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Help me understand static pressure in ducted minisplit systems

Clay Whitenack | Posted in Green Building Techniques on

Hi all,

Tried to piggyback a comment on this thread (https://www.greenbuildingadvisor.com/comment/reply/55726), but I guess the thread is too old and my comment was marked as spam.

I am planning a ducted mini split for the 2nd floor of our home. I have run manual J calculations and find that a Fujitsu 9RLFCD will fit the heat/cool load, assuming I can distribute the BTUs to all the rooms.

In talking with a Fujitsu salesperson yesterday, he was skeptical I could make it work because, according to him, the duct runs can’t be more than 15′ or so from the unit. It is my understanding that there are no rules of thumb when it comes to ducting mini splits, so I am a bit skeptical of this 15′ rule of thumb.

I have done a little research and have determined the heat loads for each room, and what I believe are the CFM requirements to deliver those BTUs to each room. I have found calculators online that turn CFMs into duct sizes, and found the static pressure calculator at Engineering Toolbox. (http://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html) The calculator returns results of static pressure in “inches water” and in “inches water per 100 ft”.

What I don’t know is how static pressure works in relation to a ducted mini split. I see on the submittal sheet for the 9RLFCD, the static pressure rating is 0 ~ 0.36 w.c.. I have read that “wc” equals “water column”. Would “w.c” be the same thing as “inches water”?

If so, does that mean that the ductwork can’t exceed 0.36 w.c.? If I have calculated CFMs and can then find the static pressure of size and length and pipe and all the fittings, I need to make sure they all add up to less than 0.36? Do I have that right?

The good news in my case, the two bedrooms farthest from the unit just need ~50CFM each (if I have done my calculations right), and I have a pretty centralized floorplan, so hopefully it will be possible to get the job done.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Clay,
    Yes:
    inches of water column = i.w.c. = water column = w.c. = inches of water

    1 inch of water column = 0.03609 pounds per square inch = 5.197 pounds per square foot = 248.84 pascals

    1 Pa = 0.0040 inches of water column

  2. GBA Editor
    Martin Holladay | | #2

    Clay,
    0.36 i.w.c for a ducted minisplit sounds better than some other options -- see this comment on another web forum:

    "The EH035 can only handle 0.125" WC. I can't imagine anyone trying to supply an entire house with that unit! That's barely enough static to supply a couple of bedrooms, and even then, the duct system would have to be almost non-existent (I hope your filter is at least 2 ft2). I've been successful using the SEZ-KDxx to supply as many as four bedrooms in a straight line (it has 0.20" WC available static), but I had to scrutinize each fitting and diffuser to shave off every possible hundredth of an inch of static."

  3. Clay Whitenack | | #3

    Thanks (as always), Martin.

    I was just on that page earlier today! I was also on another website forum today and there was a commenter by the name "dana1" that knew an awful lot about mini split specs. :) The next to last comment on that Energy Vanguard page...

    "The Fujitsu ARU series of ducted air handlers are good to .36" ESP with about a 15% loss in rated capacity. .25" will net you the full rated capacity.
    21 SEER for single head models with the ductwork in conditioned space."

    So, looks like I need to set my goal at 0.25 wc. I'll start running some scenarios and see what I come up with. Fingers crossed.

  4. John Semmelhack | | #4

    The Fujitsu rep you talked with certainly doesn't know his product or duct design very well. I routinely design + commission ducted mini-split systems using Fujitsu ducted units (over 50 systems in the past 3 years). We typically achieve our design airflows with total external static pressure in the range of 0.15 to 0.25 in.w.c. Good luck!

    Also, if you're able to move the rated airflow, you won't see a hit in capacity. Fujitsu has graphs in their design manual that show the change in capacity at airflows above and below the rated airflow. See attached example.

  5. Clay Whitenack | | #5

    Thanks for the reply, John. And I assume, by your comments, you've designed systems with longer runs than 15 feet? haha. Yeah, didn't think that made sense, because what happens when you use one of those units in a vertical position, and then branch off at the top in two different directions? I guess he thinks that cuts the runs in half, down to 7.5 feet? haha.

    I know there are now rules of thumb, but what are your typical duct sizes to keep your static pressures in check, and are you able to run main trunks and branches? I guess it depends on cfm and so forth, but are your main trunks in the 12" range, and then stepped down from there?

    And is there a good resource for the different fittings and their corresponding static pressure numbers? I think I have a ballpark on the straight ducts, but not sure how to figure the tees, 90s, etc.

