Two minisplits in one room
I just had my mini split system installed. I got multiple opinions from the installation contractor regarding the number of indoor units in one room. There is the setup:
Daikin ductless heat pump with one exterior unit and 3 interior units (7000 btu each). All installed on the 2nd floor of a house (about 900sq onthe 2nd floor). One interior unit in a large bedroom and 2 interior units in an extra-large bedroom. The reason for 2 units in the bedroom is we are going to divide the room in a few years. So there will be a total of 3 bedrooms (each with its one interior unit). So currently the two units are running in the room, the contractor initially said they were just going to turn one of the units on and the leave the other one off until the room is divided but installation contractor turned them both on saying it should not matter. Obviously its oversized for the room but is it ok to leave them both on? Unfortunately I have not been able to find any documentation on this situation. Any help would be appreciated.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Was this a Daikin authorized installer? Did the installer complete a Manual J? And where are you located?
They are not listed on the Daikin website as authorized installer. I believe a Manual J was performed. I am located in Seattle Washington.
Bryan,
Steve is correct that it's impossible to design a heating or cooling system without first performing a Manual J calculation. I can't imagine any scenario in which 900 square feet in Seattle results in a design heating load or a design cooling load of 21,000 BTU/h.
I'm guessing that your system is wildly oversized.
Step one is obvious: Turn off one of the heads in the big bedroom and see what happens.
A really leaky 900' house with single pane windows, R13 in the attic and no foundation insulation might have a heat load @25F big enough to warrant three 7K heads, but less than half that would be more likely. The cooling load for the whole house is likely to be within the capacity of a single head, barring huge "sunset view" windows. This level of gross oversizing is common with the " head per room" approach for ductless.
I'm not sure what the minimum modulated output of the (2-ton?) Daikin compressor is, but it's probably something like half the real design heat load. and at or above the design cooling load. It's typical for the minimum compressor output to equal the full max output of a 7K ,head, so turning a head off would cause the other head to run at a higher speed/lower efficiency when cycling, but unless all heads are running at the same time it's the same problem.
Try running with both heads, and observe their behavior. If they're usually blowing simultaneously at low speed rather than just one at a time, stay with two. If it's usually just one at a time,turn one off to minimize the on/off cycling numbers of the compressor.
Thanks for everyone’s input. The original design was to put a 18000 btu concealed unit in the attic with 4 duct runs (1 to the large bedroom, 2 to the XL bedroom and 1 in a medium sized bathroom). But upon installation they discovered there was not enough clearance below and above the unit per the manufacturer. There was debate that a smaller 1 ton concealed unit would not be enough to heat and cool so we went with the three head option knowing it would be oversized. I will test it out over the next couple weeks assuming we get some warm weather and see if both head are running at the same time. Thanks again.
Update. Upon further discussion with the installer we decided to turn one of the heads off.