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Mini-split system questions

jdorcutt | Posted in General Questions on

We live in a 150 year old home that we are remodeling.  Currently, the main floor of our home is heated by a basement furnace and the upstairs is heated by a furnace that sits in a utility closet upstairs. We plan to remove the upstairs furnace so we can use that space to increase the bathroom size.  We have solar panels and we’re hoping to use mini-splits to heat and cool the upstairs, however, we may find, with the help of your advice, that the system we need is out of our budget.  

We’ve had 5 different local HVAC companies come in and provide job estimates.  Only one provided us with a detailed manual J calculation which resulted in these numbers:
North BDR – HL 4709; CL1709
East BDR – HL 2977; CL 2179
Loft and Bathrooms – HL 3062; CL 2178
Master BDR – HL 9815; CL 5648
Kitchen – HL 12315; CL 6217
Total – HL 32876; CL 20367
We were hoping the indoor unit in the east bedroom would also condition the loft, but in  the bathrooms we would eventually extend the ductwork from downstairs.  We were aiming for a ductless system, but one of the companies suggested a ducted system for the bedrooms and ductless for the MBdr and kitchen.  The two companies that quoted Mitsubishi units are out of our budget at $17k, the one company quoting us Daikin units was within our budget at 10k.  The higher quote did include a kitchen indoor unit whereas the lower quote allows for that to be added later.  However, after reading several articles and questions answered by your staff, we question whether the Daikin units are just too large for our capacity requirements.  We were quoted a 
4mxl36tvju outdoor unit and 3 ftxr12tvjus units for each bedroom.  We know the indoor units greatly exceed our requirements, so we looked into the 2 smaller sizes and reviewed the capacity tables for the quoted outdoor unit.    If we used the two smallest capacity indoor units, 7,000btu in 2 bedrooms and the 9,000btu unit the heating and cooling capacity in the two smaller rooms are 7 and 8.1 respectively and the MBR is 9 and 10.4.  With a 7,7,12 combo the small rooms go down to 7 and 7.5 and the mbr is 12 and 13.29.  What would be the negatives of over sizing the units for the bedrooms by so much?  

Also, if we are able to go with mini splits in those rooms, would you recommend placing them near the floor under the windows?

I apologize for such a lengthy question, but Daikins customer support personnel are not trained to answer capacity questions and the on company who provided the manual J quote for us has not returned our calls for over 3 weeks now.  The less knowledgeable Daikin rep has been very responsive…

Please advise.


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  1. Expert Member
    NICK KEENAN | | #1

    You really don't want to oversize the heads. They will have trouble maintaining temperature, they won't do a good job of dehumidification in the summer and they won't run efficiently.

    This is really the Achilles heel of minisplits. The smallest units you can get are around 6k, and bedrooms typically need a lot less, like 1-2k in your case. They don't work well at all when they're oversized. The best way to configure it is to get a ducted unit that allows you to cover several rooms with one unit. The problem is that some of the appeal of minisplits is the simplicity of installation, which you lose with the ducted units.

    There's no easy way around this.

    1. jdorcutt | | #3

      Thanks for replying! We were hoping that the technology had improved since many of the older posts were written. Also, Daikin has a dehumidification feature which allows the unit to dehumidify the air without cooling. There is a capacity table that shows for two 7000btu and one 9000 btu unit connected to a 4 head 36000 btu outdoor unit the min unit capacity is 4,900 and the max is 41,200 when heating and 6,500 min/28,600 max when cooling. Seems that the min would be the lowest modulating btu capacity split between the 3 indoor units which would result in 1,633 heating and 2,166 each for cooling - or less for the 7K units and more for the 9K...are we interpreting the values on the capacity table correctly?
      Would it help to downsize the outdoor unit?

      1. Expert Member
        NICK KEENAN | | #8

        The thing to keep in mind is that the entire purpose of air conditioning is comfort. There is no health or safety need for it. An oversized AC doesn't deliver comfort.

        The thing to keep in mind is that the the cooling load number in the Manual J is the 99th percentile number -- 99% of the time your load is going to be less than that. An air conditioner delivers the best comfort when it can run continuously, or nearly. When the AC runs in short bursts it doesn't remove as much humidity from the air and you get temperature fluctuations. With a modulating system you can have the system sized for the maximum load but still running continuously during most operating conditions.

        So in your situation, the north bedroom has a cooling load of 1709. The compressor can modulate down to 23% of maximum. With a 7K head you're basically running fully modulated at full load, 99% of the time even the fully modulated capacity is too large and you'll have to be switching on and off to keep from overcooling the space. This will work, but it won't be comfortable.

  2. Jon_R | | #2

    > What would be the negatives of over sizing ...

    The better question is "are the various negatives significant enough for me to select another option?". Over-sizing rated capacity of inverter mini-splits by say <= 2x over design load typically has fairly small (< 10%) effects - but there is an amazing lack of quantitative data. Some even say that if a mini-split will modulate down to design load, then it's not over-sized, providing acceptable performance similar to a properly sized single speed unit. I wouldn't go that far.

    Some quantitative data here, Fig 2-5. Summary - cycling of compressor and fan has almost no effect on dehumidification!

    1. jdorcutt | | #4

      You’re right Jon_R, that is a much more useful question to ask. Considering that we would like to switch to electrical methods of climate control to move away from fossil fuels and use passive solar as much as possible...Would the oversize mini splits still be the most efficient option? Would they be able to keep the rooms comfortable through all seasons - winter low 11 F, summer high 94 F?

  3. kyle_r | | #5

    If you have existing ductwork I would consider replacing both furnaces with ducted mini splits. Fujitsu slim duct units might be a good fit. The 9k, 12k, and 18k units modulate down to 3,100 BTU/hr. They can be mounted vertically to save space, and given a low of 11 F they should be fine even though they lack a base pan heater.

    1. jdorcutt | | #6

      Thanks for your advice, Kyle. We will look into that option.

      1. kyle_r | | #7

        Carrier has similar units as well. I would get quotes for both to help with finding a decent price.

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