# Cooling calculations



## WrEkkED (Mar 30, 2013)

I was wondering if anybody knew of any calculator that could be used to determine cooling needs. I have looked everywhere on google trying to find something of this nature.

So question is, how much air (cfm) is needed for a certain intake temperature to keep a certain lamp cool?

For instance, I have a 600w being cooled by a 440cfm sucking out of a 6" cool tube with intake temps of 77 degrees. If I raised my temps to 78 or lowered my temps to 76 what would the difference be? 

Might just have to get an infared thermometer and make a chart if no one else knows of anything like this. Not that I'm too concerned about temps, more of a curiosity and would help people in designing their rooms/tents.


----------



## BackWoodsDrifter (Mar 30, 2013)

For instance, I have a 600w being cooled by a 440cfm sucking out of a 6" cool tube with intake temps of 77 degrees. If I raised my temps to 78 or lowered my temps to 76 what would the difference be? 

Bout 2 degrees I reckon 

BWD


----------



## terky (Mar 30, 2013)

I am an HVAC professional. You should see my ventilation system lol.

Too many variables to answer. 
Is the duct insulated?
whats  the actual amount of air coming in. Because I guarantee you that fan is  not moving 440cfm through a 6" duct. The 440 rating is without any duct  connected to it. Duct causes friction, slows the air down, requiring  more energy from the fan to move air through it, this decreases a fans  efficiency. 

Then you have to calculate the heat coming through your walls that the sun is shining on. Length X height X 40 BTU.

Many more calcs to get close. Like how well your house is insulated.

What  are you trying to do? Predict cooling for the summer? Get the biggest  fan you can, run the biggest duct you can. That's about all you can do  if your not getting AC. Maybe look into an attic fan thermostat. It  works in reverse so when the temp rises to say 80*, the fan kicks on.  They may also make them with a built in speed controller. So it will  speed up or slow down the fan to keep a constant temp.

My advise  is to get a good DUCT SYSTEM. DUCT is the limiting factor for most  people. These little fans just cant pull through 6" duct like people  want them to. 

My hoods condense water on the air intake because  they are so cold. They don't even get warm and I have 1000 watters. I  have to close off the duct if I dont have all the lamps on because the  whole lens will condensate and be covered in water on the bottom.


----------



## Hushpuppy (Mar 30, 2013)

I'm not sure about your question here but I think you are asking the wrong question. The reason I say that is temperature isn't affected by air speed unless there is such a force of vacuume that it lowers the air pressure within a certain volume of air. Then you would get a drop in ambient temperature due to there being fewer molecules in a given air-space. But for our purposes, we are not trying to cool the lights as much as remove the heat that is generated by the lights. That is why we put them in the vented hoods or cooltubes, so that we can control the air around the lights. If you continually mover 75degree air past the bulbs at a high enough speed and then completely remove it from the space, then the air around the outside of the hood will not get much more than 75 because it is being removed as it is heated. It is also being replaced by "unheated" air that is at 75f.

So really what you should be looking at is the amount of cube space for the volume of air that you want to replace and then see how much the fan removes. For your fan that moves 440cubic feet per minute(unrestricted by filtration), if you have a space that works out to be 220cubic feet(given by the formula lxwxh) then your fan will remove the volume of air in your space 2x every minute. If your space is 880cubic ft then your fan will replace the air in that space only once every 2 minutes.

I would say that it is best to have a fan that removes the cubeic volume of air *at least *one time a minute to keep that space close the ambient temperature of the incoming air. If you are cooling your lights separately from the whole grow space then the cubic volume of the sealed hood(s) and the hose leading to the outside need to be figured, and a certain amount of loss of volume movement figured in for the restrictions from the hose itself as well as any turns or other restrictions like a filter. I believe that these restrictions are lessened by having the fan closest to the exaust point so that it is pulling the air rather than pushing it.

If you do as many do, by having the air removed from the grow space through a carbon filter and through the lights then you are utilizing the exaust fan for 3 items at once which is the most cost effective. You are replacing the hotter air with cooler air which cools bothe the grow space and the lights, and you are filtering the grow-space air before releasing it outside, and you are continually pulling in fresh co2 for the plants.

