Mini split duct problem
Hi,
I am adding two new rooms onto my house; one room is a 13×26 with one octagon window and the other is 7×10 with no windows. The two rooms are located next to each other and would be considered “renovated attic space”.
I had 3 HVAC bids provided and the cheapest one was over $5,500 so I decided to DIY the project.
I have done all the construction myself and since my budget was so tight, I decided to at least install thehva c and hire out someone to commission it. After a lot of research I purchased a Daikin fdxs12lvju concealed duct mini split. My original thought was that I could simply add a couple small flex ducts to connect each room and it would be done. However, I am now realizing that this unit’s static pressure is so low, that it may not support my requirements. The static pressure is .12 according to the documentation.
The way my duct is currently, I have 3 – 5″ ducts coming from the supply side of the unit. Each is about 20′ long. This allows me to place one register in the small room and two in the large room.
I know you will all beat me up for not hiring it out, but I couldn’t afford it so this is where I am at.
Does anyone have any good news for me? 🙂
I’m pretty much stuck with making this unit work and just have to figure out how to make it happen. Here are my questions:
1) Will the unit handle this amount of ducting as it is?
2) Would an in-line duct fan help get the air flow up enough?
If this setup is simply not going to work, I could install the unit hanging from the ceiling in the large room and then tie the smaller room into my main home AC unit. This option is not ideal as the smaller room is a converted attic space and I don’t think it would heat and cool well off the main house.
Thanks.
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Replies
I'd run a supply and a return to each room so that room pressure stays balanced. Yes, looks like you need bigger/smoother ducts or more fan at the highest flow rates:
http://www.freecalc.com/ductloss.htm
0.12 is very low. It's water under the bridge now, but you would have been better off with a unit with more available static pressure. Fujitsu makes units with 0.4 available. You're definitely going to be limited to large rigid metal duct with very careful design and installation.
You might consider locating or relocating the unit so that it's somehow on the common wall separating the rooms, which would allow you to make the duct runs extremely short. Dedicated return air grilles would also be possible..
Beyond that, you may simply have to accept a certain amount of inefficiency or poor airflow.
I hope you have the skills to handle the refrigeration side of the installation, or else the unit will perform poorly or be a very expensive brick in short order.
My personal recommendation would have been a pair of DIY-able ductless mini-splits. These things have pre-purged and pre-charged linesets with quick-connect fittings. Some can even be plugged into a standard wall receptacle. No need for ductwork, duct design, or refrigeration skills, and maybe even cheaper to boot (These cost as low as $700 apiece).
Thanks for your help! You are correct; water under the bridge now. I think I will move the unit to a new location to eliminate the duct altogether.
My plan has always been to hire an HVAC guy to actually commission it for me. I was just trying to save money on the install.
Flex duct (especially when not stretched tight) has a lot of duct impedance. Replacing the flex duct with hard piping (and oversizing it- bumping it to 6 rounds with a can help. With hard piping use only radius elbows- never the square throat ells with sharp 90-degree corners on the inside of the turn. Make the runs as absolutely straight as you can- every turn (however slight) costs you.
See:
https://www.greenbuildingadvisor.com/articles/dept/building-science/how-move-air-quietly-through-duct-system
https://www.greenbuildingadvisor.com/blogs/dept/green-building-blog/stuff-i-learned-joe-lstiburek-s-house-part-2
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/74831/The-2-Primary-Causes-of-Reduced-Air-Flow-in-Ducts
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/56018/Don-t-Kill-Your-Air-Flow-with-This-Flex-Duct-Disease
Return paths can be ducted or not, but must be large in cross section & low impedance too.