AC Boxes Made Easy (to understand and build)

[hoosierdaddy]

You wanted to see what you could "get by with". Just why would you want to do that?
Doesn't make logical sense if you are in the design and engineering mode to design and engineer something to "just get by".
"Wow...if I use the smallest duct possible, I can be on the jagged edge of it not working, yet it may still work!" Yeah...that's the ticket.

A 5200btu unit is going to have a fan that will be pushing 120-150cfm. But this cfm is figured with air traveling through an orifice (fan housing) that is 8-10" in diameter. Basically, an 8-10 inch duct.

Boxing that up and only allowing the air to move through a 4" opening or duct, you no longer have the 120-150 cfm you started with. The fan is met with resistance because it's path has been choked down to a fraction of what it started out with.
An 8" hole provides 50.25 sq inches of air flow area.
A 10" hole provides 78.5 sq inches of air flow area.
A 4" hole provides 12.5 sq inches of air flow area.
Do you see what you are doing to the air flow by choking it down to 4"?

The unit may work, and it may appear to be fine, but it is working harder to accomplish the task, and as a result the unit will not last as long as one that has not had to labor it's whole life. Before long there will be a line met and the unit will start to cycle and the wet side will be less and less cool.

So, the unit does not work perfectly as described when no fan is helping.
And yes, you can get the air volume back by adding a fan to help, but there are a few things to consider when adding a fan. First off, CFM does not mean CFM at the exit of your duct run. It means CFM at the exit of the fan.
Take a 250CFM duct booster for example....if you place a 10" piece of flex duct to the exit of the booster, the resistance the air meets from the head of the duct causes the fan to labor, and it can no longer maintain it's 250cfm rating. With a duct booster, I could easily see it being cut in half. They are not very powerful at all...and why they cost $30.

Now, place a 250CFM S&P inline in front of a 10' flex duct run and it will barely know the duct is there, and there will be very close to the 250cfm coming from the exit of the duct.

A fan may be rated at a specific CFM, but at what power?

Nothing has changed, and the unit may work...but it won't work as long as it would, had you decided to not "just get by".

 

[what the]

It's all about the air temperature in-taking into the AC's hot side, and the mass flow rate of the air. If you don't have enough mass flow, your hot side gets too hot, and you end up melting the plastic blower wheel like I did on my 2 hose portable.

The rough formula I figured out for best possible thermal transfer values is
Degrees F rise in air temp = (BTUs Cooled * 0.0693)/(CFMs of air through the cabinet * 0.0807)

Example: My 24k BUT AC unit with a 6" vortex fan rated to 470 CFM will have an increase in exhaust temps by ~43 degrees. That makes my 80 degree room air now ~125 degree hot as shit air.

This will tell you exactly how much hotter the exhaust air would be from the cabinet assuming the AC's radiator was perfect, and air had perfect thermal conductivity. This means that this is the maximum the air will increase by, the rest of the heat remains trapped in the box, increasing the AC's cooling fin temperature until it reaches equilibrium thanks to the increased DeltaT between the fins and the moving air.


I'm building my AC box with 4 inches on both sides of the passive venting, 4 inches underneath, and 8-10 inches in back, cooled by a 6" or 8" vortex drawing air from outside the room, pulling it across the underside of the AC, then exhausting out the back top of the unit and into the attic. Based on my half-assed mass flow calculations, under full load my 24k BTU AC unit will increase the temperature of the air it's drawing by ~60 degrees F, assuming I end up getting 350 of my rating 470 CFMs out of the inline. Most people don't understand just how much airflow you can get in free air with those big flat fan discs.

And yes, I did remember to incline the box a little, and put a drip pan/drain in there so my box doesn't mold on me.

Post more please. Nice to see someone else actually crunching numbers, half-assed is much more than the standard no-assed.

 

[mrdizzle]

overkill is your friend when trying to run an AC box, 4hrs is not a test run. you could keep in running in a wooden box with no fan for 4hrs.

 

[blazeoneup]

overkill is your friend when trying to run an AC box, 4hrs is not a test run. you could keep in running in a wooden box with no fan for 4hrs.

Did you ever once hear me state overkill was bad? Overkill is always good when it comes to removing heat.

Do some math if a 150 cfm is required to cool a ac condenser and coils and I have over 175 cfm moving through the condenser etc how is that not good enough...

