Mitsubishi mini split line set question

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Bill Hulings

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Hello Hive Mind,

I'm installing a Mitsu MXZ-3C30NAHZ2(Hyper Heat) outdoor unit, with two heads--MSZ-FH15NA, and MSZ-FH18NA. Both indoor units arrived with 1/4" and 3/8" copper tubing, factory installed. HOWEVER, the 3/8" lines have 1/2" connectors factory installed!!! Is this typical? What size Line Set do I use? Do I match the 1/2" connector with 1/2" line, or do I match the 3/8" line with 3/8" line using a reducing adapter? The MXZ-3C30 has 3 ports; two with 3/8", and one with 1/2" (all 3 ports using 1/4" line as well)
20200318_131933.jpg

Above: factory lines on top of image. 3/8" line on bottom for comparison.
 

Dana

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(As stated on that other forum), I'm wondering what size room/zones really call for an FH15 or FH18 at Eugene's mid-20sF outside design temperature, and whether there will be enough load to run a -3C30NAHZ efficiently with those heads. Even mid-winter binned hourly mean temperatures in Eugene are in the 40sF. A typical code-min new construction house would have a heat load to floor area ratio of no more than 10 BTU/hr per square foot @ +25F, and maybe 6-7 BTU/hr per foot @ 45F.

The minimum output of the 3C30NAHZ @ +47F is about 11-12,000 BTU/hr, the max output at +17F (almost 10F colder than the local 99% outside design temp) is about 29,000 BTU/hr. So it has enough margin at sub-20F temps to be right-sized if the 99% heat load (of the zones combined) is about 21- 24,000 BTU/hr @ +25F. That would be serving 2100 ' -2500' of conditioned typical new construction residential space (in Eugene), and has enough load to still be modulating rather than short-cycling the compressor even into the high 40sF or low 50s F outside.

It's a bit harder to size multi-zone ductless than single-zone, but to hit the nameplate efficiency numbers both the heads and compressor need to be less than 1.5x oversized for their design loads. Bigger is the opposite of better here.
 

Bill Hulings

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Here's the official response from my Mitsubishi supplier: in order to keep the warranty valid the line set size must match the connector size at the indoor unit. Any reduction or enlargement will be made at the outdoor appliance. Thanks, Dana, for your input.

Bill
 

Dave17

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di d
Here's the official response from my Mitsubishi supplier: in order to keep the warranty valid the line set size must match the connector size at the indoor unit. Any reduction or enlargement will be made at the outdoor appliance. Thanks, Dana, for your input.

Bill
Did they give any logic for the oversized line? Seems that would effect the efficiency of the unit.
 
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