Hey Everyone!
Background:
ICF Home - 36k Btu Mitsubishi Hyper Heat system.
Load calcs completed system installed two years ago, worked well. This year when we switched to cooling I started having problems.
Outdoor Unit: MXZ 4C 36 NA HZ
3 Indoor Units: 18k, 12k and 9k
Run through a Branch Box: PAC-MKA30/50BC
No Central Controller
Here's a description of the problem.
Power down system.
Let sit for 30 minutes.
Turn On system.
Set remotes to 68
Ambient temp 78
Ambient RH 60 - 65%
All three units kick on nicely. Output air temps drop as expected. They all achieve between 58 and 62 output air temp measured on a gage hanging in front of unit. Technician confirms that return temps of refrigerant are 48 - 52deg. Everything works well.
After precisely 60 minutes the compressor ramps down to very low speed. All the output air temps climb.
This is repeatable... I've done it 50 times at least. System runs perfectly for exactly 60 minutes, house temps start to drop (never reach set-point) and then right at 60 minutes, compressor slows and output air temps climb.
18k unit will climb to about 73
12k unit will climb to about 66
9k unit will climb to about 64
With that 18k unit not pumping out cool air, the great room (where it's located) never cools off.
More facts:
1) We've completely pulled the charge, recalculated the proper weight and installed based on line set lengths, etc. We needed just under 18lbs in total. I'm confident the charge is correct.
2) Indoor units have been maintained - Filters, and coils are clean.
3) All settings on computer board in branch box and outdoor unit for addressing, and all dip switches are set to how the manual describes. (At least as far as I can tell with the vagueness of some of the instructions...)
4) The LED read out shows 1 1 1 1. Which, is not an error code, but an indication of what's running. But I'm honestly not 100% on that. The manual is unclear.
5) The company that installed this unit is out of business (not surprised). They talked me into a system I didn't spec (I wanted three individual outdoor).
6) House located in MD.
7) Even with the compressor running at super low speed, my awesome ICF house doesn't get above 80, but I do want to pull more humidity out and get it running correctly.
So here's the question: What happens at EXACTLY 60 minutes after the compressor activates that would cause it to go into super slow mode regardless of what the set-points are in the house?
This has happened regardless of outdoor temps (70 - 92) or indoor temps (72 - 80).
I know this is a long shot, but this forum has been invaluable in the past. Maybe I get lucky. I'll be ohming out all the thermistors today to see if I'm getting the right resistance. Is there anything else you can think of?
THANK YOU ALL!
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