# How to size a cooling system for your garden

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#### MrBlah

1000 watt lamp inc radiant heat 4,000 BTU
1000 watt magnetic ballast 3,500 BTU
1000 watt digital ballast 2,500 BTU

600 watt lamp inc radiant heat 2,400 BTU
600 watt magnetic ballast 2,100 BTU
600 watt digital ballast 1,500 BTU

This information right here is grossly inaccurate.

First off, lets talk for a short bit about the amount of energy that is emitted from a light bulb. A 1000W light bulb will emit 1000W of energy, for all intent and purposes. As most high power bulbs age(>10,000hrs of run time), they increase the power draw, usually peaking around 1100W. So, lets use this 1100W number for the bulb. Photosynthesis is a process that is approximately 1-2% efficient in most plants(sugar beets are an exception). HPS bulbs emit light at approximately 20-25% efficiency. So, the plants are absorbing 2% of 25%, or 0.5% of the energy emitted from the light bulb. 99.5% gets converted into heat. Basically, you need to account for 100% of the energy emitted by the bulb.

The unit conversion from W to BTU/H is 3.412. 1000W=3,412BTU/H.

1000W HPS lamp near end of life, 1100W = 3,753BTU/H.

Now lets talk about ballasts. A ballast is a transformer, essentially. They are typically 92-97% efficient. The worst of them at maybe 80% efficient. This means that a ballast that is 80% efficient that delivers 1000W of power, draws 200W more from an outlet and emits that 200W as heat, hence the cooling fins ballasts tend to have. Or 1200W in total. So, lets assume we are at end of life again, 1100W of HPS lamp means 220W of ballast power usage/heat.

220W = 751 BTU/H

For a 600W lamp, end of life, 660W end of life power draw = 2,252BTU/H
+ ballast @80% efficiency = 450BTU/H

1000W Bulb, Combined: 4,504BTU/H
600W Bulb, Combined: 2,702BTU/H

The last thing you need to consider is energy entering the room from equipment like pumps or fans. A pump in the grow room to circulate water in the hydroponic system? A fan to ventilate air? Account for 100% of the power rating of the motor that drives it.

A 1hp pump = 746W
A 1/8hp ventilation fan = 94W

You get the idea.

Lastly, you need to consider the temperature of the outside of the grow room. If you are growing in a tent, you shouldn't have much trouble as long as you are ventilating that room really well in addition. Basically, you want to keep the area outside the tent the same temperature as the inside. Maybe 75-80°F. If you can do that, you aren't adding heat to the tent. If you aren't in a tent, and are in a complete room, you need to pay attention to the temperature outside. Hottest days are 95-105°F? These need to be accounted for. Ventilation will not suffice as you are bringing in hotter air from outside during these summer months. A rule of thumb that I have used in the past as a mechanical engineer works like this:

A = Measure the Area of the walls on the exterior.
dT = Maximum temperature differential between outside and your grow room. Usually a 40°F differential is the largest you'll see. That's assuming 75°F grow room at 115°F Arizona summer heat.
R = insulation value of your wall. Don't know? Assume R3 insulation. R = 3. If it is just a singe piece of plywood wall, with no drywall(like an attic) assume R = 0.62. 1/2" plywood, R = 0.62.

A*dT/R = BTU/H heat gain due to environmental factors.

Example: Attic grow room. 1/2" plywood + shingles = approx R1.5. Total roof and upper wall area = 600 ft^2. 40°F dT
600*40/1.5 = 16,000 BTU/H

Add everything up. Add the total amount of lights. The total amount of motors. The heat gain from hot summer days.

Now, it is always good to have a larger cooling system than the load that it is design for. This is how every commercial HVAC system in the world is designed. Always implement equipment that is 15-20% greater in capacity than the design load.

Multiply the number you just got from adding everything up by 1.2. Go find the next largest piece of HVAC equipment you can find. It can be a window unit, a split system, etc.

#### Budgod

Great questions. To make sure I understand everything please answer these for me.

1. I understand that the veg room has a bare bulb and is cooled by outside air being pulled through the basement. For the flower room are you pulling air from the basement, through the hoods, and then venting them outside? Or is it setup similar to the veg room where you just exchanging the room air?

