Issue with attic-located Mitsubishi ducted cooling
We have been struggling with a poorly performing Mitsubishi ducted mini-split setup since 2016 that we purchased shortly after moving into our 3 story, 1896 townhouse. The ducted unit is in the attic is for cooling only the 3rd floor. Whenever it would get hot, like over 90F outside, we would see that the 3rd floor temp would go way over the 73F temp setpoint, going up to over 78F. The installer, who was a Mitsubishi diamond dealer, tried adding another duct and even added a second outdoor unit to move over two other ductless units I have in the house off of the main outdoor unit but it never solved the problem. Since then, the Mitsubishi dealer was acquired by a company that does not sell Mitsubishi so the level of support is minimal. I have not had any luck discussing this problem with Mitsubishi.
At this point, the setup of interest is (all fans are set to auto speed):
– MXZ-5C42NA2 outdoor unit
– MSZ-FH06NA ductless on 1st floor – MHK1 Setpoint 74F
– MSZ-FH09NA ductless on 2nd floor (in bedroom) – MHK1 Setpoint 72F
– MVZ-A18AA4 ducted unit in the attic (it isn’t a conditioned attic so it gets hot) MHK1 Setpoint 73F
Deciding to take more of this into my own hands to understand the problem, I set up temperature loggers in various positions related to the attic ducted unit:
– Inside the ducted unit (in the filter area)
– At the outlet of the ducted unit in the plenum
– At the MHK1 thermostat that controls the unit
– In the attic near the ducted unit to measure the temp around the ducted unit
– I also put an inexpensive anemometer on a duct outlet to get a sense on if there was a drop in airflow that could indicate icing in the coils
Also, to see how the MSZ-FH09NA ductless on 2nd floor (in bedroom) was performing while the ducted unit was running I also set up temperature loggers:
– Inlet of the unit
– Outlet of the unit
– At the MHK1 thermostat for the ductless unit
Attached are the graphs of what I found (including some other details). Some key findings:
– The attic temp peak temp was 101F
– The temperature at the thermostat for the 3rd floor ducted unit went up to 77.4F (73F setpoint)
– The 3rd floor ducted unit was only providing 11F of cooling on the warmest day (6/23/20)
– Looking at a longer period of time (from 6/23/20 08:00 until 6/27/20 0:00) I can see that the typical AC In-Out varies from 9.5-14F when the unit is working to cool the 3rd floor.
– The airflow (the cheap Bluetooth anemometer I used is not very good so doesn’t have much resolution and is a bit noisy) showed that the airflow was increasing when the temperature at the thermostat was 76.5F so I don’t think that there is icing plus the outlet air isn’t very cold.
– On the hot day, the 3rd floor ducted unit was essentially full on until 4:56 the next morning at which time there was a noticeable temperature response at the 2nd floor ducted unit.
– Also included is the house power during peak temp at the 3rd floor thermostat. No obvious power spikes, I think, to indicate a serious issue with the unit.
Regarding refrigerant charge, the total line length is approx. 176 ft. The company that took over the original Mitsubishi dealer, in order to try to figure out the issue, emptied and refilled the unit last year with 9 lbs 9 oz of refrigerant (a number that they calculated). The unit’s factory charge is 8 lbs 13 oz. Using the calculation in the service manual, it looks like the adjustment for line length would be 1.08*(176-98)/5 = 16.9 oz which would indicate that the charge probably should be closer to 9 lbs 14 oz (so 5 oz higher than what they filled). Also, keep in mind that this was filled last year so there is always a chance for a leak that might have dropped it below the fill level.
My questions are:
– Any idea why I am only getting an 11F temp drop across the inlet and outlet of the ducted unit on the hot day (and up to 14F in the best typical case)?
– Would being 5 oz low in refrigerant explain this?
– Is there more data I need to collect? Anything I might be doing wrong with my data collection so far?
– Is there a known issue with having the MVZ-A18AA4 in an unconditioned attic?
– Does the interaction (sudden temp drop at 4:56) for the 2nd floor ductless important? Looks like too much is being diverted to the ducted unit but at the same time the ducted unit wasn’t able to provide enough cooling.
– Any suggestions on getting help from Mitsubishi? The original Mitsubishi dealer that sold me the unit that it would control to a setpoint of 72F and that has never been the case on hot days.
I’m in climate zone 5A. Much appreciate any suggestions anyone can provide to hopefully finally resolve this drawn out situation.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part