Question about mini split sizing

[VioletMary]

May I ask a question about a room in a house, and not my garage?

If so, I'm deciding between a 24000 BTU or 18000 BTU mini split for a
large open livingroom/diningroom/den. The total area is about 700 sq ft.
The windows are single pane, the house is old and poorly insulated.
One contractor insists I need a 24000 BTU, but according to all the sizing
guides online, I should get an 18000.

Can any of you weigh in? Thanks so much!

 

[billconner]

Primarily heating or cooling climate? I'm guess contractor is right. I wasn't sure I could get away with 12000 for 500 sf, primarily heating, double glazed and very well insulated and tight. But, I'm no expert, just went through same decisions for a new room addition.

 

[VioletMary]

Thank you, billconner! I'm in Los Angeles, so primarily cooling.

 

[pcmeiners]

"The total area is about 700 sq ft."

24,000 btu for a 700 sqft room is ridiculous. At the moment I am cooling a 1080 sqft garage, and a 1800 sqft attached house with a Fugitsu 12000 Btu very efficient minisplit. The garage is cold, the house is reasonably cool as a small fan is all that pushes cool air down a 70 ft long house. So that is 2880 sqft.
If I had multiple fans driving the cool air from the garage, the attach house would be fully cooled. Granted my garage and home are insulated to 1986 standards. It has been 90 degrees for over a week.
Now if you left the windows and doors open then I could possibly see a 24k unit.

 

[VioletMary]

pcmeriners,

Thanks so much for confirming what I thought!

 

[mike93lx]

Mini's are inverters, which allows them to throttle and greatly minimizing the risk of oversizing that occurs with traditional split systems.

I bet 18k is fine, but also that 24k would likely work OK too.

 

[VioletMary]

Thanks, Mike. That's what I've heard, but I also read online that de-humidification can be a problem
even with inverters.

 

[American Locomotive]

24k is way, way oversized for 700 sq ft. 18k is oversized too. A 12k mini split will easily cool that space with headroom to spare. It should also be mentioned that the smaller capacity mini splits are typically much more efficient than the larger ones. Plus, minisplits always have headroom. Mitsubishi's 12,000 BTU unit will do nearly 14,000 if it has to.

 

[VioletMary]

Thank you, American Locomotive! Much appreciated.

 

[428PI]

I'm easily cooling my 1300 sq ft main floor with 27k and that's in an old house with leaky windows and 9 ft ceilings. Tomorrow will be 99. We like to keep bedroom down to low 70s and rest of house mid 70s.

 

[Loose Ctrl]

I was about to ask this same question. I have a mobile home (14x70) with poor insulation and bad single pane windows. I was looking at a split unit with a 18,000 btu unit to cool the living room/kitchen and main bedroom, along with a second 9000 btu unit that would cool the small back bedroom and small bathroom. The bathroom is a walk through from the small bedroom and into the short hall. I live in upstate SC and would use it for heating and cooling. I currently have three window units from the late '80s. A 10,000 btu unit in the main bedroom, a massive industrial commercial 28,000 btu on a 240v 50 amp circuit for the living room/ kitchen area, and a 9000 btu for the small bedroom and bathroom. The old window ACs kept it fairly comfortable as long as the summer temps stayed out of the upper 90s+ and the humidity stayed realatitively low. Over the past few years, the window units have died. It's a good thing because my power bill was insane. In the winter, I heat with two kerosene heaters, an Omni 105 at 18,000 btu and an Omni 15/Moonlighter at around 6000 btu. They kept it comfy until the temps got down around freezing and stayed there. An extra layer of clothes kept things bearable. My thoughts are that the newer split systems are way more effiecent than my old worn out window units which worked way better back in the day. Plus, the split systems circulate air better especially when aided by a fan or two. What say the congregation? I know other people that have installed split systems with less BTU, and they say it stays more comfortable than with the old window units.

 

[VioletMary]

I'm easily cooling my 1300 sq ft main floor with 27k and that's in an old house with leaky windows and 9 ft ceilings. Tomorrow will be 99. We like to keep bedroom down to low 70s and rest of house mid 70s.

Thanks!