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Concealed duct minisplit in attic, 2 rooms for a total of 470sf….NJ

phanley | Posted in General Questions on

Hi Folks,

I currently have a Fujitsu concealed duct system which was installed 15 yrs ago, and has been leaking for the past three years. HVAC tech said that he could replace the line set, but that didn’t make sense to me since he reflared and reconnected the ends, and it still leaked. 

So, I figured it might just be time for a replacement. The question is, right now the system heats and cools two rooms, using a two head system. One is 12k the other is 9k, both mid static. The smaller room always got damp, it seemed like the 9k unit might even have been too big and was short cycling perhaps, even though the heads are mounted in the attic, and connected via about 8-10 ft of flex duct. 

The question is, if I replace this system, would a high static single head design make sense, given the low amount of space it would try to cool, and just run a pair of supply and returns for each room from it, or would that annihilate the necessary CFM? I was thinking 18-24k BTUs.

I know very little if anything about this stuff, so I just want to make sure I can at least educate myself enough to know what’s going in there might be “right sized” so to speak.

Thank you for your help!

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  1. ohioandy | | #1


    Other than the leak, if the system was working well it would be a shame to trash it. A 15-year-old lineset may need to be replaced, especially if the tech reflared the connections at both ends and it's still leaking, but this is where it gets tricky; there may be internal leaks in the units, which can be as hard to spot as lineset leaks buried in the wall.

    If you decide on the upgrade, it's a great chance to get a better system. A total of 21K of capacity for under 500 sq.ft. is absurdly oversized in almost any circumstance. A single mini-ducted, right-sized and (if possible) mounted cleverly BELOW the ceiling in conditioned space, would serve far better. Plus, collect the $2,000 tax credit on an efficient model.

    You didn't say which Fujitsu models you have. If you actually have the mid-static, that's overkill. Serving only a single room, unless the installation is designed and installed by a sloth, a regular model will do. Typical residential installs involve only regular and mid-static. Check your model numbers and nomenclature. And FOR SURE, get an approximate idea of your actual heat/cool loads before accepting a bid on another oversized monster!

    Good luck!

  2. phanley | | #2

    Hi OhioAndy, thank you for your reply.

    The current system is cooling two separate room with two separate heads. The system is comprised of one Fujitsu ARU12RML and an ARU9RML connected to a AOU24RML. The blowers are in the attic, and flex ducted into the two separate rooms below, both rooms equate to just under 500 sq ft.

    It's a 50's home, and the top floor is tiny (as shown above), which does not provide me with much flexibility other than shoving units into the attic, or somehow squeezing a pair of small floor/wall units in each space. So, I opted for the less intrusive mounting in the attic.

    To be honest, the system never really worked properly. The smaller of the two rooms was always damp (aforementioned potential short cycling), and the system never really measured temps well at either the controller or in the blower.

    The smallest one can really buy in this case are generally 7-9k units, and that's why I figured "downsizing" to an 18k single head system might be the answer for this small space. However, the tech would have to send two ducts for supplies and two ducts for returns off of the single head. So, I wasn't sure if moving to a single blower would work since each flex duct would face about two 90 degree turns before getting to the registers.

    I'm also considering the fact that the heads won't be in a conditioned space, even worse, an attic which probably "sees" 15oF on a 95 degree day (single ridge vents along the top). So I figured 18k would be reasonable, and the duct work plus vent dampers would control flow to each of the two rooms.

    1. ohioandy | | #4

      "However, the tech would have to send two ducts for supplies and two ducts for returns off of the single head. So, I wasn't sure if moving to a single blower would work since each flex duct would face about two 90 degree turns before getting to the registers."

      That kind of install is perfectly reasonable; this is not rocket science but you need a tech who does it right.. for example, NO FLEX. Plus, you gotta downsize. 18K is still way too much. As you'll read on this forum, people who attempt to cool small bedrooms with individual heads face no end of trouble. Fujitsu mini-ducted can be mounted vertically. If this is a Cape Cod, I assume there's a small hallway between the bedrooms; put a single unit against a wall there with a common cold air return (undercut the bedroom doors to ensure airflow) and run a supply to each room. It's simple, but I also understand how hard it is to find a contractor who's willing to play with mini-ducted.