Before I was an idiot who knows nothing, Now my math is clearly valid and still you try to make it seem like I know not what I am speaking about. I know I am pushing it, I knew that long before I found this thread, I didnt come in here for help or opinions I come in here to share my box and thats about it. I keep it real, If the box dont work I'll be sure to share that with the community....

The main reason larger ducts is important isnt for airflow in cfm, If you hook a 150 cfm fan up to a 4" duct and then also a 8" duct the fan will only push 150 cfm no matter what the differencce in duct size, The difference is pressure and velocity, In the 4" pipe the same 150 cfm would be moving at much higher pressure and the larger 8" would still be moving 150 cfm but at a lower pressure so the air generaly moves slower cause its not restricted.

That itself is what makes the larger ducts better, The lower the pressure and velocity the more heat it can pick up from the condenser and coils as it passes, The faster the less heat it will remove, the longer the air is in contact with the condenser and coils as it passes the more heat it will remove.

The real gain I would get with larger vents isnt in cfm, Its in pressure and velocity and if I were to double the intake size and exhaust size airflow in cfm would be very simular, The difference would be lower pressure less velocity and in turn the same 150 cfm of air movement would remove more heat because its passing the condenser and coils slower allowing more contact time which in turn equals more heat removal.

You guys can quit acting like I am some newbie who has no clue what I am talking about....

Pressure and velocity is what allows larger vents to give better cooling with the same rated fans. I'm done debating my math was fine... yeah I went minimal with it but to say I am clueless is a nothing more then a laugh. To tell me I'm gonna melt something and the ac will not last long is quite stupid because that's not based on anything more then opinion. I done the math and the math dictates I meet the minimal requirements to cool it, Yeah its a may or may not work...

4 hrs is not a test, I was being sarcastic because our hoosierdaddy was stating I am an idiot and have no clue.

Then he post my ac requires 150cfm to properly cool it, I have the fan built into the ac and the 170 cfm inline blower and somehow that's not near enough lol.

You guys keep acting as if I am saying bigger isn't better when, Its obvious bigger is better. It's also obvious my box is meeting the reccomended airflow to cool the condenser and coils actually probably slightly above the needed 150 cfm.

Now if you want to say that because I have such small openings the pressure and velocity increase will make the air cool the unit less effeciently because its moving past the coils faster.

I would agree with that! But to say its gonna melt the fan and blah blah that's nothing more then hater status....

I dont need anyone to tell me the obvious, I know my situation you do not, So acting as if your positive I could have went with 6" etc etc is funny, I built my room and I have meter readers who need to walk past them to get to the meters, I have one wall available to me for ventilation. I am limited in that space and how large my ducts can be, If you knew my situation you would know why I went with the minimal possible.

You do not know my situation so anything you tell me I could or can do is speculation...

 

[DenseBuds]

Wow, this thread has soured. Used to be fun to come by and look at people's takes on hoosier's initial postings. Not really sure what the in dept discussion is truly about as I admittedly skipped a lot of it in recent days.

 

[hoosierdaddy]

bou,
When you choke down the exit port and restrict the airflow, your AC unit fan is no longer pushing at 150CFM. And when there is ducting connected to your inline fan, it is no longer pushing air out the end at it's rated CFM either. You need to grasp this part first.

 

[frikn squirrel]

Thanks Hoosierdaddy for the excellent tutorial you have provided! The AC box I build is doing just a fantastic job cooling my grow tent. I have a 5000btu AC in there cooling a 1000w in 85-90 degree ambient temps no problem.

One inquiry I have: is my AC unit doing an efficient job? the AC unit seems to draw approx. 450-550 watts at any given time. It's an older model -it has low cool and high cool settings along with a dial 1-10. I have it set on high cool at 5. It doesn't seem like it ever turns off, even though the room is quite cool it continues to pump more cool air in. Is this normal? Though sometimes I hear it making louder noises and then suddenly go quieter (as if shifting gears or something), electrical draw stays the same. I was expecting it to be able to cool the tent to a certain temp and then turn off for a while.

