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Old 07-26-2011, 12:04 PM   #1
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Oversizing Mini Splits


I'm considering placing a Fujitsu 9RLS Mini Split in each of the two bedrooms for my children due to the rooms being uncomfortable during extreme weather (heat & cold in the Midwest). Each is ~150 sq ft, with 9 ft ceilings (~1350 cu ft) and well insulated, so I am concerned about them being over-sized during moderate weather. Since this is an inverter unit, I realize that it will decrease output when it reaches it's set point to maintain temperature, but estimated cooling/heating needs during moderate weather are <=2000 BTU/H. With 9,000 BTU/H capacity at standard cooling conditions (for example), I'm concerned this will still be excessive if the unit can not "scale back" it's output enough to even maintain. We are considering using these throughout the home in replacement of our HVAC system to provide zones and increased energy efficiency, over the long-term, starting in these rooms since they lack the most comfort (and keeping our little ones comfortable is a prime concern at night since we don't want them waking us up for being too hot or cold). Zoning for the other rooms will not be an issue as they are larger and/or more open.

Anyone with experience using a mini split in a moderately-sized bedroom? Any suggestions on settings for the unit that may make it work comfortably and efficiently under these conditions? Thanks

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Old 07-26-2011, 05:55 PM   #2
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Oversizing Mini Splits


Do the bedrooms have their own returns? May only need to add returns to them if they don't have their own.

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Old 07-26-2011, 11:12 PM   #3
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Oversizing Mini Splits


Thanks for the reply beenthere. Yes, the rooms do have their own returns. As long as these are only being used for supplement cooling/heat, that shouldn't be a problem since that will help better even out the air throughout the house. However, if we decide to use a mutli-zone system exclusively (and not use our HVAC system) then the returns would be stagnant. I've considered leaving the fan running continuously to circulate the air, but I'd like to stay away from that if possible. However, that is an option.

Another plan would be to install a unit in one room and use a register in each room that links the two (these would be independent of our HVAC ducts), with an in-line duct fan to pull from the room with the unit and blow into the room without the unit (assuming the doors are open, this should help circulate between the rooms also). Any thoughts?
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Old 07-26-2011, 11:13 PM   #4
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Oversizing Mini Splits


I guess I should also state that I realize these units will shut off the compressor when they reach their set point and are still going too strong to maintain the temperature. However, I've heard that the fans will continue to run, and am therefore considered about the humidity being added back into the air. This is my main concern with oversizing (in addition to performing less efficiently)
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Old 07-27-2011, 04:31 AM   #5
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I would see if running the fan continuous is enough, before spending all that money on a ductless system.
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Old 07-27-2011, 09:09 PM   #6
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Oversizing Mini Splits


We have tried running the fan continuously, both in the summer and winter. It does help in the summer, but the downside is the significant increase in humidity throughout the home. Although it does even out the temperature, it does not improve the comfort. In the winter time, the fan running continuously kind of gives a "chilling" effect throughout the home. The ductwork runs through our basement, and even though it is insulated, the 70 degree air still feels uncomfortable to our 98 degree bodies.

Additionally, it doesn't help our long-term energy-efficiency goals. Our current heat pump has a SEER rating of about 13, while these Fujitsu's are around 25-26. Even if we don't get that full efficiency, it is much more efficient, even when you consider the loss of efficiency through our ducts. We are also considering a geothermal install, perhaps with a better measured Manual D(?) calculation to improve our duct system, but the cost of that (I think) would significantly outweigh the ductless systems. I could also replace the existing 6" ducts with 7" ducts to improve the airflow and comfort to those rooms, but again, it doesn't solve our long-term energy-efficiency goals.

I've considered the pros and cons of a ductless system, and I'm ultimately stuck with this issue with the bedrooms. One unit for each room (which may lead to problems if they are too over-sized) or one unit to service both with the "workaround" previously mentioned. If I can get that figured out, I think it would give me workable project to compare to a geothermal install, which would really be our other option.
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Old 07-28-2011, 04:29 AM   #7
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Oversizing Mini Splits


Increase the size of the supplies to those rooms. then, take the money that would have been spent on a ductless or geo. And improve your homes efficiency. Seal up the house. Get your infiltration rate as low as you can. Seal the wall switches and receps, replace or repair all door seals. Caulk around the door frames, and all of the windows. Seal around any can lights. Caulk any holes in the plates in the attic. and or basement. Insulate and where you can.

A 13 SEER that runs for 15 minutes is using less electric then a 23 SEER that runs for 30 minutes.

Then down the road, when it is time to replace your current unit, you can do a load calc on your house to see how much smaller of a unit you need. And install a smaller high efficiency unit.

Spending 20 grand on a geo doesn't make a lot of sense, if it won't give a ROI for 30 years.

Next. Within 5 years, most manufacturers will be using inverter compressors on central systems. Bring up their SEER to 20 plus, for less then what a geo cost. carrier is releasing theirs soon, if they haven't already.

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