for if insulation is needed or not and what type.

    I prefer to use Dew Point. If the duct surface in question is in an area where it is below the Dew Point then it needs to be insulated.

    Metal MUA ducts in a commercial kitchen may operate below the dew point so i would insulate it.
    PVC supplying a boiler probably not!

  3. #3
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    Thread Starter
    I'm not really concerned about what should or shouldn't be insulated. That is the engineer's job to figure out although from experience, we do know what should be insulated. The argument with the client is whether a duct that has a MUA unit as its starting point should be called a Supply Air duct or a MUA duct. Usually a MUA will feed another type of unit before the output is considered a Supply Air. But since this duct goes from outside and straight to a room with only a MUA unit on that line, our position is that it's a MUA duct.

  4. #4
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    Well I consider MUA as fresh air coming in to replace exhaust air being exhausted (I.e.) commercial hood system.
    I suppose though it could be replacing any exhausted air within its given space.
    Fresh/outside air I consider as air supply to fuel a gravity type apparatus.
    I also suppose it could be to provide the minimum outside air requirement.

  5. #5
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    Thread Starter
    Quote Originally Posted by VTP99 View Post
    Well I consider MUA as fresh air coming in to replace exhaust air being exhausted (I.e.) commercial hood system.
    I suppose though it could be replacing any exhausted air within its given space.
    Fresh/outside air I consider as air supply to fuel a gravity type apparatus.
    I also suppose it could be to provide the minimum outside air requirement.
    The problem I have is that for insulation purposes, when it comes to insulation specs, if done properly, each system will be listed showing what the engineer wants on each one as far as type of insulation, thickness and jacketing. When it comes to insulation typically SA is only insulated on concealed areas, FA often insulated entirely and MAU may or may not be insulated depending if conditioned and where it passes. This is why it is important not to group these ducts as just supply. Most don't but once in a while, an engineer does and it causes problems as in this case. Now the burden is on me to prove that what that system is specifically... SA, FA or MAU. If my initial assumption in my initial post is correct, then this duct is a MAU. My understanding is that supply air is air that has been treated for "room temperature". On this project, the air enters the MAU unit outside and goes straight to the paint booth. It's purpose is not to cool or heat the room and as such, not a supply.

  6. #6
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    Yes, for a paint booth with a exhaust fan I would consider that as a MUA.

  7. #7
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    Thread Starter
    Quote Originally Posted by VTP99 View Post
    Yes, for a paint booth with a exhaust fan I would consider that as a MUA.
    Thanks you! Now how do I convince my HVAC client and the engineer?

  8. #8
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    Convince an engineer?? Do you have a 4 year masters Degree form an engineering school? If not forget it even if it is in black and white.
    There is only one truly right way to do something, but there are thousands of wrong ways to varying degrees to do it.
    So the question is: If you don't do it right, then how wrong is it going to be???

  9. #9
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    Quote Originally Posted by Lukus2019 View Post
    The problem I have is that for insulation purposes, when it comes to insulation specs, if done properly, each system will be listed showing what the engineer wants on each one as far as type of insulation, thickness and jacketing. When it comes to insulation typically SA is only insulated on concealed areas, FA often insulated entirely and MAU may or may not be insulated depending if conditioned and where it passes. This is why it is important not to group these ducts as just supply. Most don't but once in a while, an engineer does and it causes problems as in this case. Now the burden is on me to prove that what that system is specifically... SA, FA or MAU. If my initial assumption in my initial post is correct, then this duct is a MAU. My understanding is that supply air is air that has been treated for "room temperature". On this project, the air enters the MAU unit outside and goes straight to the paint booth. It's purpose is not to cool or heat the room and as such, not a supply.
    Most MUA units in general do condition the air, not for the purpose of heating or cooling the space, but for the purpose of reducing the heating and/or cooling load in the space.

    My area is primarily heating so we don't see a lot of MUA's with cooling. Think about winter time and dumping 0F temperature air into a kitchen line up. If you did not condition that incoming air, you are adding to the heating load for that space because to keep the temperature comfortable a lot of that cold air has to be heated up. Most MUA units here are gas fired heating. I would consider the duct feeding the space from the MUA a supply air duct in that case.

    Now a paint booth is quite specialized case and is likely different, although I doubt paint dries well in freezing air.

    Sent from my SM-G965W using Tapatalk

  10. #10
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    Make up air may or may not be conditioned and is used in conjunction with exhaust.
    Fresh air is passive
    Supply air is conditioned to cool or heat the space.
    Ducts that supply conditioned air should be insulated. Ducts that supply conditioned air that run thru an unconditioned space need to be insulated or you risk serious condensation issues during the warm months and above average heat loss during the cold months.
    Officially, Down for the count

    YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET

    I know enough to know, I don't know enough
    Liberalism-Ideas so good they mandate them

  11. #11
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    Using insulation company's quite a bit it, usually the mechanical contractor that subs the insulator and they bid on what they need covered. For service work the insulator comes out show him what is needed and alot of times they pick the material it's their trade. Service also show the insulator other items that need to be addressed and a price so we can notified the customer. Spec jobs everything is spelled out. Most insulators after working in the field that i have dealt with know what is needed for most comfort cooling or heating in particular area of the country.

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