[mattnkara]

Hi,

I am adding two new rooms onto my house; one room is a 13x26 with one octagon window and the other is 7x10 with no windows. The two rooms are located next to each other.

I have done all the construction myself and since my budget was so tight, I decided to at least install the hvac 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?
3) If so, what "auto" booster fan would you recommend?

Thanks.

 

[pecmsg]

Good luck
Warranty Void on Internet sales.

 

[rider77]

There are some ways you can increase static, slightly. However I'm not familiar with this product. Maybe contact the technician support line. Though they are likely to tell you they don't offer support to DIY installed equipment.
Your really trying to do something with this unit it wasn't really designed to do. Where did you come up with that duct size? What's your heat loss and heat gain for each room? What static did you design the duct system to run at? How did you determine the tonnage needed for this space? What did you spec the design temperature at?
Is your commissioning guy no help in resolving this problem?

 

[hedrash]

Daikin ducted units have VERY strict low static tolerances. You'll need to redo to ductwork to maintain .08 MAXIMUM otherwise you're not going to get it to work. I'm assuming the company you used to commission this was a Daikin rep & pointed this out to you?? Or did you bypass a Daikin certified rep to save money.
One thing about saving money..... it costs more in the end.

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[pecmsg]

. I'm assuming the company you used to commission this was a Daikin rep & pointed this out to you??

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Now that's funny!

 

[rider77]

Just a question, going off the above question. Did you hire someone that specializes in these units to perform startup? Or was it someone you found on the side?
Do you still have 60' of flex on this unit? Because you'll want to have the absolute minimum amount possible, flex kills airflow. What's your current static on the supply duct?

 

[hedrash]

It's DIY I wouldn't go much farther. This is why you hire a professional that's certified in specialty equipment.

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[mattnkara]

Just a question, going off the above question. Did you hire someone that specializes in these units to perform startup? Or was it someone you found on the side?
Do you still have 60' of flex on this unit? Because you'll want to have the absolute minimum amount possible, flex kills airflow. What's your current static on the supply duct?

You have been much more help than any other person on this thread. I am a professional IT engineer and if someone gets into something in IT that they lose control over, I am glad to offer some guidance where I can to help them out. I don't see why everyone reverts to "you should have hired a pro". I had 3 bids form "pros" and the cheapest was over $5,500 to heat and cool two small rooms right next to each other. I simply didn't have the money to do that, so I am trying to DIY everything. Thanks for your help.

To your specific reply above, yes, I purchased this product based on calculations that I knew to use such as the amount of square footage that would be heated and cooled; the insulation R value that would be utilized for this space, etc. I had considered purchasing two ceiling cassettes, one for each room. However, the second room is so small, the smallest ceiling cassette would have still been way too large.

I decided to use the concealed duct (obviously it was a bad choice) because I thought it was designed for this type of application where I could run a limited amount of duct between the two rooms.

As far as the duct size I chose, I didn't realize that it would be so critical to use a specific size for this small of an application. Obviously I was wrong.

Today, I ran a duct calculation and it says that I would need to run a duct size of about 8"? Does that sound correct?

Looking around online, it seems that I may be able to help my problem with an in-line duct booster fan on the supply line and the return line? I didn't know if that would help or not so I thought I would post the questions here. Is my primary problem that I don't have the CFM necessary to push the air? In my reading it seems like I could remedy this with a current sensor relay and two duct booster fans? Maybe that's not a good idea?

Another option for me (although I don't really like this one) is to do away with all the duct work and hang the unit from the ceiling in the first 3rd of the biggest room. However, this means I would have to run some duct from my main home AC unit to the smaller room (which is in the attic) and I didn't think that would work very well.

Any additional help would be appreciated.

Thanks.

 

[rider77]

Booster fans are a huge waist of energy, and never really solve anything. Running that unit with no duct, will cause issues as well. It isn't meant to run with no static.
Your going to kill that poor unit.

 

[hedrash]

You have been much more help than any other person on this thread. I am a professional IT engineer and if someone gets into something in IT that they lose control over, I am glad to offer some guidance where I can to help them out. I don't see why everyone reverts to "you should have hired a pro". I had 3 bids form "pros" and the cheapest was over $5,500 to heat and cool two small rooms right next to each other. I simply didn't have the money to do that, so I am trying to DIY everything. Thanks for your help.

To your specific reply above, yes, I purchased this product based on calculations that I knew to use such as the amount of square footage that would be heated and cooled; the insulation R value that would be utilized for this space, etc. I had considered purchasing two ceiling cassettes, one for each room. However, the second room is so small, the smallest ceiling cassette would have still been way too large.

I decided to use the concealed duct (obviously it was a bad choice) because I thought it was designed for this type of application where I could run a limited amount of duct between the two rooms.

As far as the duct size I chose, I didn't realize that it would be so critical to use a specific size for this small of an application. Obviously I was wrong.

Today, I ran a duct calculation and it says that I would need to run a duct size of about 8"? Does that sound correct?

Looking around online, it seems that I may be able to help my problem with an in-line duct booster fan on the supply line and the return line? I didn't know if that would help or not so I thought I would post the questions here. Is my primary problem that I don't have the CFM necessary to push the air? In my reading it seems like I could remedy this with a current sensor relay and two duct booster fans? Maybe that's not a good idea?

Another option for me (although I don't really like this one) is to do away with all the duct work and hang the unit from the ceiling in the first 3rd of the biggest room. However, this means I would have to run some duct from my main home AC unit to the smaller room (which is in the attic) and I didn't think that would work very well.

Any additional help would be appreciated.

Thanks.

Because this is an open forum that is available for anyone on the Internet to see. If we give open advice that results in injury to a non-professional the site would be legally culpable. This is a litigious society & we're fine with professionals doing work but limited in DIY advice. There are applications you can fill out to become a professional member & gain availability to locked sections. Nothing personal but we're restricted from going to far into discussing technical issues.

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[jrbenny]

20' each run. Ouch.