  6. Clay Whitenack | | #6

    I see Dana and others recommend that you "oversize the ducts" when dealing with ducted mini splits. What does this mean, specifically? Upsize from Manual D, or upsize from typical "rule of thumb" duct sizing? If a Manual D calculation results in a specific duct size, go up one size? Or is that just a "rule of thumb" to try to counteract the typical "rule of thumb" of just using rule of thumbs for duct sizing?

  7. Clay Whitenack | | #7

    "The Fujitsu rep you talked with certainly doesn't know his product or duct design very well." -- JOHN SEMMELHACK

    Yeah, I am beginning to understand this. This guy just ran some manual J calculations for me and his results came back about twice what I (and others) have calculated, plus he sized the equipment based on the cooling capacity, thinking it was the heating capacity. In other words, he thinks I need 51k BTU of heat for the whole house and he sized 5 tons of equipment for a total of 64k BTU of cooling cooling and 71K BTU of heating capacity.

    I can't express how thankful I am that I found this website.

  8. Clay Whitenack | | #8

    OK, am I stressing out a little too much about having enough static pressure? I did some studying on total effective lengths, and looks like my ductwork will be around 115 for my longest supply and return. I'm not sure how much the seven grills, dampers, air filter, etc., will take away from that .36 figure. I know I have to calculate those items specifically, but would the ballpark be somewhere between 0.1 - 0.2, leaving me with an available static pressure of 0.16?

    I'm trying to figure out if i can run the ductwork through a coffered ceiling and the warnings about low static pressure equaling large ducts has me worried. If I can keep my ASP around 0.16, my supply ducts will be around 6", which will allow me to hide them.

  9. Isaac Savage, HvacDesignPros.com | | #9

    Hi Clay,

    You should be stressing out ;-). This is important stuff! The rest is almost pointless if you don't get the duct sizes correct. Here's a quick overview of the process that may fill in some of the gaps for you, and hopefully serve as a helpful guide to other readers.

    To size your ducts, you'll need to determine the friction rate (FR). To get to the friction rate, you need to know the TEL of your ducts (equivalent length of fittings+linear distance), the ASP (available static pressure) of your blower, and the static drop of the other components in the airflow stream (grilles & filters in this case).

    So, in this case, you have 0.36 ASP & 115 ft for the TEL. For the components, you can use 0.1 to account for grilles and balancing dampers (assumes ~0.03 for each). For the filter, it'll depend on what type you're using. You'll want to consult the product specific info for it. For the sake of this example, let's say it's going to produce a drop of 0.05" at the max airflow for your unit.

    You'll first subtract the component drops from your 0.36 (0.36-0.05-0.10 = 0.21), giving you an available static pressure of 0.21". This is the blower power you'll actually have to move air through the ductwork.

    Next you use this equation: ASP/TEL*100 (0.21 / 115 * 100 = 0.18).
    This is your friction rate - 0.18

    Now you can use a duct wheel to match up your desired CFM with the FR to determine the required duct size. Be sure your duct wheel is providing you with sizes based on the duct material you plan to use. It'll be different for sheet metal vs flex duct etc.

    NOTE: This FR is pretty high, which will produce relatively small duct sizes. Be sure to double-check your TEL calculations and make sure you've taken all fittings into account. And, you'll want to adjust for the correct filter data as well.

  10. Clay Whitenack | | #10

    Isaac, Thanks for the reply. Yes, that helps fill the gaps. I may be a little off on my TEL. I made a good attempt at finding the effective lengths of the fittings correctly but I very well could have an error somewhere. I admit I am over my head.

    You are currently double-checking my manual J calculations, so maybe when you are finished with that you can double-check my duct design!

  11. MKCF | | #11

    I know this thread is a bit old, but as there is a recent reference to it I figured I might as well see if anyone is still viewing it.
    Clay, where did you find TEL's for fittings and such? Sounds handy. Also, I'm guessing by now your ducts are installed and up and running. How did it go?
    Did you put a balancing damper in every run, or no?
    A piece that I can contribute to the above information on how to calculate friction rate, is that the CFM requirements for each room can be determined by taking the BTU's for each room (from Manual J) and figuring out their percentages relative to each other. I was missing this for a long time, for some reason. These are the percentages of the available CFM of the air handler blower as listed on the submittal sheet. You are supposed to use the highest blower speed because this will give you the largest duct sizes, and also add 20% to each CFM number to make sure you have a bit of extra capacity.
    (Do I have this right, anyone...?)
    Also, an online version of Manual J can be had for free at coolcalc.com. Make sure to do the inputs manually (the software actually pulls data from Zillow etc. if you let it).

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