Adding a second mini split help
Last Post 11 Oct 2015 12:47 PM by chrs. 38 Replies.
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Dana1User is Offline
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04 Aug 2015 02:09 PM
If using the basement as a return plenum it's means you just put a register grille in the floor as the dedicated return. You can use panned joist bays for the returns to limit the amount of light & sound transfer between rooms and some privacy, rather than just a hole looking straight down to the basement floor.

The size of the duct needed just isn't that big for a ~15-20,000 BTU/hr mini-duct cassette. Even the 18RLFCD is only rated at a nominal 550cfm at 0.1" of water at it's maximum speed, and ~450cfm at min-speed (where it would be spending all it's time.) If it's just a couple of rooms an 8" round hard-piped duct to each room that separate fairly closely to the mini-duct cassette, (a medusa or spider, not a trunk & branch), each with a tweakable vane to balance flows/temps should work, and it might be fine with 6" duct. If it's more rooms the cfm per room is lower, and you can drop to 6" runs.

How many rooms would it be serving, and what is the calculated heat load in each room?

Unlike most of their competitors, it's fine to mount Fujitsu mini-duct cassettes vertically, with the air flowing straight up, if that makes the duct routing any easier, avoiding full U-turns.

Read up on all of the "slim-duct" ARUxxRLFxx stuff in the tech manual before getting down to designing the ducts:

https://www.goductless.com/hvac/pdf/fujitsu/fujitsu-8-tm.pdf


jonrUser is Offline
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04 Aug 2015 09:39 PM
As an example, 75 CFM with a 1 pascal pressure differential requires a 12"x12" opening between rooms.
ToadmanUser is Offline
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05 Aug 2015 01:19 AM
It's goin to be between 2 to 4 rooms. I wouldn't have any U turns and would like little turns as possible knowing It messes airflow up. So of I would use the basement as a return I could do floor vents on a few rooms so the air is pulled down to compensate for the air loss? That defiantly help putting a lil heat down there and take dehumidify it when its warmer out.
Dana1User is Offline
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05 Aug 2015 01:16 PM
The heat loads of just 2 rooms is probably way below the 0F output of a 3/4 ton mini-duct cassette. With 4 rooms, maybe not. Without the heat load numbers it's hard to make good suggestions.

Once you figure out the loads and cfm required to serve those load you can estimate the duct impedances with handy tools like this:

http://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html


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05 Aug 2015 08:55 PM
Thanks for the link. I'll take a look and read up on it. I was originally thinking a 9 or 12 but read an 18 could work. I wouldn't mind the extra btu power in case I ever need it or want to vent some into the basement in the future it wasn't it want. I did a heat load calculation last year and gave me an estimate. I don't have the kii papers with me but it was around 2500- 3000 for being 0 outside and 60 inside. I believe it's more more since a space heater struggled heating the bedroom.


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05 Aug 2015 08:57 PM
I thinking a 6in diameter pipe would work so it have more airflow and be able to circulate the air. I used box fans to help this past year to do that, especially the bedroom.
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09 Aug 2015 02:42 PM
I'm goin with either one straight duct or a floor mounted unit. Talked to one of the seller's online and the ducted units aren't made to go more then 10 ft. So the 15 ft would have very little air flow by the sounds of it but probably enough heat coming out to keep some heat in the room
jonrUser is Offline
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09 Aug 2015 04:34 PM
You can make up for any amount of distance by using larger ducts. For example, using the calculator above, you will find that a 6" duct running 10' has more resistance (ie, less flow) than a 7" duct running 20'.
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16 Aug 2015 03:11 AM
Update. I had some extra foam board so I used it on some joists that were easy to get to. I notice one of the rooms down there will be impossible to do because the ceiling is a finished one so not worth messing it up. The other parts will be a pain since it is all drop ceiling and has batts insulation in. I did notice the was behind the wood panel is no more then an inch thick and now I doubt they insulated it at all. It's probably because must of the pipes are around an inch away from the wall in the exposed areas.
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20 Aug 2015 04:59 PM
I decided to go with a 9rlffh. I would've did a ducted one but I going do most of most of the installing myself to save. On my future projects redoing the basement is on the list. Everyone that helped out thanks and I'm going to save this info for use when I don the basement.
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20 Aug 2015 05:00 PM
I decided to go with a 9rlffh. I would've did a ducted one but I going do most of most of the installing myself to save. On my future projects redoing the basement is on the list. Everyone that helped out thanks and I'm going to save this info for use when I don the basement.
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23 Aug 2015 06:45 PM
One thing I am puzzled about is the SEER and EER rating of ducted air handlers. Lets say you have a large closet behind your bedroom. You install the unit in a drop ceiling in the closet which will also serve as your return. Your conditioned air is dumped directly into the bedroom with two feet of duct work and the pathway back to the return is bedroom, bathroom and to the closet return. Return no duct length only 2 feet of duct to bedroom.

