Minisplit Set Point and Actual Room Temperature
kyoung
| Posted in General Questions on
Brand new Mitsubishi 4 zone mini split heat pump. The first night I set a head to 61 degrees for an overnight set back temperature. The next morning I get up to a room at 73 degrees. What problem(s) should I be looking for to explain why I have such a mismatch between set point and actual temperature?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Which climate you are in and how big is your place and what is each zone supplying? Most likely the issue is oversized equipment.
I believe I am in zone 4. (Brooklyn, NY) The house is a two floor row house of about 1100 sq ft. One large open space on the first floor and three bedrooms on the second floor. The compressor is a 36k unit. The first floor has a head unit of 18k. The two small bed room are 6k and the master bed room is a 12k.unit. I believe the most important factor in this situation is the fact that this is a row house. IE 2/3 of my "exterior" wall area are somebody elses interior wall so I lose minimum heat through them.
I'm not an expert but I'll predict what the experts will tell you:
Your 3-ton unit is likely 2-3x oversized for your 1100 sq ft space that has limited exterior wall area.
As for the system itself, do a search of this forum. When you match multiple indoor units to 1 outdoor unit, the indoor units have essentially zero modulating ability, and they are ALWAYS circulating some amount of warm or cold air -- even if they are set to be off -- because hot or cold refrigerant is always flowing through the refrigerant pipes (if the ODU is on).
So yeah, I hate to say it, but you might have trouble getting a lot of improvement. Sounds like your contractor needs to learn about manual J and needs to take some extra training courses on multi-splits.
Yep, Aun nailed it. When any of the heads call for heat, warm refrigerant circulates through all of them. Just the one calling for heat runs the fan, but you will get some convection in the others. Your bedroom heads are probably 10x oversized, so even without the fans running, they're going to warm the rooms. Bad system design.
I would guess your place has at most a one ton heating load. If your outdoor unit is one of the hyper heat ones, it can deliver close to 4 tons of heating in your climate.
It is beyond ludicrously oversized, well into plaid territory.
The bottom line is the system will never work properly as is. You are seeing already the issues with heating, next it will be problems with cooling and humidity removal. Because of the large oversizing and cycling, the operating cost for both heating and cooling will also be about 50% more than it needs to be.
The bend aid fix for the temperature overshoot is to configure the indoor units to disable the fan once setpoint is reached. This might require a remote temperature sensor or thermostat on some units. You can check the tech note bellow on what applies to your equipment:
https://web.archive.org/web/20210204174849/https://nonul.mylinkdrive.com/files/Application%20Note%203048%20-%20How%20to%20turn%20the%20indoor%20unit%20fan%20off%20when%20set%20point%20is%20met.pdf
The proper fix is not simple. You need a proper manual J for your place to figure out your actual heating loads then size equipment from there.
My guess is that you would end up with either a two zone setup with a 9k wallmount in the living space and a 9k or 12k ducted unit shared for all the bedrooms or the better setup, wallmount and the ducted unit both on their own individual outdoor units.