Ducted Minisplit and Static Pressure
4khz
| Posted in General Questions on
Edited for clarity
is .16″ wc external static pressure enough to serve multiple rooms with a central return grille ? A contractor is proposing a Bosch 3/4 T compact ducted AH with this esp.
My guess is that these air handlers aren’t designed for multiple rooms. I saw an article by Bailes who used a mitsubishi unit with .20″ for multiple rooms, but the duct work looked quite large – plus he is an expert who likely had a top tier contractor.
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What is the CFM of the indoor air handler in its highest speed? What size ductwork will be installed and roughly how much footage including any fittings or bends? Use an online duct friction table to roughly calculate if 0.16” w.c. Is enough or not for your situation.
CFM, H/M/L
352.94 / 282.35 / 176.47
matthews25 - I raised my concern to the contractor and asked if they could install Fujitsu which are .36". They agreed. With everything I've read on GBA regarding low static systems, .16" doesn't leave too much/any room for error. I feel lucky to have found a contractor that's willing to work with me (what a journey).
P.S. I edited my post realizing that my original post was a bit convoluted.
.16" is definitely not mid static but can be made to work. I've done .1" rated units and they work quite well.
To make it work, you generally spec a much larger air filter (about 2x the area of a typical furnace rule of thumb) and use larger ducting.
For example in a low load home usually a bedroom would need a 5" duct, with a low static that jumps to to 6" rigid or 7" flex. Keep the bends to a minimum and generally it is much easier to home run these system to a box plenum right on the air handler.
This gets you into the rough ballpark, but I would still go through the exercise of calculating the friction loss for each feed to make sure it is well within limits.
Does 'home running' make the ducts paralleled, which reduces rather than adds to total static pressure? Same as parallel circuits. Or do you lose some of those gains with the box plenum?
Akos - Good to hear this could actually work. on two of my zones, I have very short / straight duct runs. However, the third zone would serve three bedrooms, bath and laundry - about 800 sf box with the AH/ducts in a not so tall crawl space. CFM is about 100 cfm for two bedrooms, 75 cfm for guest bed.
I did go back to the contractor and requested 3/4 T fujitsu with .36" esp or possibly a 1 ton with .60" esp. They agreed to it. However, per neep.org, the Bosch modulates down to 1,800 btu versus fujitsu at 3,100. The difference in low load cycling isn't insignificant. Hmm
I do like your idea of home runs. I've been doing duct design this way - mostly because it simpler for me to calculate (novice). Good to hear that it might be simpler from a installation standpoint too. And thanks for the tips on the return side/ filter. This seems to be a crucial component.
Not my area of expertise either, but .16 does seem like 'low' static.
Have you seen these:
https://www.energyvanguard.com/blog/busting-myth-about-low-static-ducted-mini-splits/
https://www.energyvanguard.com/blog/path-low-pressure-drop-across-high-merv-filter/
Maine - I did see these articles, ty for referencing. Actually, this is what got me comfortable with the low static systems (I actually edited my post for clarity and referenced Bailes). I read many articles here on GBA about low static AH's and it being tricky for multiple rooms - most of them referencing the mitsubishi's at .20 - .26. So, .16" seemed undoable and/or totally out of my league.
I did go back to contractor and propose Fujitsu at .36. They were ok with it and have some experience with Fujitsu. Initially, they were pushing Bosch. I have had a very hard time finding a HVAC contractor within a reasonable budget and was concerned I might lose them over this.
Per Akos in a response above, .16" for multiple rooms can be done in many cases. Bosch does have an advantage on the minimum btu of 1,800 which is the lowest I've come across for a slim duct.
I wouldn't sweat over not getting a Bosch system. As far as I can tell, Bosch is rebadged Midea or Gree. This doesn't make them bad, but that's perhaps not as fancy sounding as "Bosch".
I have a Fujitsu mid-static ducted system that I contracted the install of, and multiple Fujitsu ductless units that I installed myself. They all operate exceptionally.
As far as sizing, what are your cooling and heating loads? Unless 9,000 BTU/h is wildly oversized as is, I wouldn't worry about the Fujitsu turndown, because 3:1 is not bad in the grand scheme of things.
Patrick - I have three (single) zones for a 2,800sf two story, each with about a 10K heat load and about equal or a bit less on cooling. Per neep.org, on the heating side for a Fujitsu 3/4 T, the minimum threshold is 45 degrees. Bosch 3/4 T threshold is 51 degrees, or 15% annual low low cycling versus 5%. Not sure what difference really means in terms of comfort or efficiency.
Initially, I thought the Bosch system was something fancy because early on in my construction I had received a quote for $90K. But yes you're correct, it's a rebranded Midea.
The revised quote for the (3) three Fujitsu systems remained the same at $32K. and I'm leaning towards this - primarily because of all that i've read here on GBA, including your feedback, ty.
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