Another Mini-Split sizing question

[Titus]

I am about to order a mini-split for my shop, and I am afraid to order something too small despite all I have read here. I have a 400 sqft metal building, which most seem to say I should get a 12000 btu unit for. Despite this, the little voice in my head is telling me to get an 18000 btu unit based on the following extenuating circumstances:

- I am in North Texas, so the summer days are usually over 100 and the evenings rarely get below 80.
- The shop has a gambrel roof that peaks at around 14'. Probably a bigger issue for heating than cooling, but still a consideration.
- It is not very well insulated- it has the supper thin bubble wrap insulation
- I use it on weekends and a couple weekday evening a week, so I won't be keeping it supper cool all the time, but want to be able to go out there and be able to drop the temperature by 20 degrees in no more than an hour.
- I am not too concerned about added humidity from an over-sized unit as I already have a dehumidifier out there that is piped to a drain and kicks on when needed

So what do you think? Do I listen to the little voice and get the 18000, or will I be fine with the 12000?

 

[JMURiz]

I'd probably go with the 1800, I have one in my ~500 sq/ft vaulted ceiling space. Mine also has a dehumidifier function (Mitsubishi). Mine has R13 walls and R30 ceilings too.

I think 1800 would be best, but hopefully an expert will chime in.

 

[jm1fd]

Are you getting an inverter driven mini-split? If so, get a 24k BTU unit. Inverter units vary their capacity to meet the heat load, so it will run at 6k or 9k BTU or whatever, if that's all that's necessary to keep things cool.