Minisplit wall mount vs. Slim Duct
I have pretty much decided on 2 slim duct units for my upstairs (to keep the master suite and the east/west rooms separate), so I am good there. Both mounted in closets with only 1 having a single duct in the attic.
My question is on downstairs. I can mount a wall mount 15k unit that will blow across the 2 rooms with the most heat gain/loss. That leaves the bathroom at the other end of the house and dining room as not getting enough cooling/heating….with no good place to mount a wall unit.
My thought was I either run a duct from that area and have it suck or blow air across the house with a small inline duct fan to the other side of the house by the wall unit….for circulation. This would help distribute pellet stove heat in the winter as well….since those same 2 rooms are left pretty cold in the winter.
Is this a bad idea to run a single duct across the house for air distribution and use 1 wall head? Or should I be installing another slim duct unit for the 1st floor instead and suck it up….losing the headroom for ducts in the basement and spend time on running the extra ducts.
As everyone has said here, the Fujitsu slim ducts come out much less expensive then the P series Mitsubishi’s and using a MXZ-C condensers. Which is why I have pretty much settled on Fujitsu’s. The Fujitsu 1:1 condensers are small…so I can survive with having 3 of of them outdoors vs 2 of the Mitsubishi MXZ-C smaller units.
I found with the Mitsubishi Diamond Builder that the larger Mitsubishi MXZ-C units with branch boxes have a large temperature derate when you take them down to 63 F wet bulb, 75 F dry bulb…the Fujitsu’s do not…surprised me.
EDIT: Added the first floor plan below. Red box would be the wall head. Orange box being a duct run to draw air across the house. The Living Room (now office/spare bedroom) I plan to run a duct down from the closet/bedroom above it. Dining room has 2 openings to hallway/kitchen area and hasn’t had a problem with the pellet stove heat (next to the wall head location). Second Floor plan shows the 2 slim duct units and a general image of ducts.
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Paragraph breaks make it more readable.
Without careful room by room Manual-J numbers it's hard to say if any of that works. It's unlikely that the individual zone loads are big enough to rationalize even an AOU/ARU9RLFCD mini-duct to manage either the loads of the two upstairs zones as drawn. Maybe a multi-split with two ARU7RLFs upstairs would work... (or not).
Run a serious room by room Manual J (or have an engineer or RESNET rater and NOT an HVAC contractor run it for you) and put the heating/cooling numbers on each room in your drawings, and specify your 1% & 99% outside design temps.
Dana,
You're right about the paragraph breaks. I added them.
Thanks for the reply and fixing the paragraphs. Here is he Manual J for 1st and 2nd floors.
So background, I am a mechanical engineer getting into HVAC...more for fun with family friends at the moment but we will see where it leads...although i have designed liquid cooling systems for commercial/industrial. I have Manual J, S and D on my desk and got my EPA license and have done 1 mini split install (have all the equipment). I will admit I am learning, as I have only done 2 Manual J calcs so far for 2 different houses (one being mine). But have read through the majority of Manual J. Learning something everyday and greatly appreciate the help and pointers!! :)
You can't just scribble the load numbers on the drawings rather than make us work for it? ;-)
I don't see 1% & 99% outside design temps in the Manual-J summaries.
The heating load numbers for the bedrooms seem high, making me wonder what the infiltration + ventilation assumptions are.
Good idea on the loads onto the floorplan. I'll work on that tonight when not on my cell phone.
Indoor db 70
Indoor design cooling db 75
Indoor Cooling RH 50%
Outdoor Winter 99% db 2
Outdoor Summer 1% db. 87
Grains diff 24
HTD 68
CTD 12
Daily Range Medium
ACF 0.993
I know I should blower door test. But I have so many projects in work I'd have to again after. Need AC this spring. Have air infiltration set at average due to big and old crap leaky windows and canned lights esp on cathedral ceilings. Strong wind will blow door in one area. Gotta make due with what I have and keep on making this place more efficient as I have been. I like the Fusjitsu slim ducts where then turn down to 3k for the 7k, 9k, 12k and 18k...helps design for my situation a little.
>" Have air infiltration set at average due to big and old crap leaky windows and canned lights esp on cathedral ceilings. Strong wind will blow door in one area. "
That's worth fixing, independently of the HVAC sizing issue.
A single AOU/ARU18RLFCD pumps out 19,700 BTU/hr into a 70F room @ +5F outside, and 18,400 BTU/hr @ -5F outside. Your calculated load for the entire second floor @ +2F is 19,262 BTU/hr.
Given that your numbers are probably higher than reality now, and will be much higher than the heating load after fixing the obvious air leakage I'd call that a pretty good fit on the heating end, despite having zero margin at your leaky calculated number.
Undersizing will be better for comfort and efficiency than going with a pair of 9RLFCDs. A single 9RLFCD puts out 14,000 BTU/hr @ -5F, 15,400 BTU/hr @ +5F, and you'd be at more than 1.5x oversizing for the total load even at the exaggerated "before air sealing upgrades " numbers.
A single 18RLFCD has capacity to spare on the cooling end.
A pair of 7RLFs are nominally 16,200 BTU/hr, but only work with a multi-split compressor (at lower efficiency). That would probably cover the "after air sealing" load just fine (run the Manual-J using "tight" assumptions), and at 7K cooling each still covers 98% of the calculated cooling load.
https://www.fujitsu-general.com/us/resources/pdf/support/downloads/submittal-sheets/ARU7RLF.pdf
If one of the zones can really use a 9RLF cassette it wouldn't change the multi-split used, but may affect how much space is needed to mount it.
Dana, do you know if there is a drain pan heater available for the 18RLFCD? I've been looking and it seems odd that it goes to -5F and doesn't have an optional drain pan heater....unless I'm missing something.
