Attic AC Help needed

I've got an 8x4x8 R12 insulated box I'm trying to cool in my attic. Tried a 12000 btu portable AC with about a 20 foot duct venting out and it won't cool the space at all. Only lows hot air during hottest part of the day.

I have a central AC duct in the space I could unhook and use to help cool it and it would also allow cool air from the house to come into the box.

Considering using a window AC and venting it out of a soffit or a Split system.

Any help, particularly as to why the portable unit isn't working, would be appreciated.

It's gotta stay in the attic unfortunately. Money isn't an issue if I know spening a lot will fix the problem for sure.

420dreamer said:

I've got an 8x4x8 R12 insulated box I'm trying to cool in my attic. Tried a 12000 btu portable AC with about a 20 foot duct venting out and it won't cool the space at all. Only lows hot air during hottest part of the day.

I have a central AC duct in the space I could unhook and use to help cool it and it would also allow cool air from the house to come into the box.

Considering using a window AC and venting it out of a soffit or a Split system.

Any help, particularly as to why the portable unit isn't working, would be appreciated.

It's gotta stay in the attic unfortunately. Money isn't an issue if I know spening a lot will fix the problem for sure.

Click to expand...

Where is the portable a/c located at? Is it inside the box? Is it a single duct or dual? If dual, where is it getting air from? The primary reason that an a/c will stop working, is because it can't get rid of the heat. In the case of a dual vent, is it drawing hot air from the attic? If it is a single vent, is the unit sitting in the attic? I wouldn't even screw with a window unit inside of an attic, I can't imagine it going any better than than the portable. As for the stealing of a/c from the central a/c unit, why screw up the balance of your entire house? If you want the problem just "SOLVED", get a small mini split installed, and be done with it.

PS, if you can add more insulation, I would.. if that attic is as hot as I think it might be, r12 walls inside a 120 degree attic would pose a serious heat load.

The portable unit is sitting inside the box and it is a single vent exhaust that runs out of the box and exhausts about 15 feet above through a table vent.

The portable ac works when the temps are low.

How difficult is it to install a split system? Finding qualified help that I can trust might be a challenge.

You need to vent the attic with a fan. In my attic there is a small window at each end. Open both windows and place a fan in one to pull the air in from the other window and out the window it is mounted in. You need a fan that can get the attic down to outside ambient temp. That will be the best you can do with the attic temp.

Next you need to be able to cool the light with its own separate sealed loop. Hooded light with glass seal or cool tube. A fan needs to draw air from the attic right through the light and out the cab into the attic.

This will put you in the best position to be able to keep the inside of the grow box cool with the portable a/c. It might still be tough to do.

420dreamer said:

The portable unit is sitting inside the box and it is a single vent exhaust that runs out of the box and exhausts about 15 feet above through a table vent.

The portable ac works when the temps are low.

How difficult is it to install a split system? Finding qualified help that I can trust might be a challenge.

Click to expand...

Well, that is a little unexpected.... When you say temps are low, do you mean in the box, or do you mean in the attic/outside? If the a/c is doing its job, the box should stay at a pretty consistent temp. If it is just a gradual thing, where the box heats up, and the output of the a/c goes down, to the point of not working..... I would bet that the vent just isn't exhausting enough air. This could cause the output of the a/c to be greatly reduced. Then combine that with a rise in the temp of the box, and it will reduce the output even more... to the point that you have a hot box, with little a/c being delivered. I would see what would happen, if you just run the exhaust vent far enough to get outside the box, and see what happens; or try putting a booster fan on the exhaust. Hmmm .. you do have a fresh air supply into the box, right? Was just thinking that if the box was built tight enough, that it may be under enough neg pressure to reduce the a/c exhaust enough to cause issues.

Trying to do a DIY mini split .... ugh... it is in theory possible, and actually some of the kits are designed for it with quick disconnects on a pre-charged line set. Those could be installed without much knowledge/tools. But you will pay a premium for that feature, and I don't care for the quick connections all that much. There are a lot of youtube videos on people installing them, you should be able to get a pretty good idea if it is something that you want to pursue.

iTarzan said:

You need to vent the attic with a fan. In my attic there is a small window at each end. Open both windows and place a fan in one to pull the air in from the other window and out the window it is mounted in. You need a fan that can get the attic down to outside ambient temp. That will be the best you can do with the attic temp.

Next you need to be able to cool the light with its own separate sealed loop. Hooded light with glass seal or cool tube. A fan needs to draw air from the attic right through the light and out the cab into the attic.

This will put you in the best position to be able to keep the inside of the grow box cool with the portable a/c. It might still be tough to do.

Click to expand...

Ahhh different approach... instead of trying to ramp up cooling, decrease heating ... maybe try a little of both.