I hope this helps more than confuses


----------



## WeedHopper (Mar 30, 2013)

Way to many Variables in different growrooms in different places,, to do that. Besides,,they make controllers to raise and lower Fan Speeds to help with what yur talken about.


----------



## WrEkkED (Mar 30, 2013)

I myself actually have very little ducting. I have no duct to the tent, or duct attached to the intake of the cool tube (at the moment) and then one piece of flexible ducting about 3' with one 90, then my fan and one more rigid 90. I'm exhausting into the same room as I'm sucking in, but I'm in a small enough condo that I keep the air on at 77 and the room it's in actually gets 2 degrees colder than where I located my thermostat.

I did some testing today and found that with 77 degree air going in, reading from outside the tent just before the intake vents, my exhaust temperature is 84.

I want to experiment to see if it's exponentially different or linear. I have tonnes of moving air since it's only a 2' x 3' tent and I will soon have 2 x 440 fans going. One with 2 90's going straight through the tent and trough the cool tube and one sucking through the tent, through a filter and exhausting. 

The main reason I'm curious is because I want to be ready for when I put my new babies into the tent with a mh after my current plant is finished flowering. I find that regular thermometers are unreliable as far as ambient temperature because at the canopy they get way too much radiant heat. Infrared will tell you the temperature of the leaf but that doesn't tell you anything either. Outside on a 90 degree day in direct sunlight, it will sow way hotter than 90 as well, but this has no bearing on the plant (I wouldn't believe). Seeing as these plants grow in hot locations, I'm really curious as to how it all equates. With my current setup I don't even need a fan inside the tent as the airflow from the air being pulled into the tent has my girl looking like shes getting 15mph winds.

I guess for data, if you took the temperature reading at the intake of the cool tube, what would the exhaust be on the other side of the cool tube given a certain air flow, and obviously certain wattage ratings and how much of that heat would radiate through the cool tube. I know it sounds like an impossible task but I feel like there's probably some data out there.

It's funny how certain trades master that part. You should see the electrical work in this place I did as I'm a union electrician and obviously have no clue about hvac! That's why this site rocks. So many people for different walks of life that everybody has good useful information to help out everybody.


----------



## WrEkkED (Mar 30, 2013)

So to clarify even further. If we calculate the losses and say this 440 is pulling 200 and 200 cfm at 77 degrees cools the lamp exhaust to 84, If I started moving 400 cfm of 77 degree temps, what would the change be? 

So in theory, how many cfm of 77 degree temps does it take to lower cool tube temps as close to 77 degrees as possible?


----------



## terky (Mar 31, 2013)

Hell yeah! I am Union too Brother! I used some relays and photo sensors to turn my exhaust fans on and off. Then I realized I could have used relays to trigger EVERYTHING with 1 timer. You Sparkys got some cool toys.

OK. IF everything is assumed we need to think about the BTU (heat  output) output of the bulb and the capacity of the air to absorb that  amount.

From a quick google one watt equals 3.414 BTU. So 600x3.414= 2048.4 BTU. Not exact but close enough for our purpose.

Now it gets complicated because the amount of heat the air can hold depends on the humidity. But .018 is close enough. .018 BTU to raise 1 cubic foot of air 1 degree.

2048 x .018 = 36.864 CFM

So  to theoretically stay the same temp,  you need 37 cfm. But that is only  considering the heat of the bulb, not the radiant heat produced or the  heat load that is coming from outside on a hot day( which could be substantial).

From  my experience with 600's in a cool tube, I found 75CFM to be a good  number for my old 4x4 tents. I had three tents each with a cool tube. 8"  insulated duct from the cooltube to a metal duct with a carbon filter  inside. Through carbon filter, through 15' of 12" insulated duct,  through the 6" 440 cfm fan then out a 7" bathroom fan vent that was in  the wall. 