  3. mgensler | | #3

    The cool thing about Fujitsu slim duct is you can mount them vertically. Is there a closet in a hall or in one bedroom that shares a wall with another? Getting the equipment out of the attic is key. My guess is you can probably get one 9k or 12k slim duct to handle it. You'll save $$ on operating costs and a properly sized unit will dehumidify much better.

  4. phanley | | #5

    Thanks folks.

    THere is a very small landing at the top of the steps. From that landing, you're surrounded by 3 doors. One for the bathroom directly in front, then to the left and right, the bedrooms. We actually have a small supply from the 9k mini split ducted there, with another supply and the return for that 9k unit in the smaller bedroom.

    The 12k unit handles the other bedroom, it's about twice the size of the smaller room, dedicated supply and return, performed fairly well, but again, damp. This is what lead me to believe the system was oversized to begin with.

    Unfortunately, there's no shared wall between the two bedrooms. I was also concerned that placing a single return in the hallway/landing would bring in too much heat from downstairs and throw off the unit's temp measurements or overwhelm the unit, not sure if that concern is justified.

    I assumed a pair of 8" ducts for supply and the same sized pair for returns. The tech I have said he would do it, but it would cost $1500 more than a rip and replace with another pair of units, but I still think that in the end, the cost of the single line set vs a pair, and a better sized system would provide an ROI in no time.

    The problem with going down in BTU is that you also go down in CFM. If he has a few 90 deg turns in there, I'm worried about losing too much CFM. I felt like the 18k units were the best compromise, not sure if that makes sense though.

    1. mgensler | | #6

      Maybe mount the slim duct to the ceiling of the landing and then build a dropped ceiling around it. If you have them use a nicely radiused double tee, your friction losses should be ok.

      We have a 12k slim duct that does 3 bedrooms, 2 baths, a living room, and foyer. The 2 bedrooms at the end of the run actually have the highest airflow.

      The key to proper sizing is the unit will run pretty much continously when you have it turned on. That way you will get good dehumification and it should be extremely quiet.

  5. phanley | | #7

    Thanks for your reply!

    I'm very surprised 12k can cool that much space, must be a straight shot throughout the duct. May I ask which air handler you're using for that? I'm not sure I saw any 12k units that I would have confidence to push air over a run that long, but perhaps that's really the solution.

  6. mgensler | | #8

    It's the Fujitsu cold climate slim duct unit. It has one of the main trunk supply runs doing a 180 degree turn and then a 90 before the supply boot. Longest run is around 38'. You can hire an engineer to design the ducts or find enough info on this site to diy.

  7. phanley | | #9

    Thanks for this info. It really appears that my space won't need much then. I'm really beginning to consider the 12k units now.

    1. ohioandy | | #11

      This is not a matter of opinion. Equipment sizing is an objective thing that you should calculate. Duct design is an objective thing that should also be calculated. And know that the old conventions and rules of thumb used by most HVAC salespeople were problematic even with old conventional systems; you're using mini-ducted units which allow much more targeted and even distribution, but ONLY when ducted properly and not grossly oversized.

  8. walta100 | | #10

    Consider 2 separate smaller systems. If you did 2 12,000 btu or even smaller 6000 they would run without cycling at 10% each and you could turn one off.

    Your old multi systems can’t run below 33% or 8000 BTUs.

    Be sure your bid includes wall mounted hard wired remote temp sensors for both units.

    Please be smart and keep your HVAC equipment out of the attic.

    To my ear changing the line set without proof that it is leaking part would be foolish.


  9. phanley | | #12

    Thanks folks. I don't have a way to get the unit out of the attic, but I do believe that I am going with a 12k Mitsubishi system. Their PEAD heads seem to be just what this configuration needs, with a good balance of CFM and pressure. I almost pulled the trigger on the 15k system just to get the extra 115CFM that it offers, but my tech said he doesn't believe I'll need it.

    I was going to get a manual J calc, but I'm not entirely certain what insulation is in the walls, though the attic has blown in, so I suppose some must have made it into the walls. :)

  10. davidsmartin | | #13

    I faced a similar problem in a newly constructed 2 bedroom cape cabin. There is a bathroom between the two bedrooms and I tried to convince the installer to put a small ducted unit in the closet with two five foot ducts to the bedrooms. He wouldn't consider the idea and wanted to do a multi-split. So we compromised and put separate 6K units in the bedrooms, each with its own outdoor unit. We haven't been through a winter yet (in Vermont) but I am hopeful that it will work well

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