Another question I had was in regards to fresh air. My room exhaust is on a temp thermostat and turns on whenever the top of the room gets to 78. Normal room day temps are 73-76 and night temps are 65-69. On cooler days, when the lights are on, sometimes the top of the room doesn't ever get hotter than 75. Should I tweak my settings so that my temp controlled exhaust goes on more? I also have a repeat cycle timer and was throwing around the idea of using it to have the exhaust fan come on 1 min/30 mins off or something like that... this could help provide fresh air consistently and also lower the humidity of the room which hovers 48-58%. I would like something lower for flowering to prevent mold issues.

Anyways, just hoping to get some feedback with others who went through a situation similiar to mine... thanks!

 

[LiLWaynE]

One inquiry I have: is my AC unit doing an efficient job? the AC unit seems to draw approx. 450-550 watts at any given time. It's an older model -it has low cool and high cool settings along with a dial 1-10. I have it set on high cool at 5. It doesn't seem like it ever turns off, even though the room is quite cool it continues to pump more cool air in. Is this normal? Though sometimes I hear it making louder noises and then suddenly go quieter (as if shifting gears or something), electrical draw stays the same. I was expecting it to be able to cool the tent to a certain temp and then turn off for a while.

Anyways, just hoping to get some feedback with others who went through a situation similiar to mine... thanks!

that noise you hear is the compressor kicking off and on... the compressor is what pushes out the cold air... the ac will always be on, and when its on, the fan will always be blowing... it will only blow cold air when the compressor turns on... for example, my AC has a digital thermostat, so whenever it goes above 75 which is what I have it set at, then the compressor kicks on and cools the air back down to 75 then shuts back off while the fan continues to operate... when the compressor kicks on, the ac pushes out tons of heat through the back...

 

[frikn squirrel]

Thanks LilWayne, good to know... that also explains why my AC box exhaust is sometimes really hot and sometimes not so much.

I'm still tossing around the idea of hooking up my exhaust fan to a repeat cycle timer instead of a thermostat control. My AC box has absolutely no problem keeping the room cool and the temperature even at the top of the room is only 1-2 degrees higher than the top of the canopy.

It's not a problem right now, but once the plants start to smell I will be worried about odor control. Right now my exhaust fan (which pulls air from a carbon filter hanging at the top of the room) does not turn on frequently at all because the air conditioner keeps the room too cool to activate the room exhaust thermostat.

 

[Eschertias]

Thanks LilWayne, good to know... that also explains why my AC box exhaust is sometimes really hot and sometimes not so much.

I'm still tossing around the idea of hooking up my exhaust fan to a repeat cycle timer instead of a thermostat control. My AC box has absolutely no problem keeping the room cool and the temperature even at the top of the room is only 1-2 degrees higher than the top of the canopy.

It's not a problem right now, but once the plants start to smell I will be worried about odor control. Right now my exhaust fan (which pulls air from a carbon filter hanging at the top of the room) does not turn on frequently at all because the air conditioner keeps the room too cool to activate the room exhaust thermostat.

If you AC box is drawing air from outside the room, then you can do what a lot of larger growers do, and make a little tower of a single 6" carbon filter and a 6" Vortex fan. Plug the fan it and it'll basically scrub all the air in the room every 2-3 minutes, which will do wonders for the smell in there. My room used to get pretty funky, to the point where I would smell like I had a bowl after I spent 15 minutes in there. Now I can open the door and there is only the faintest hint of sweet sweet green.



My AC box is nearing completion, soon I shall have a 32*22 box, with internal partitioning, two seperate 6" intakes in the passive vent partition, and a 8" vortex fan for exhaust in the hot waste heat partition. I should have between 550 and 750 CFMs blowing though my vortex and out into the attic, which according to my calculations, will keep my exhaust air well below 50C, which incidentally is the maximum rated air temperature for the inline fans. Most people with powered exhaust systems like that forget how hot that air gets.

 

[blazeoneup]

bou,
When you choke down the exit port and restrict the airflow, your AC unit fan is no longer pushing at 150CFM. And when there is ducting connected to your inline fan, it is no longer pushing air out the end at it's rated CFM either. You need to grasp this part first.

I completely agree with this post, But do the math 170 cfm + 150 cfm = 320 cfm, Even if how I am running it cuts the performance down by 50% I'm still golden!

I am happy to say I have done all the testing I need to, To insure the box works. I know your just trying to help everyone and offer constructive critism if you will, But I've tested the box and it performs just fine.