Each of the three cooltubes pulled about 100 cfm, I  measured them with an industry standard air measurment device. Because I  had oversized duct the one fan could do it all. At normal pressures, 6"  duct will move about 100 CFM, 8" is about 200, 10" is 300- 400 and 12"  is about 400 - 500. Significant performance gains can be had by changing  to larger duct. Also Insulated duct will get the heat OUT of the building. Otherwise the duct is a huge radiant heater.

My  advise is to get ready for this summer. It gets hot fast with this  hobby. You may be able to find insulated flex duct and all the reducers  and stuff at home depot or lowes. If not just call a heating contractor.  They can get all the stuff and have stuff custom built if you need it.  Its not as expensive as paying for nutes and electricity and then  cooking your whole crop.

Figure out a way to get the heat  outside. I had duct hanging from the ceiling, running from a bedroom,  through a bathroom. Whatever gets it done. Put the fan as close to the  exhaust hole as possible. Then run the biggest duct you can to your  cooltube. At least 8", bigger if its a long run from the fan to the  cooltube. Hook directly to the cool tube, it will still pull heat out of  the tent and its the most efficient way to cool the bulb. Also lights  on at nite helps with the temps.

I am horrible with written communication so I hope this makes sense.

And Trailer Park Boys is AWESOME! "Its just a big kitty! I know all about kittys!"


----------



## WrEkkED (Mar 31, 2013)

That is an awesome formula. So the most air you can pull through a 6" duct is only 100cfm?? That's nuts. My tent doesn't have any 8" holes. If I added a duct booster to aid the inline would it actually make much difference or is that 6" maxed as it is? With AC on my humidity drops to around 31% with humidifier on, sometimes 22% if it's running constantly.

As I say I'm not toooo concerned for myself. As my ac is cheap to cool this place. I have ducting I could go into but my neighbors would def know as the duct is a shared bathroom exhaust that vents to outside. I believe it's attached to the laundry rooms as well.


----------



## terky (Mar 31, 2013)

100CFM isnt the most. A fan can pull as much as the duct will let it, if the fan is powerful enough. A big commercial air handler can literally collapse duct. Smash it like a soda can from the negative pressure. I have seen people say to use 400 CFM per 1000w for air cooling. I think they are not sizing the duct correctly and just assume "If I use this 800cfm fan on these 2 hoods thats 400cfm each." but it doesn't work like that unless the duct is big enough. I have 1 8" duct for every 2 lamps. The air is ducted into a hood, cools one lamp, ducted to the next hood, cools that lamp, then up and out the room into the larger ducts. I have not measured the airflow but I assume I am at 100cfm per 8" duct. So 50 cfm to cool a 1000w. But I am in a cool climate. Could be different for peeps in SoCal.

For our fans the best way to use them is an appropriately sized duct system. If you have that fan hooked to one 6" duct it will move about 200 CFM. But if you have a big main duct say 12 " and then 4 6" ducts coming off of that, that fan would easily pull 400 CFM. What your shooting for when designing duct is about .08" of static pressure in the duct. That's where these fans are going to be the most efficient. .08" of static pressure in a 6" duct will move about 100cfm. Its more about pressures and resistance (friction) in the duct than the size of fan. But you can always get a bigger fan.

Edit: I always think of stuff like I am building a commercial system. Sometimes I go a little beyond what most would do. My buddies call me Tim Allen when it comes to my op because its way over engineered. So yeah if it gets hot just make that fan blow outside lol.


----------



## WrEkkED (Mar 31, 2013)

haha, ya I'm the same with overkill. I do all my installs the way that is done commercially. I used to own a hobby farm and I had the whole thing automated. You would think it was a Tyson chicken plant or something.

Anyways, I can put my hand right on the cool tube and hold it there without it getting too hot that I have to move. I live in a cooler climate too but they keep this place heated so much that I actually run ac all year. Right now it is 80 degrees in here even though it's a little under 40 degrees outside.


----------



## WrEkkED (Apr 4, 2013)

Now I'm a little confused. I now have one 6 inch inline going through the tent to the cool tube and out and one 6 inch inline pulling through the tent, through a carbon filter and out. My temps raised from 84 to 94 degrees!!! I don't get it? I should now have more airflow through the tube and have additional pulling cool air into the tent and exhausting old air.


