Cooling BTU's

[Retired Guy]

I need to cool a bedroom and am considering a 12000 BTU portable AC unit. The room only has one window and I am unable to manhandle a window unit for the nights that I need the AC. We will put in a split system when we build a dormer in about three years.

Would a portable unit really provide the advertised BTUs much like a window unit or should I over size?

[Shophound]

If you go with a portable a/c unit, do not get the single pipe style often found for sale at home centers. These units must suck air out of your house to cool the condenser, so you're basically sucking air you just paid to cool and sending it outdoors. Look for a two pipe model.

As for sizing, if you really wanna nail it you can download HVAC-Calc at the link above and run your own heat load calculation for that bedroom. Some portable a/c manufacturers have square foot sizing guidelines on the box. If you go that route you're at the whim of whatever thinking they used to arrive at those guidelines.

As for BTU ratings, if portables are rated at the same conditions as permanent split systems are... 80 degrees indoor dry bulb, 67 wet bulb, 95 degrees outdoor dry bulb, you won't be seeing rated tonnage unless you are at those conditions. You can have more than nominal tonnage, you can have less than nominal, all from the same unit. The operating conditions determine the true output.

[BaldLoonie]

Listen to the hound!

One mfr of these suckers did a test. If using a 1 pipe unit on a hot day, it could cause so much infiltration by using room air instead of a 2nd pipe, that the heat that "leaks" in could be greater than the cooling capacity of the unit!

[hangfirew8]

I need to cool a bedroom and am considering a 12000 BTU portable AC unit. The room only has one window and I am unable to manhandle a window unit for the nights that I need the AC. We will put in a split system when we build a dormer in about three years.

Would a portable unit really provide the advertised BTUs much like a window unit or should I over size?

Retired Guy,

I have the 13K BTU Fujitronic portable in my 3-car insulated garage/workshop. It is undersized for that job and cannot keep up 72 degrees on a hot Summer day. It can, however, dehumidify and knock 10 degress off the top, which is good enough for me. If it was a 1.5 car sized workshop, I'm sure it could keep up.

Professionally installed mini-splits come in every size, and would certainly do the trick, but don't have the versatility or mobility that the portables do.

Mine has two pipes. The second pipe uses outside air to cool the compressor and provide make-up for the exhaust air. In addition, the system uses condensation water to cool the pump and then evaporates it out the exhaust to improve overall efficiency.

For your situation, you don't give a room size, construction, floor, roof type, climate/location, so it's impossible to answer your question. If properly sized it will cool a bedroom just fine. I'll tell a story.

I used to live in a 100 year old row-house upstairs apartment. It was about the worst energy pig you can imagine. Huge, leaky windows, tar shingle roof, plaster walls with no insulation. My bedroom was maybe 10x12, and I had a tiny 4,000 BTU window A/C in the kitchen where it did no good.

One sweaty 95 degree night after a 102 degree day, I ripped the A/C out of the kitchen window at about 3:00 a.m. and installed it in my bedroom with cardboard and duct tape, took my third shower of the night, and when I came back, that room was cool and dry and stayed that way for the rest of the summer.

-HF

[jlcash26]

I would recommend a mini-split cost more up front but there efficient, quit, and do a great job.