I went and picked up a indoor outdoor thermometer and drilled a small hole in the box to fit the probe down into the box and cranked the ac to 64f which is its maximum setting and put the fan speed on max, I left the probe in the box for the past 5 days or so...

The maximum temperature the box reached was 107f which is perfectly acceptable. No way will 107f cause any components to melt or the ac to work less effeciently! I'm sure as long as my intake air stays in the 75-79f range the unit and box will perform perfectly fine.

I know you were just offering constructive critism, and in the end with an ac box intake and exhaust bigger is better. The numbers dont lie and if the box wasnt working properly the temps inside the box would have to reach 110f or higher for the unit to become less effecient and risk ruining any of the components.

Seeing the max temp inside the box was 107f, I have to say the math panned out and the box works. I dont wanna argue with you or anyone else I have no need or time to waste with such chilidish arguments. Overkill is nice but its not a must, the minimal setup is working just fine.

Here is the completed box and such, I'm going to be monitoring the minmum maximum temps permanantly on this box seeing you are so positive it cant work, I will make sure if it gets above the 110f mark to increase the intake and exhaust vent size. If I do I will do a follow up post to let you know the temps rocketed above the 110f mark.

As it stands the maximum temp in 5 days of running it full blast 12/12 is 107f which isnt to bad IMHO....







This is the minimum maximum for the past 5 days, Of course this unit runs at night and only during the 12/12 cycle although I do let the inline fan run 24/7.


This was the current operational temp when I took the photo's!


Until next time be safe and take care!

 

[calamus]

poly film, gorilla tape and foam insulation board of your choice. Dont duct the cold side if you can manage. try incorporating your hot out on ac to a ducted reflector and then out, this collects all hot-side air and poops it out in one solid chunk, oh and insulate the hood

 

[Eschertias]

New box is finished and hooked up, 18,000 BTU AC unit is keeping my room at a stately 83 degrees with 4 KW of lights in it.

Exhaust temps never got any hotter than 125 F, which is right at the upper limit of the Vortex fan's temperature rating. I might get some inline duct boost fans to pressurize the two 6" intakes to help cool the unit down when I crank it down some more, or to help boost the pressure in the light hoods to help evacuate more air.

 

[Lone Wolf]

New box is finished and hooked up, 18,000 BTU AC unit is keeping my room at a stately 83 degrees with 4 KW of lights in it.

Exhaust temps never got any hotter than 125 F, which is right at the upper limit of the Vortex fan's temperature rating. I might get some inline duct boost fans to pressurize the two 6" intakes to help cool the unit down when I crank it down some more, or to help boost the pressure in the light hoods to help evacuate more air.

lets see some PIX!

 

[louie]

So I have a question for you guys that have built the AC box. I just built one of these boxes with the insulation board about a week ago. I have a 15, 000 btu AC. I am running about 15 ft. of 8 inch insulated ducting for the exhaust to the outside and the intake is coming for inside the outer room (the back of the AC hangs out a different room from where the cold air blows into). When I initially built the box I had an 8 inch. vortec blower helping the exhaust. The problem however was that the back of the AC hangs into the same room as my natural gas water heater. And since I had some much sucking power with the vortec blower assisting the intake of the AC, it actually caused a major problem with my water heater. The exhaust and heat from my water heater was being sucked into the room and out through the AC because of all this extra sucking pressure from the AC and vortec. I was lucky I noticed it right away because my water heater was almost junk (brand new) and it was exhausting the Carbon monoxide and heat improperly causing the water heater to get extremely hot. I eventually diagnosed the problem by disconnecting the water heater exhaust ducting which then gets vented out of my chimmney and put a lighter to the end of it, and it should be sucking out naturally and suck the water heater exhaust up and out the chimmney. The problem was when I had my vortec and AC running, the water heater exhaust duct was blowing air back into my house because of all the suction from the AC and vortec. (It took me a while to come to that conclusion, but when i turned off the vortec, the water heater exhaust went back to normal) So now I basically can't run the vortec to assist the AC intake, but I am still getting good exhaust, but not as good as when the vortec was in use and I think this is compromising the efficiency and overall coldness of the AC. What got me worried was last night I put my hand on the back of the AC after running straight for a few hours and it was HOT. Hot enough that I could keep my hand on it for maybe 3 seconds. Is this normal? Is it too hot? If it is too hot what can I do to assist the exhaust without messing with the exhaust pressure of my water heater? I thought about making another box around the intake and then pull the intake for the AC from outside, but can't because I have no where else to tap into outside to so. Ive got about 3500 watts of light and it is only keeping the temp at 86-87 degrees right now. What do you think?