If using a portable AC with a single exhaust then your box is pulling hot air from the attic through every gap in your box to replenish the air it's pumping out. Attic air is very warm. Mini split is only option. As someone else already mentioned window shaker won't work for same reason attic air too warm for cooling.

ideal air minisplit

ideal air minisplit

precharged lines
pretty simple install
And mr cool they do minisplits with 23 ft precharged lines for attics

420dreamer said:

I've got an 8x4x8 R12 insulated box I'm trying to cool in my attic. Tried a 12000 btu portable AC with about a 20 foot duct venting out and it won't cool the space at all. Only lows hot air during hottest part of the day.

I have a central AC duct in the space I could unhook and use to help cool it and it would also allow cool air from the house to come into the box.

Considering using a window AC and venting it out of a soffit or a Split system.

Any help, particularly as to why the portable unit isn't working, would be appreciated.

It's gotta stay in the attic unfortunately. Money isn't an issue if I know spening a lot will fix the problem for sure.

Click to expand...

What type of ducting are you using for the 20' run? If its flex duct with all the ruffles in it, your duct is causing to much resistance and restricting the airflow of the unit and not properly cooling the condenser further reducing the efficiency of an already less efficient unit.

Although they are not as efficient as window or split units it should function well enough to cool the space.

If you have a room built inside the attic and its insulated from the surrounding attic space, just vent the split unit to the outside of the room shortening the length of the duct to only whats needed to get the through the wall. This will heat up the exterior space, but I am assuming you can use your household ac to reduce the heat in the space or add an active exhaust somewhere to help remove the added heat from the ac exhaust.

That 15-20 foot run of hose might need an auxiliary fan to help exhaust that hot air. Those portable units have trouble if the exhaust isn't kept at an incline, as short as possible and with as few bends and turns as possible.

Also a 15-20 foot run of exhaust hose is gonna give off a lot heat right back into the attic unless extremely well insulated.

Like b4urtime mentioned, that flex hose with all the ruffles is causing a lot of resistance which would be helped by an auxiliary exhaust fan (at the end of the run pulling the hot air while the unit pushes) but the added surface area of the flex hose might be working just like a heat sink if not insulated.

I would use a squirrel cage fan so the motor isn't in the heat. Maybe it doesn't matter. Check first. These are just random thoughts off the top of my head. I don't want you to listen to me and burn your house down!

Smooth wall exhaust vent would help a great deal. Plus insulation of course.

I don't know why people think a window shaker wouldn't work,I use a 12,500 to cool an 8 by 8 room and the temp outside in Fla is over 90 degrees in the summertime normally.I need a unit that bottoms out at 60F most go down to 65,and even then I must use co2 enrichment but I manage pretty well.I know 90+ degrees is a far cry from 115-120 degrees so maybe a mini-split would be the only option,not everyone can afford one of those.I would try a 12,500 that bottoms out at 60F before spending a lot of money.If you had one to borrow for a couple days that would be ideal if not you can get one for around 300 Beebs.

Many thanks guys. Lost of good thoughts here.

The ac exhaust is insulated flex hose. It's a 5" exhaust and I got a 6" hose.

All I have are soffit vents to potentially vent the attic space with. Should I blow a box fan out of there?

Here are some pics. Also made this fake bookcase to hide it.

One issue (someone already touched on this) that's not always considered is not only does the unit suck in air from out side your grow area to cool the condenser coil and then exhaust back out of you grow space, it's realistically gonna be exhausting cooled air out and filling the room with uncooled air, or a mixture of both.

It's also gonna sometimes continue to draw air from outside your growspace even after the thermostat has stopped it from blowing air over the evaporator coil. If it can ever get to cool the space enough (depending on the temp of attic air).

I think the best case scenario for portable a/c's and small grow spaces is a lung room. That's when you cool the air outside your area and draw from that space to cool your grow space.

If the a/c doesn't put out cool air with the current setup, you can't easily exhaust a window unit and a split system is overpriced or overkill for the space, the easiest way would be to go with a lung room.

An exhaust fan for the grow space, maybe a temp/humidity controller if ya wanna get fancy. I would just constantly recirculate the air myself. Insulation for the lung room. it's not the hardest thing to pull off.

There are other added benifits as well like allowing the a/c to rest longer. Not having to cycle on and off anywhere near as often adds a lot of life to those portable units.(although replacing the capacitor isn't the hardest thing to do).

It also allows for less spikes in temp. which the plants are always greatful for.

Your house will always be better off with more insulation.

More space in grow area for growing.

Maybe just cooling the attic with the house a/c is the easiest. I don't wanna offer any advice on this option because I'm an idiot! Haha

I fucked up more houses than I'd like to admit!haha

On a side note.... I really appreciate the ingenuity displayed by my fellow growers.

I grew in a space above the kitchen in a house for two years before telling my chick!(gotta find another word for her!).

This is the space above the kitchen I grew in long ago.lol