----------



## Sin inc (Apr 4, 2013)

well famly iam ruinng 5 muffin fans and my temps dont get above 80 most of the time. each fan is rated 12v 2.5 amps. i have one one each end of the cool tube and then about two feet of ducting and then another fan all fans are sucking and not pushing . i have another on the front of the box for intake.and one more on the side of the box for extra outtake. my cool tube stay around 3-5 inches away form my plants at all time.take a look at my current grow you will see my homemade box its about 2.5x2.5x6.and working just fine i am running a 400 watt hps


----------



## Hushpuppy (Apr 5, 2013)

I don't really understand why you are having so much issue with heat. I have 2 5x5x7 tents and 1 4x4x7 tent, each with 1 6" centrifugal fan(540cfm) connected to a pair of cooltubes. Then it is connected to a carbon filter. So the one fan pulls air from inside the tent, through the filter, through 2 cooltubes in series, then through about 5' of hose before reaching the fan that is mounted to the exit from my building. I can put my hands on my tubes and hold them without getting too hot. My tent stays below 73f unless I slow the fan with a speed controller to get the temps up to about 76f.

I pull air from within my building so that I can control the temp and humidity before it is pulled into the tents. I have measured the exaust air temps and they are around 80-85f, but that is fine. Given that your air space that you are pulling from is 77f, it is reasonable that the exaust air(from cooltube) would be 85f. If it was above 95f then I would be concerned.

Is the intake air for either fan being taken from up high in the room or down low? All my intake air is pulled into my tents from down low in the building so that it is cooler. Then the filters are placed in the ceiling of the tents so that the warmest air in the tents is pulled out first. For the size space you have and and only having 1 light, the 1 fan should be plenty to cool the tent and the light through a filter without issue.


----------



## terky (Apr 5, 2013)

Do you have a fresh air intake? In theory you should be pushing more air with 2 fans BUT if there is not enough air coming in for both of the fans they will fight over it and neither will move enough air.

You can pull about 50 cfm from under a door. Any more exhaust than that and you would want to crack a window.

Do like hush puppy is saying. Have the filter as the intake, Ducted to the cooltube, Ducted to the fan, Ducted outside. It should be plenty cool.


----------



## WrEkkED (Apr 5, 2013)

Turns out it was my thermometer being dumb. I returned it to original configuration and it still stayed the same temp. It said it was 96 today and I put my hand under the light and on the tube and the exhaust air was all cool. So I knew something was up.

I wanted to do the carbon filter before the light but I thought that was a bad idea because particles could break the bulb?


----------



## terky (Apr 6, 2013)

You should be able to put it before the cool tube. Nothing should come loose from the filter.

Or what I do is put the filter inside a duct. Then you can tap off of it to run duct to many lamps. Then of course its after the lights.

Cheap little thermometers! I had one for a couple years. Started having mold on the buds but humidity said 50%. So I was like ***. I finally replaced the thermometer with a new one. As soon as I turned it on it read 95% humidity! Now I replace them yearly just for insurance.


----------



## WrEkkED (Apr 6, 2013)

That is seriously an awesome idea. I only have the two so maybe I should get a Y connection and have both fans pull through, or is that too much and just pull with one?


----------



## terky (Apr 6, 2013)

Just the one fan will be sufficient.


----------



## Canna-Bliss (Apr 12, 2015)

My setup. Jardin ds 60. 2X2ft. X 5tall i think. 250Watt hid. Both passives on the bottom open. 6 in can max dialed down to 1. Has a 3 speed controller built in. Bad *** fan. 201 cfm on low, no filter as of now but when needed i can turn it up! I dont get the whole exchange air once a minute deal....it takes 201cfm to cool a 250 in 2x2 tent with straight as an arrow exhaust. Day temps are 78-82 and ambient temps are 68-75. Its not summer yet. Im in a closet too, and rambling....hope it helps.


----------



## Canna-Bliss (Apr 12, 2015)

No cool tube or reflector though. Bat style!


----------



## budz4me (Apr 14, 2015)

terky, nice to see another HVAC pro here....CHEERS!


----------