 

[hoosierdaddy]

louie,
Your outer room that has the AC intake, and also has the water heater unit, is a lung room of sorts for the AC. As such, it needs to have sufficient intake of air to satisfy anything within it asking for air flow, such as the water heater exhaust and your AC unit.
What I am reading tells me that there is not sufficient intake into your outer lung room, so the water heater line is trying to supply it...or you could look at it as if the AC and fan wants air, and can't find it except through the WH ducting.
And no, the AC unit should not get so hot on top at the intake that you can't keep your hand on it.

I would first find a way to get more air into the room. A vent through the roof, or to the subfloor, or outside. Or maybe a door gets a vent cut into it? Some way to increase the air volume into the room.

 

[Gold123]

So I have a question for you guys that have built the AC box. I just built one of these boxes with the insulation board about a week ago. I have a 15, 000 btu AC. I am running about 15 ft. of 8 inch insulated ducting for the exhaust to the outside and the intake is coming for inside the outer room (the back of the AC hangs out a different room from where the cold air blows into). When I initially built the box I had an 8 inch. vortec blower helping the exhaust. The problem however was that the back of the AC hangs into the same room as my natural gas water heater. And since I had some much sucking power with the vortec blower assisting the intake of the AC, it actually caused a major problem with my water heater. The exhaust and heat from my water heater was being sucked into the room and out through the AC because of all this extra sucking pressure from the AC and vortec. I was lucky I noticed it right away because my water heater was almost junk (brand new) and it was exhausting the Carbon monoxide and heat improperly causing the water heater to get extremely hot. I eventually diagnosed the problem by disconnecting the water heater exhaust ducting which then gets vented out of my chimmney and put a lighter to the end of it, and it should be sucking out naturally and suck the water heater exhaust up and out the chimmney. The problem was when I had my vortec and AC running, the water heater exhaust duct was blowing air back into my house because of all the suction from the AC and vortec. (It took me a while to come to that conclusion, but when i turned off the vortec, the water heater exhaust went back to normal) So now I basically can't run the vortec to assist the AC intake, but I am still getting good exhaust, but not as good as when the vortec was in use and I think this is compromising the efficiency and overall coldness of the AC. What got me worried was last night I put my hand on the back of the AC after running straight for a few hours and it was HOT. Hot enough that I could keep my hand on it for maybe 3 seconds. Is this normal? Is it too hot? If it is too hot what can I do to assist the exhaust without messing with the exhaust pressure of my water heater? I thought about making another box around the intake and then pull the intake for the AC from outside, but can't because I have no where else to tap into outside to so. Ive got about 3500 watts of light and it is only keeping the temp at 86-87 degrees right now. What do you think?

You have negative pressure in the room with the vortex on. As HD said you need an open door, window or vent for that room.

 

[louie]

Thanks for the fast replies! So I do have (2) 4 inch drier hose vents that I am not currently using in the lung room, do you think if I left those uncovered (essentially (2) 4" holes bringing outside air inside) will that be enough extra intake into the lung room to accommodate and not cause the problem with the Water heater exhaust while running the vortec?

 

[Gold123]

Thanks for the fast replies! So I do have (2) 4 inch drier hose vents that I am not currently using in the lung room, do you think if I left those uncovered (essentially (2) 4" holes bringing outside air inside) will that be enough extra intake into the lung room to accommodate and not cause the problem with the Water heater exhaust while running the vortec?

Try it use smoke to check flow/backflow of air. If necessary you could add a duct booster or another vortex to the dryer vent/vents pushing or sucking into the room.
It's possible to hook them directly to the ac box you made, but there may not be enough air from that small of ducts.

 

[slaveinc]

real quick

real quick

I am having a problem with the fact that even with the box the air is still being pulled from the room. Then exhausted out the back. i must be missing something. I have a sealed room and have my central ac routed to it but dont want it running all night. So i bought a 15000 btu window ac thinking i could fabricate a duct that would just fit over the outlet of the ac. I could then have the whole unit outside of my room to supplement my cooling, Would this work? any help would be appreciated.