Oversized mini split system?
Hello,
Approx 2 years ago while doing a gut renovation on our home, we had a new Mitsubishi hyper heat mini split system installed. We installed spray foam insulation throughout and all new high quality Marvin windows, so the house is very tight. We use the system for standalone heat and cooling. While it has warmed and cooled the house perfectly fine, we have had very high humidity in the warmer months when cooling the house. It does not go below 60-65% humidity in the summer.
The house is 2800 sqft. Here is a breakdown of the room sizes and units size that was installed by the HVAC company.
Living room 357.75 sqft. 18,000 BTU
kitchen 270 sqft 18,000 BTU
Family room 320 – 12,000 BTU
Primary bedroom 225 sqft. – 12,000 BTU
3 guest bedrooms 125 sqft per room 9,000 BTU per room
Can anyone tell me if the units are over sized for my house?
Thanks in advance.
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Replies
We cannot answer definitively. However, 99/100 times when a ductless head is in every room, you’re drastically oversized. I’m sorry it appears like that’s the case here.
Does the 2800 sqft include the basement?
The 2800 does not include a basement. I have an encapsulated crawl space running a dehumidifier 24/7.
I am in climate zone six, and a single 18,000 BTU mini split in my basement heats my entire house (basement and floor above it, 2200 square feet total) for most of the year.
It sounds like an interesting way to go. You said heating the basement only works for most of the year. What do you do for heat on the main floor in the coldest months?
I’m zone 6 as well. If you’re heating your house on one 18k unit then sounds like I’m way oversized. I have two 18ks and one 12k just on my first floor!
Square footage and zone tells you nothing. It is all about windows, how tight and well insulated the two homes are.
Walta
What are your outdoor units? Most likely it is oversized which would create the humidity issues you describe.
There is no easy solution to it, the best option is to try to run one of the heads turned down so it runs near rated capacity to do the bulk of the cooling (say guest bedrooms with the door open). This would improve humidity removal on that head which might be enough to control the the RH in the whole house.
The outdoor units are as described. The contractor (whom is Mitsubishi diamond certified) told me that the most efficient system is 1 condenser per indoor head and not have multi zones.
I have a house about the same size cooled by 3 units, two 9k and one 24 k[and still oversized really]. In the summer room temperatures do not vary badly and it is the dehumidifying that you will notice.
When not at peak cooling try shutting off the guest bedroom units and whatever other ones and manually drive the fan low on whatever units you leave on
I'm not even going to napkin this with the usual "it depends on X Y Z". Simply, they grossly oversized you, even if you were Climate Zone 6 and your primary usage is for heating. For heating, almost every room should be half what they installed. For cooling it's probably oversized by a factor of 3.
Thanks for your response. I’m in zone 6. Now to see if I can hold them responsible and get them to replace the units. They are Mitsubishi diamond certified, I’m not sure if that’ll make a difference or not.
It sounds like you have one outdoor unit for each indoor unit if so simply turn off all but one units and set it to the “dry” mode this should force the unit to run at its highest speed and remove a lot of moisture. If the indoor temp get to high turn on a second unit.
Walta