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AC, Calculating air conditioner size

  • by jzchillin
  • Apr 01 2011 03:18 PM
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Posted 01 April 2011 - 03:18 PM

Calculating Heat Load:
Heat is measured in either BTU or Kilowatts. 1KW is equivalent to 3500BTUs.
The heat load depends on a number of factor's

1.The floor area of the room
2.The heat generated by equipment
3 the heat from lighting
4.The number of room occupants
5.The ambient temp (your room's starting temp)

1.Floor Area of Room

Room Area BTU = L x W x 40 ( H = 8foot + 5btu per foot after that)

If you have a wall that is facing the sun add in for the extra heat

Sun facing wall BTU = L x H x 40

This is trickier to calculate than you might think. The wattage on equipment is the maximum power consumption rating, the actual power consumed may be less. However it is safer to overestimate the wattage than underestimate it.

Equipment BTU = Total wattage for equipment x 3.5

Ballast in room BTU = ballast wattage/2 x 3.5


Lighting BTU = Total wattage for all lighting x 4

Air cooled hood BTU = wattage x 4 / 2
400= 200-250 cfm
600= 250-300 cfm
1k = 300-350 cfm
^ is just a guide cfm per hood will be on your ducting/SP....

even being that im only there for 2 or so of the 12 hrs i like it to be able to handle the extra sweat when i smoke one.

Total Occupant BTU = Number of occupants x 400

Total Cooling Required

Add all the BTUs together.

Total Heat Load = Room Area BTU + Total Occupant BTU + Equipment BTU + Lighting BTU

If your using a portable a/c:

Total Heat Load BTU x 2

^this has been from threads if you are planning on getting one i think one that's bigger then you thought about Posted Image


so if we have a 3 x 3 room with 1k and 200 watt's in fan's/pump's

for room we need 360 btu
for fan we need 700 btu
1k = 4000btu
ballast in the room = 1500 btu

we would need a/c that is 6560 btu i would round up to the next size 7kbtu

air exchange
aka intake/exhaust
complete air exchange every 4-5 minutes is average for a grow room/greenhouse (co2 control)....
for heat control (no a/c) i like to use 2.5 minutes {1M is best imo}
m=air exchange in min's

l x w x h = cf / m = cfm

lets use are room from above as a sample
3 x 3 x 8 = 72 /2.5 = 28.8 (30)
highend turn over = 72cfm
lowend turn over = 30cfm
now for the light 1k = 300-350cfm

exhaust system and air-cooled hood should be separated but most ppl cant/dont ( it's spendy) so add them together if your using 1 fan for both

the fan size we need is 330-422 remember you have duct loss (SP) in the sample room i would use a 440 cfm fan

SEER & EER ratings NO they are not the same.....

EER, or the Energy Efficient Ratio, is a measure of how efficiently a cooling system will operate when the outdoor temperature is at a specific level - usually 95°F. EER is calculated as a simple ratio of BTU's to the amount of power a unit consumes in watts. Here is an example using an air conditioner with 12,000 BTU's and consuming 1500 watts of power:

EER = BTU's / Watts

12,000 / 1500 = 8

EER = 8

While it is true that the higher the EER and BTU's, the more efficient the cooling system, many make the mistake of purchasing oversized air conditioners and ignoring EER ratings. The following is an example of an air conditioner with 12,000 BTU's and 1200 watts:

12,000 / 1200 = 10

EER =10

This would mean that this second unit can produce the same amount of cooling but more energy efficiently. Therefore, to save money on your monthly electric bill, choose a cooling system by getting an appropriately sized unit with a high EER.

I'm not taking credit for all this info.

Edited by jzchillin, 02 April 2011 - 03:20 PM.

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Replies (47)

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Posted 03 April 2011 - 04:55 AM

thanks for the info man

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Posted 03 April 2011 - 11:46 AM

nice! good info!

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Posted 04 April 2011 - 03:02 PM

Thank you bro, excellent wealth of information here. I'm sure this will help and be appreciated by many.

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Posted 04 April 2011 - 03:04 PM

Thank you bro, excellent wealth of information here. I'm sure this will help and be appreciated by many.

I hope so. Took me a minute to find this.

