PACKAGED TERMINAL UNITS AND THROUGH-THE-WALL UNITS/ Mini Split system

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    Mark Baker

    Wow that is a great question that NCARB will not give you the answer to.

    Package Terminal Units - (PTACs) are a CHEAP option that have a "short" lifespan - id sy 10 years but I do not have a factual document to back that. I do however know in practice that PTACs (package terminal units) are the VALUE option for prrojects. They are not quiet or effiecient. They are cheap (in real life) and get put into projects where the owner needs to save money and does not want to use full roof top whole building options. (think: motels, apartments, and communal living facilities).

    Mark, Archizam - ARE 5.0 Practice Exams

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    Kawai Yam

    Thank you for replying mark. 

    yeah. I am just trying to find out what answer NCARB wants us to answer. I did find out a lot of subject matters have different opinions on different topics for NCARB exam. I guess I will take Wall units as "short" lifespan, "noisy" and "cheap" for now.

    Just my two cents tho, my parent's house has the same wall unit since I was little, maybe like 20 years+, I personally think it's actually quite quiet compared to my central ac in my current home. little fun fact. 

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    Gang Chen (Edited )

    A wall-mounted split unit is a valuable option for remodel job:

    I have done some residential remodel jobs where it is almost impossible to run ducts to a new room added. A wall-mounted split unit is ideal for this situation.

    Gang Chen, Author, AIA, LEED AP BD+C (GreenExamEducation.com)

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    Michael Ermann

    Kawai, remember that the compression-refrigeration cycle makes the coolth, and that a separate system of fans and pumps move coolth around. Each of these three systems looks like a box, so folks often get confused between them, but the technology inside them varies considerably.

    PTACs are essentially window units that are set into the exterior wall instead of the window. You've seen them in hotels, apartments, and hospitals. They look like this. They're self-contained because they have their own compression-refrigeration cycle and don't rely on a remote chiller and pumped chilled water.

    Advantages: Inexpensive, no ducts needed, super-easy to install (no trained HVAC professional needed), high level of thermal control (I can have mine on heat and you can have yours on cooling and someone else can have their's off)

    Disadvantages: Very noisy (fan and compressor in the room), ugly (visible both inside and outside), inefficient (all those separate compressors, each one only serving one room!), and that's a lot of compressors to run and maintain over time.

    Mini splits: smart split-system A/C with one outdoor unit connected to one to five indoor units, so one indoor unit for each thermal zone. You've seen them too, but primarily in newer buildings and newer renovations. They look like this. Each indoor unit--and the outdoor unit--can serve as the condenser (high-pressure refrigerant, hot) or the evaporator (low-pressure refrigerant, cold), so each indoor unit can heat or cool through variable refrigerant flow (VRF) technology. Think of an old-time telephone operator patching through calls. . . . VRF technology works like that, moving refrigerant to zones where heating is needed (high-pressure, hot) and moving refrigerant away from where cooling is needed (low-pressure, cold). Heat can then be moved from inside to outside, from outside to inside, or from room to room.

    Advantages: no ducts needed (just refrigerant pipes connect the units), can heat and cool different rooms simultaneously (because each unit can be hot or cold), efficient, the noisy compressor is in the outdoor unit, and the indoor fans are quiet (more because someone prioritized quiet in their design and branded them as such than anything inherent in this specific technology. . .there's no reason that fan coil units couldn't be just as quiet as mini splits, it's just that no one expects them to be so they aren't).

    Disadvantages: More expensive and more complicated to install than PTACs (but the price keeps coming down), needs a spot outside the building for that outdoor unit, and if you have 500 dorm rooms to cool, you'd need 100 of those outdoor units. . . so these systems don't scale well to very large buildings. It may be difficult to get fresh air into the building through this system necessitating a separate fresh air system.

    Fan coil unit: a remote chiller--on the roof, in the basement, or in some nearby building--makes chilled water and pumps it around to all the thermal zones. A remote boiler--on the roof, in the basement, or in some nearby building--makes hot water and pumps it around to all the thermal zones. Each thermal zone has a fan coil unit (FCU), which looks like this, sits in the ceiling connected to ducts, in a closet connected to ducts, or as a box in the room without ducts; each fan coil unit includes a chilled water coil and a hot water coil and a dumb fan that blows room air over one coil or the other when the thermostat tells the fan that the room is too hot or too cold. If it's a big residential building, hotel, hospital, etc. with different simultaneous heating and cooling needs, lot's of thermostats, there's a good way to get fresh air into each room, and you don't see grilles from PTACs on the outside under each window, there's a good chance each dwelling has fan coil units. . . likewise for spaces in large non-residential buildings--like a server room or storeroom--where fan noise isn't a problem and fresh air for occupants isn't important. In other occupied, noise-sensitive areas of large non-residential buildings, we'll typically install remote ducted air handling units that bring coolth and fresh air down the corridors, then branch into human-occupied office suites where one thermostat may control 10 rooms. People are grudgingly willing to give up control of their room's thermostat at work. . . but folks get weird when you take away their thermal control at home.

    FCU Advantages: no visible ugly grilles on the facade and no ugly outdoor equipment (other than a remote cooling tower for the remote chiller), no ducts needed (just hot and cold water pipes connected to a remote mechanical room), can heat and cool different rooms simultaneously (because each unit can blow over the hot or cold coil), efficient (because FCUs rely on large central equipment), quieter than PTACs (but noisier than mini splits when there are boxes in the room,...though they wouldn't have to be noisy if some company put some R+D into quieting the fans the way that the mini split manufacturers have). Relatively noisy FCUs may be just as quiet as mini splits with long duct runs between the fan and the room (but I don't think that NCARB knows that because that's not in any of the books they use). These are scalable and make the most sense for large buildings. Only one compressor to maintain for the whole building, located at the central chiller.

    Disadvantages: More expensive and more complicated to install than PTACs (lots of piping), each zone needs fresh air from somewhere, so we'll need to duct that in with a separate system or open a window.

    I wrote this assuming you own the concepts behind the compression-refrigeration cycle. If you don't, you can see the animated videos I made explaining that here, here, and here. (more than a million views on YouTube).--Michael Ermann, Amber Book creator.

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    Suchitra Van

    Great explanation Mr.Ermann thanks.
    In addition one can also question the infiltration problems in each one of this. Assume Mini split has least problem and PTAC has most infiltration problems being those are slid in. We had one in our high rise apt. It was always cold and we felt air come through the sides.

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    Shikha Subramanian

    So I've seen clients go with a ductless mini split system several times (at least thats what I'm told) but I still see in our mechanical drawings, ducts. Is this because mini splits do not provide fresh air and we need ducts to provide fresh supply air and take out exhaust (like kitchen smelly air and bathroom air)? Also, do split systems fall under an air system or a hydronic system?

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