Another Mini Split question.
 

I need cooling in my guest house which is above my garage. It roasts in the summer. It’s about 900 square feet of open space. Would a 12,000 btu unit be sufficient? What is the advantage of a bigger say 18,000 btu unit?It’s a 2500 dollar difference. Thanks

I'm not a HVAC tech, but not an easy question to answer without a bunch of other info, how well are the roof and walls insulated how much direct sunshine? How many windows does that sunshine get through? If the room stays at a pretty reasonable temp for most of the day say until late afternoon the 12k could be enough, if it heats up early in the day likely need larger unit.

I see you’re in California, and I’m not sure the design parameters there. In Georgia, as a rule of thumb, it is 600sq feet per ton. Obviously, lots to factor in that formula. Maybe someone in the HVAC biz in your area can chime in for square footage versus tonnage. I love mini splits, but buy a name brand that is serviced in your area. I’ve had several customers buy cheap junk online only to have it fail in a few years with no parts availability. Best of luck.

Bugs, here is a pretty good sizing guide for mini-splits. BTW this company (part of Power Equipment Direct) is legit, as I've bought from them with no issues.

Oh, and they have other tech and selection articles worth looking at on the website.

https://www.ecomfort.com/stories/1185-How-to-Properly-Size-Your-Mini-Split.html

DX units are tricky because if you undersized they won't provide enough cooling.
If you oversize they will cycle too much and not dehumidify.
900 sf is really pushing it for a one ton unit.
Especially with any glass or doors, or lots of direct sunlight.
1.5 ton at a minimum.

^^^^ What BK said .... this isn't in my arena of expertise .... but it is in his ;).

This.

The unit is very well insulated with a sliding door facing the south side. It’s has a sliding shade that has helped keeping the direct sun from beating on it. I leaning to the bigger unit. I’d rather make a $2500 mistake and over size then a $4000 mistake and undersized. Thanks for the reply’s.

Looking at a Fujitsu unit.

Rule of thumb, 800 s.f.per ton. 1.5 tons as bk911 says. You want to dehumidify the air, otherwise you’ll feel cold and clammy, not fun. Look up hvac load calculation form on the web, they ‘re easy to fill out, and you’ll have a better idea of what cooling requirements you’ll need. Any competent hvac contractor will usually do that for you.


H

I guess this is as good a place and time to ask this question as any. It comes up every time I read something about air conditioning:

What is the origin and purpose of rating AC capacity in "tons" instead of BTU? I know 1 "ton" equals 12,000 BTU, but why not just say "12,000 BTU" the way you would for a furnace?

Metric vs standard.

Just a unit of measurement?

My garage is about 900 sqft and I have a 2 ton mini-split. It keeps it cool but not 'living condition' cool. It's likely not as well insulated as your guest house but I'd still say 2 ton is about right.

I’m having an HVAC contractor supplying and installing the unit. They recommended the 18,000 BTU system but also said if price was an issue they said the smaller unit would work but it would take longer to cool the area. I’d rather spend the extra money and not have any regrets. 18,000 BTU unit ordered!

Bug's humidity factor is low. But it gets hot in the middle of the day. In August it just stays hot. What heat do you have, Bug's?

BTU is the amount of heat required to raise the temp of one pound of water one degree F. Latent heat of fusion(hidden heat) is 144 btu/lb. The Brits determined It is the amount of heat required to melt one ton of ice in 24 hours. 144btu/lb.X2000lbs.=288000btu./24 hours=12,000btu’s or one ton of cooling. Heating just uses btu’s as a constant. Heating devices are rated in btu’s only.

I hear this question a lot and the best answer I have heard was "because".

Also...

12,000 btuh = 12 mbh.
Why not 12 kbh?
Because!!

I am surprised at the $2500 extra for the 18k BTU, should not be that much more, do you need to run power from a different electrical panel or something?

It has a gas wall furnace and a wood stove. The wood stove works great. Only a few pieces of wood and it’s quite toasty.

Good question…. I’ll ask but I’m questing more profit.