2. You mentioned several max-fans and 8" chiller boxes (Ice Box?). How many of these do you have and are you putting any in the veg room?

3. Where is the res going to be? Its a non issue as long as it isn't outside where its hot.

4. What about your pump? Are you going to be pumping up from the basement? Lift has to be considered if you are. Not having enough water flow could greatly effect the performance.

LMK
I'm having the same problem my tent is 7 *7. Two 1 thousand watt lights .. A carbon filter . And two fans going my temp is 85 so I shutdown the tent and my humidity is really low any advice please

I'm just wondering how this figure was calculated as it is higher that I have previously been informed from other cooling specialist. I was under the asumption that a 1K bulb puts off around 3410 of btu.

1000W x 3.41 BTU's = 3410 BTU (this is the formula I was told works)

thanks

Both are wrong.

Bottom line is 6000BTU/1000W

6900BTU/DE's

This includes running co2 burners all other equipment and NON COOLED lights in the hottest day of southern cali. Doesn't matter where you live really unless its in a desert somewhere and you constantly get 110+ days during the summer. IF you run glass you will be losing 15-25% Yield when they are spotless.

#### MrBlah

Both are wrong.

Bottom line is 6000BTU/1000W

6900BTU/DE's

This includes running co2 burners all other equipment and NON COOLED lights in the hottest day of southern cali. Doesn't matter where you live really unless its in a desert somewhere and you constantly get 110+ days during the summer. IF you run glass you will be losing 15-25% Yield when they are spotless.

SweetTooth is correct on the conversion between watts and BTUs. But, you're right in that he has not taken into account any other equipment. Unfortunately HVAC sizing isn't as cut and dry and you are trying to make it.

My experience as a mechanical engineer doing HVAC sizing for commercial buildings and specifically for marijuana facilities puts the number between 4,500-5,500 btu / 1000W lamp. And there is no difference when you switch to a double ended HPS vs any other 1000W light bulb.

So, is 6,000 BTU/1000W a good safe number? Yes, especially for small scale. And it doesn't matter if you use a DE HPS or not.

Yes it does de use more watts so.how do you account for all.of the equipment when its said and done where do you stand

Also in what type of climate do you get your figures.

HVAC sizing is not cut and dry at all and in some cases under estimated.

1000-1500BTU is a lot of cooling that is left out of the equation. Not only does this put even more strain on the units but it also uses more power.

Sir if you recall 1000W lights are not the same as a DE. Yes you can turbo charge 1kW bulbs and have a shorter lifetime.

DE's run 100% at 1055W (so no its not the same BTU's at a 1kW) even more so at 115% DE's run at 1210W so how can you claim its the same BTU requirement?

I'd love to know how many BTU's per 1000W you use including ALL equipment. dehues, pumps, fans, controllers etc in a place say southern california during the hottest day of the summer. Cheers!:ninja:

#### MrBlah

HVAC sizing is not cut and dry at all and in some cases under estimated.

1000-1500BTU is a lot of cooling that is left out of the equation. Not only does this put even more strain on the units but it also uses more power.

Sir if you recall 1000W lights are not the same as a DE. Yes you can turbo charge 1kW bulbs and have a shorter lifetime.

DE's run 100% at 1055W (so no its not the same BTU's at a 1kW) even more so at 115% DE's run at 1210W so how can you claim its the same BTU requirement?

I'd love to know how many BTU's per 1000W you use including ALL equipment. dehues, pumps, fans, controllers etc in a place say southern california during the hottest day of the summer. Cheers!:ninja:

Alright. So, lets look at an attic in Southern California.

600 ft^2 on the ground, 20' x 30'.
3 ft. tall walls, 30 ft long, x2. Total Wall Area = 180 ft^2
12ft peak of the roof, from the floor, so you've got a roof that is 30 ft long and 13.45 ft wide, x2. Total Roof area = 807.25 ft^2
Attic sides are a 20'x9' triangle with a 3'x20' wall below it. Total side wall area = 300 ft^2

Net exterior wall area = 1,287.25 ft^2

Wall insulating value: R value = 5.5, estimated. 1.5 for for plywood and shingles + a minimum of R4 because that is what California building code requires in unventilated attics.

