Mitsubishi System Low Delta T

Hi all,

New to the forum and also not an engineer or an HVAC professional (so PLEASE do not yell at me for my flippant and likely incorrect use of Delta T, below), but I was hoping you could help me with an issue that has stumped my installer.

The basic issue is that when the system should be working the hardest to cool the home, it does not seem to be doing that (even when it is capable of it). See details below.

House/Install

• A 3 story rowhouse. Old but fairly well insulated (due to brick walls and having neighbors attached on both sides). HVAC is probably 2-3 ton ducts even though it is a ~2200 square foot house because the HVAC system was installed 100 years after the house was built.
• We installed a Mitsubishi split system last year to try and fix the fact that the primary bedroom on the top floor was completely underserved by the air throughout the house. The idea was also to eventually go ductless in ALL of the top floor bedrooms (2 remaining) and then cap the duct to the top floor and just use the ducted unit on the first floor and basement.

System

• Mitsubishi Smart Multi hyper heat pump
• Branch box: PAC-MKA52BC (it is the 5 branch box, so that we can eventually go with 3 ductless and 1 ducted, but currently only has 2 units attached).
• MLZ-KY06NA ductless unit (in master bedroom on top floor). [I'll call this the "ductless unit" below]
• SVZ-KP30NA air handler [I'll call this the "ducted" unit, below]

Problem

Basically, the system is fully capable of producing a Delta-T of 20 degrees, but this changes throughout the day, and in particular, in the late afternoon (i.e. 5pm-7pm) on hot days, the delta-T drops dramatically both on the air handler and on the ductless unit. Here are some measurements on a hot DC day (90 degree outside):

1. 12:21pm: thermostat at 69, indoor temp at 70 at thermostat, ductless upstairs measures 45 degrees at register, ducted measures at 52 at register (so VERY good performance)
2. 7:15pm: thermostat at 70, indoor temp at 73 (it has been at 70 the whole time, so the system has allowed the air temperature to rise 3 degrees throughout the day while running fairly consistently). Ductless measures 57 return (up 12 degrees), ducts measure 60 (delta T of 13, when it was 18 earlier).
3. 9:54pm: thermostat set to 70, now indoor temp at 74, ductless measures 61 (up another 4 degrees since 7:15pm), ducted measures at 63 (delta T now at 11).
4. 11:38pm: thermostat at 70, indoor temp is 73, ductless measures 63 (up another 2 degrees since 10pm) and ducted measures at 65 (delta T of 8).

At SOME POINT the system resets overnight and the air gets super cold again. Then every day during the summer this cycle rinses and repeats.

Something that does seem to fix it: A hard reset

When I shut it down myself at the breaker and then restart it, I can sometimes get it to drop. For example:

1. At 7:30pm on a hot night, the AC was at 70 (air temp was 71) and the temps coming out of the register were only 62 (so 9 degrees difference).
2. Then I flipped the breaker for 10 min.
3. When I flipped it back on the ducted had temps of 55 degrees (Delta T 16) and the ductless had temps of 52 degrees (Delta T 20)

Tech Troubleshooting

• So far, they have assured me that the pressure seems fine and the measure of the coolant in the system is fine (no pressure problems?).
• They took high tech measurements using a computer for 36 hours and sent it to Mitsubishi and Mitsubishi has said that it appears normal (though my tech is pushing for them to send it to the second level of engineering troubleshooting).

Any ideas what is going on?