Venting a Protable A/C in Attic HELP?

Need help venting a portable A/C in my attic. Most instructions say the flex house shouldn't be more that 7-9ft. I need 15-20ft. So I was going to use 4inch round duct with a inline booster fan. Anyone running a portable A/C? Will my portable A/C work properly?

This will be a back up source of cooling when it's really hot. I have a 9000btu mini split as my primary

It will still work but an extension will create more load on your portable's exhaust fan motor.
If its just a back up it might be okay.
I would get the booster to be on the safe side.

Is it a single hose or dual hose?
A single hose will draw air from the room and expel it out the duct.
Which means air from outside the room will be sucked into the room.
Depending on your outside temps, this will create more load for your primary.

If odor is an issue then you might want to put a filter on the portable's exhaust, then a booster would be a really good idea.

I tried this w and the ac would shut down, circulate air but stop cooling.
It needs the same sized hose.
I did manage to extend it to almost 15 feet using the same sized hose as the ac needed.
It's hot hot air, it needs to be exhausted. Install a roof vent.

Smtty025 said:

Need help venting a portable A/C in my attic. Most instructions say the flex house shouldn't be more that 7-9ft. I need 15-20ft. So I was going to use 4inch round duct with a inline booster fan. Anyone running a portable A/C? Will my portable A/C work properly?

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Is the AC cooling the attic or venting the hot air into the attic? Venting into an attic is generally a bad idea. When it's cold outside the warm humid air condenses in the attic quickly causing black mold.

If you want less static pressure with a duct run you should use galvanized snap seam duct, it's smooth walls don't cause turbulence like accordion duct will. Also upsizing the diameter of the duct will help as well, never downsize it. Most portables I have seen use a duct that is larger than 4 inches in diameter, 5 inches being common.

I should have explained myself better. My grow room is a sealed room in the attic. My main source of heating and cooling is a 9000btu Mini split. The portable A/C would be extra help cooling in the hot summers.I would run a 4inch hard pipe vent with a booster fan 15-20ft to outside. My question is the length of the vent? Is that too far? Will the A/C work properly? Again I'm installing a booster fan to help push the air
QUOTE="Renfro, post: 15773634, member: 1007445"]
Is the AC cooling the attic or venting the hot air into the attic? Venting into an attic is generally a bad idea. When it's cold outside the warm humid air condenses in the attic quickly causing black mold.

If you want less static pressure with a duct run you should use galvanized snap seam duct, it's smooth walls don't cause turbulence like accordion duct will. Also upsizing the diameter of the duct will help as well, never downsize it. Most portables I have seen use a duct that is larger than 4 inches in diameter, 5 inches being common.
[/QUOTE]

Smtty025 said:

Will the A/C work properly? Again I'm installing a booster fan to help push the air

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Only if the booster fan actually moves enough air to make up for the static pressure losses on the long and small duct. For a duct run that long I wouldn't advise 4 inch.

Unless there are a lot of bends in the system (like 3 or more) I would think 6 inch with a fan will do the job. If you have 3 or more bends I would go to 8 inch. Know that most of the axial "booster" fans are actually not that effective. I like to call them duct restrictor fans lol. This is where your system will need the most help too.

Renfro said:

Unless there are a lot of bends in the system (like 3 or more) I would think 6 inch with a fan will do the job. If you have 3 or more bends I would go to 8 inch. Know that most of the axial "booster" fans are actually not that effective. I like to call them duct restrictor fans lol. This is where your system will need the most help too.

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I will be using hard pipe 4inch round duct with just 1 90° fitting. And whatever small bends I will have with the flex house. I was hoping to get away with 4inch.

If you are going to use the smaller 4" duct then I would suggest a more powerful fan to makeup for it. It gets expensive if we cook the compressor in the AC unit because we aren't getting enough air flow over the condenser coil.

If you have a wind speed meter you can check the air speed at the AC unit's open port and then calculate the CFM of the air coming out the back of the unit with no restriction. Then when you have your duct system in place and running you can measure the air velocity coming out of the exhaust end to see if you are moving enough CFM. Thats the best way to know for sure that your moving enough air. A quality 4 inch centrifugal blower might be able to overcome the static pressure and do the job. One of those cheap axial "booster" fans would probably be more of a hindrance than anything lol.