Also, thank you for stating the width...the spec sheet is wrong and shows the sizes for the smaller units on the diagram. Luckily you stating the size made me pull up the design manual and see the correct matching dims in there....forces me to install on a different place...good to know up front.
The RLFCD series does not have the controls or a pre-planned location/channel to support a drain pan heater. It's possible to make up your own pan-de-icer using heat tape or a pan heater from another mini-split of similar dimensions and control it manually (plug it in when upon you see the drain has developed an ice plug), or devise your own controls.
The RLS3s don't come with a pan heater either, but I believe a pan heater can be installed after the fact as an optional accessory. Not so for the RLFCDs
Alright, I just had some time to place all the numbers right on the Floorplans. I also decreased the window loading...you are right, they were high and I was being conservative with my old windows. Screens are mostly broken, so didn't have them on there and didn't have shades 1/2 drawn down.....screens on and shades half drawn in the calc now :)
So one thing I'm wondering is if I should be putting a split wall head in for the master bedroom and/or bathroom on the west side of the house...these rooms get awfully hot/cold in the summer/winter. Master Bathroom has a cathedral ceiling, which doesn't help. It would be REALLY nice to have these 2 rooms zoned off the others....for comfort and for noise with kiddos.
Then maybe have a small ducted handler for the 2 front bedrooms and maybe cover the Office/Bedroom with it too? It seems the load in these 3 rooms may be too small for 6k wall heads on a multisplit, correct?
Thoughts?
Be sure to provide a return path that doesn't create excessive pressure.
I struggled with that at first. I managed to get 2 slim ducts figured out into closets so there is only 1 real tricky bedroom. Hoping a large jumper duct does it there? Or is that a bad idea with the slim ducts.
There is also an upstairs bathroom that will have to return via the hallway or add jumper duct...but I'm not too worried on that one as that bathroom has a small exterior wall and doesn't heat up too much.
The xxRLF ducted cassettes can be mounted vertically, and if you make the end of the Bedroom #2 closet the "utility room" the return plenum can have big grille facing the foyer as the common return, with partition wall jump ducts for the room returns. It will take at LEAST 32" of closet depth to accommodate even the 7RLF in that closet in any orienation, and 40"+ to accomodate an 18RLF.
A single 18RLFCD mounted vertically in the corner of the Laundry can probably cover the whole shebang, using hard-piped 3" x WIDE soffited hard duct below ceiling level where it has to cross over to Bedroom 2 and Bedroom 3.
This is a vertically mounted 18RLFCD in a ~8 square foot micro-closet heating & cooling a whole house in Berkeley CA using the big-grille common return approach:
https://uploads.disquscdn.com/images/2ffa6e108a7ded9f51130ff14126239b275b1244b7d53138beb63b4182d68f13.jpg
https://uploads.disquscdn.com/images/7843213f27734395e6ede8ea696552a8eafd3a2dd7f62c2b61241bb23189a293.jpg
Notice that they used flex (not recommended for the turns) and built out a soffit to accommodate the run down the hall.
It's worth flipping through John Semmehack's PHIUS slide presentation for tips on designing around Fujitsu RLFC cassettes:
http://www.phius.org/NAPHC2018/Think%20Little%20Slide%20Deck%20-%20NAPHC%202018.pdf
Download and archive the design & tech manual for bedtime reading:
https://hvacdirect.com/hvac/pdf/(DT)ARU9-18RLF-AOU9-18RLFC2015061.pdf
Thanks for all the info!
I just looked at the Bedroom 2 closet and the depth is too short. I'd have to put the unit vertically on the back wall and create a large plenum box under it to get the grille on the Foyer wall perpendicular to the unit. Doable but not ideal.
Just measured and with the 18RLFCD it looks like I would block the dryer from coming out unless i took the washer out of the room first and jogged it over :( BUT, I could mount the 18RLFCD horizontally on the ceiling of the master closet...that wouldn't really hamper space...again makes a centralized return filter to the hallway a pain in the butt, but I could make it happen. Seems like the least of the evils.
Do you think I'd have enough airflow with a single 18RLFCD?
What are you thinking on the 1st floor?...seems the 18RLFCD wouldn't quite cut it. Makes me wonder if I shouldn't discount the Mitsubishi multi's. A PEAD 24 downstairs and a PEAD 18 upstairs may work. I get a 0.81 temp derate with 52% RH on the 42k Hyperheat unit (so really ~34k BTU)...which is as low as you can go at 75F db w/o it saying incoming coil temp too low. The price is sure A LOT more than the Fujitsu's.
Another option is a PEAD 18 upstairs and a MLZ 18 and FH06 downstairs. The derate really kills you here unless you go with the hyperheat 24k or 30k multi condensers :(
For some interesting analysis, review Fujitsu's ducted air handler in terms of latent removal. Don't know if it has automatic fan speed control and in any case, it has little range listed (554 to 442 CFM). Unless it can actually go lower, it's not that far from a single speed system. So no humidity removal (1750+ CFM/ton) if the outdoor unit is modulating to low output.
In any case, plan for separate dehumidification (only some Daikin AC dehumidifies when sensible load goes to zero).
So I looked through the operation manual. For fan speed I see auto, then 4 speed settings. Do you mean the fan never shuts fully off?
I'm more used to the Mitsubishi controllers.
I just compared the Mitsubishi PEAD and SVZ CFM ratings and you are right, larger range than Fujitsu even though only 3 speeds.
OK, it has an automatic fan setting. But I see no indication that it can modulate airflow down to levels that produce reasonable SHRs at low compressor outputs. Ie, there is a fundamental problem when using a 6.7:1 range compressor with a 1.25:1 fan.
For heating, the more than needed airflow is beneficial - it maintains proper throw and mixing.