Mini split size question

[Violetmary]

I need a mini split for an open living room/dining room/den area that's about 700 sq. feet.
The house is old and poorly insulated and the windows are single pane.

A contractor says I need a 24000 btu mini split, but all the sizing guides I've seen online
say 1800.

Do any of you have a suggestion? Thank you!

 

[gthomas785]

24k seems large for a space of that size, but the only way to know is to do a manual J calculation:
https://loadcalc.net/

Btw, for cooling, oversizing the unit will result in poor dehumidification.

 

[begreen]

It's best to not oversize the unit. A good quality 1.5-ton (18,000 BTUs) should handle that area well.

 

[EbS-P]

How is the space being cooled (and/ or heated)now.

 

[Violetmary]

24k seems large for a space of that size, but the only way to know is to do a manual J calculation:
https://loadcalc.net/

Btw, for cooling, oversizing the unit will result in poor dehumidification.

Thank you, gthomas! That's what I thought.

 

[peakbagger]

Unless there are humidity issues like cooking, open water like indoor pools or wet basements which add latent (hidden) heat load being used to remove moisture from the air (unlikely in LA) , go with the smaller unit. Also keep in mind that its far better to precool an area earlier in the day and keep it cool than to come home to very hot house and try to cool it in short period of time as all the walls, ceilings and furnishings are all hot and have to cool down in addition to air in the room. An adequately sized unit will feel too small during the initial cool down stage. In many cases it makes sense to have a AC on timer to kick on a few hours before someone gets home rather than cool all day but your electric rate plan will decide.

Keep in mind sealing air leaks and reducing heating load from the outdoors is important. Outdoor shading is the best as it keeps the sun out of the building. Insulated or curtains with reflective surfaces help but they are less effective than external shading as a big portion of the heat coming in the windows do not reflect back outside.

 

[Violetmary]

peakbagger,

Thank you for such an informative, helpful response!

 

[Highbeam]

Be careful not to just base sizing on the 700 foot room but on the entire area that will be conditioned. We see the same thing with woodstoves where people only consider the actual stove room. I have a 24k minisplit in a tiny room that is open to the rest of the house so it conditions everything. Works well.

 

[Violetmary]

Highbeam,

Thank you for this! It's exactly what the last contractor told me.

 

[fbelec]

mini split ductless are very forgiving they are not like central units of the past. if you have a higher ceiling than most and if you are in southern California where the sun shines bright and if you have a lot of people over (people put out 400 btu each) then i would go with the 24000 btu one. all of them have lots of sensors on them and only draw what is needed to cool the space that it is in. a half a ton like you are talking is good 1 to 2 tons of extra cooling is not.