You would think the SEER/EER rating would be about the same as a non-ducted head unit. Calculation?
jonrUser is Offline
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23 Aug 2015 06:55 PM
If the air flow is maintained at the same flow rate, then the only impact is the energy used to move the air. That's not so hard to calculate (and with short and, more importantly, large ducts, is quite small).
ricky_005User is Offline
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23 Aug 2015 07:04 PM
Why do they rate ducted handlers in the 15/16 SEER/EER range when if mounted with lets say 2 feet of duct which would be virtually no resistance and very short distance on the conditioned air delivery?

SEER/EER ratings for ducted handlers are based on maybe on the maximum recommended duct run for the handler?
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24 Aug 2015 06:25 AM
I wondered that as well. I was going to put a ducted unit in a dropped ceiling in a centrally located closet, but the efficiency hit was too great, and the unit also didn't have the low outside temperature capability that the wall hung units have.
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25 Aug 2015 07:06 PM
My main problem for example the MXZ-C multi zone outdoor units is they are built to only power down to only 6,000 btu's in cooling mode. The smallest SEZ-KD09 ducted handler states it can power down to 3800 btu in cooling mode which would be great if the outdoor unit could do the same. I guess what they assume is that more than one air handler will be running? One of the problems I think some may have depending where the thermostat is located is excessive power cycling, instead of long extended run times.

Would be great if they would design and sell a SEZ-KD06 unit that could power down to 3,000 btu's and a outdoor multi zone unit that could power down to 3,000 as well.
Dana1User is Offline
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26 Aug 2015 02:02 PM
The modulation range is important, and that's why multi-splits are generally less efficient than single-head/cassette mini-splits using the same heads/cassettes married to a dedicated compressor. The minimum modulation of the larger multi-split compressor is well above the min-modulation of a single head, so when serving a single zone it can't operate at it's lowest speed/highest efficiency.

The Mitsubishi FH09NA has a really nice low min-modulated output of about 16-1700 BTU/hr which makes it a better choice than the Fujitsu 9RL_ _ _ _ for low-load applications, since the min-mod of the Fujitsu untis is always 3100 BTU/hr (heating or cooling).

http://www.younits.com/media/wysiwyg/Literature_PDF/Mitsubishi/MSZFH09NA_MUZFH09NA_Submittal.pdf
ToadmanUser is Offline
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10 Oct 2015 09:20 PM
The wall mini split is short cycling. I read that adding the thermostat would keep it from doing that by maintaining the exact temperature and the outside temperature doesn't matter. The afternoons I have to turn it down and evenings turn it up but it still over shoots the temperature by a few degrees. The heat coming out of the unit gets up to 108 when running ( can sometimes hear the humming sound of the compressor at that point ) and it seems you be pushing well above the minimum 7500 btus. Is the thermostat worth investing or just adjust the unit like I did for the past year.

Also I'm looking to get some 2in foamboard to put in the inside of the basement Windows to help cut airflow.
chrsUser is Offline
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11 Oct 2015 12:47 PM
From what I understand and have heard the thermostat would be well worth it.

If you don't mind blocking the light, foam board over your basement windows, tightly caulked or taped in place, can do double duty by stopping the air leaks as well as the heat loss. Clear plastic can also be used to block airflow and a little heat, without blocking the light.
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