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Posted 05 April 2011 - 06:39 PM

Thanks :) very helpful info

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Posted 11 May 2011 - 11:09 PM

For AC units 1 Ton does roughly 900 square feet. If you are working with a larger scale its better just to remember that. So in an 1800 square foot home you would need a 2 Ton unit.

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Posted 31 July 2011 - 01:21 AM

So real good info man.............

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Posted 10 August 2011 - 09:11 PM

if you put a 2.5 ton evap in your will have more cooling cause of your coil size it will only effect the refrigeration charge a tiny bit if at all / great info bro!

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Posted 20 October 2011 - 10:47 PM

Damn.... Thats a lot of math up there. Instead of getting all crazy with the math, I just look at how many lights you have and if there vented or open. If there open, the very minimum would be 3K btu per 1,000 watt light. 6K btu is perfect.

If there vented/air cooled, then shoot for at least 2k btu for every 1000 watt light. 4K Btu is perfect.

And 12K btu is about equal to 1 ton. Square footage is important too, but not as important as how many lights your running, since they generate much of the heat.

Nice post... even though I got lost in it all.

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Posted 25 October 2011 - 11:47 PM

please help! i have a 2.8mtr x 3.2mtr space 25 babies (tomato obviously) a 10" inline extractor, and 4" ducting intake(no fan just ducting) i use hydro and have 6x 600w hps! i have tried to use an ac unit but after a min or so it cuts out and blows hot air(i think its too hot to run in there)! the temp just read 112farenheit!!!! i can open the door but this is VERY temporary! i need help! any ideas! all input appreciated! cheers oh yeah my humidity is only 20% i left 3 big bowls of water open in there but its not helping at all so i think i am gonna have to buy a humidifier(myster)! thanks again

Edited by GrowPro1, 25 October 2011 - 11:49 PM.
forgot something

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Posted 08 November 2011 - 09:33 PM

^^that sounds like to many lights for the amount of space..
and ur ac sounds like it "shit the bed"
i run 10k btu on 1650w i can keep temps around mid 70's with ac set on 69

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Posted 18 March 2012 - 06:07 PM

Thanks a lot for the info. I wish I had read this BEFORE trying my first grow... Would have saved me a lot of time, money, and pain.

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Posted 26 March 2012 - 09:17 PM

nice info a lot of help...

maybe this calculator can help more people like me

BTU Calculator

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Posted 07 June 2012 - 11:19 AM

I am very much confusing regarding the size of the AC. But, You relief my confusion very informative way. I am really thankful to you that You suggest me such a nice information. Its totally worth to read it through this community.

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Posted 06 July 2012 - 04:28 PM

Solid AC info. Given that global warming has set in hard, go for the most tons and highest SEER you can get. Beware of how your split unit communicates with your interior air handler, especially if you have a furnace blower that your AC has to communicate with. USA is going to get way hotter, so invest in the best AC now.

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Posted 09 September 2012 - 06:51 PM

basically its 3.41 btu per watt. and digital ballasts dont generate enough heat to divide by two. i have six ballasts running and combined they dont generate the heat of one lamp.

exhausting the cool air out of the room also needs to be taken into consideration. if you take the cold air out through the lights... you need to be replenishing that also.

great post but a lot of the figures are waaaaay overkill. i conservatively figured my room according to your guide and it says i need 42,770 btu of a/c, which equates to a 2.5 ton unit. thats 23,770 more than what i use right now (11k chiller and 8k hvac a/c) and my room will stay 79 degrees as long as i have the a/c turned on. thats with three external walls and outside temps at 80. 10 degrees outside is two degrees in for me.

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Posted 10 September 2012 - 03:46 PM

Shit thats alot of in depth but +rep for that. most of that math is pretty spot on too.

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Posted 21 October 2012 - 12:36 AM

I have a grow room with the following dimensions:

2 feet wide, 1 foot deep, 2.5 feet high, I will have 200-240 watts of CFL's in there at all times, I will be using passive intake holes at the bottom right, at the top left I plan on having a exhaust. I was going to use a computer fan, But for some reason when I use the formula in this thread it gives me really strange CFM ratings. With the dimensions of my grow box plus the total CFL watts, what would be an adequate size fan to get fresh air in there every 2.5-5 minutes? Also what is the CFM rating for that?

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Posted 15 January 2013 - 04:33 PM

very good read brother

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