0.4% design temperature for Palm Springs is 111.3°F, so that's our design temperature. It is the highest design temperature for any city in California. This is not based on high temperatures but instead based on the amount of hours throughout the year where the design temperature will be exceeded. 0.4% design temp means 0.4% of year, temperatures will exceed 111.3°F. This is standard practice in HVAC design.

Our grow room max operating temperature is 80°F. So, temperature differential, dT = 31.3°F

Total heat gain due to environmental factors = Area*dT/R

1,287.25 ft^2 * 31.3°F / 5.5 = 7,326 BTU/H = Total environmental heat gains

Onto the lights. 600 ft^2 on the ground with the roof structure the way it is really only give us about 360 ft^2 of growing area (30'x12'). We're 30 ft long, have a 3 ft center aisle, so, we have two rows that are 30 ft x 4.5 ft. 6 lights in each row. 12 total.

12x 1000W DE HPS fixtures. According to Gavita, the power usage for the Gavita Pro 1000e DE US is either 1085W for 120V or 1055W for 240V. Being a residence, probably only 120V readily available. So, we're assuming 1085W power usage for each fixture. And for reference, the Gavita power usage numbers include the ballast power usage.

12 x 1085W * 3.412BTU/H/W = 44,424 BTU/H = Total lighting heat gains.

You've got two 24" oscillating fans. Each are 1/3 HP. Bought from this store. Specs says 267W.
You've got one 0.5 HP water pump you bought off Amazon for your irrigation system. Their specs days 370W each.
You've got one 50 pint dehumidifier you bought off Amazon. Their specs says 520W each.
You've got one CO2 burner at 6,000 BTU/H of heat because you don't want to ventilate it.

2 x 267W + 1 x 370W + 1 x 520W = 1424W
1424W * 3.412 = 4,859 BTU/H

4,859BTU/H + 6,000 BTH/H = 10,859 BTU/H = Total equipment heat gains.

Net heat gain from all sources = 7,326 + 44,424 + 10,859 = 62,609 BTU/H

62,609 BTU/H / 12 lights= 5,217 BTU/H per light.

An uninsulated attic means environmental heat gains rise to 26,860 BTU/H which bring net heat gain to 82,143 BTU/H, or 6,845BTU/H per light.

like i said 6900 btu and the de do put outmore heat. palm springs gets hotter than 111. and who would not ventilate an.attic lol

i cant drive....55

111.3? Where did you get that number. Record High for Palm Springs is 123... it hit 122 this summer. So you are off again by a significant amount. I'm not an engineer but anything over 10% is significant. How about considering the thermal heat put off by a few humans in the room? About about if you bring a shopvac in there? What if you need to run additional dehues? Theres not much wiggle room and overestimating that I've seen with your work that I have seen in the HVAC field.

#### MrBlah

111.3? Where did you get that number. Record High for Palm Springs is 123... it hit 122 this summer. So you are off again by a significant amount. I'm not an engineer but anything over 10% is significant. How about considering the thermal heat put off by a few humans in the room? About about if you bring a shopvac in there? What if you need to run additional dehues? Theres not much wiggle room and overestimating that I've seen with your work that I have seen in the HVAC field.

I'm sorry I couldn't add on a few other things I wanted to, I had to run out the door. Bear with me here, this is a little long winded, but I hope we can understand each other here. I just want you to see all the sides an engineer sees when designing HVAC systems.

111.3°F is the 0.4% design temperature as laid out by ASHRAE (American Society of Heating, Refrigeration, and Air conditioning Engineers). It is not based on record high temperatures. So, the temperature will rise above it. The 0.4% design temp means that 0.4% of the time, throughout the entire year, the outside air temp will be above 111.3°F. That's a total of 35 hours. So, for a few hours on 10 or so days throughout the summer, your room will rise above your design temperature of 80°F. These design temperatures are a part of a very large set of tables that is available for almost every county/major city in the country. It is meant to make the engineer's job easier. There a lot of intangibles that you can't account for when designing HVAC systems, so, being good enough for 99.6% of the year and then being a couple degrees warmer for 0.4% of the year is something we humans and also a few plants can deal with. If you have any more questions on design temperatures, let me know. I tried to be thorough.

And yes, we can definitely consider the heat put off by 2 humans in the room. It is 100W per human. 200W total, 682 BTU/H. The reason I didn't include that and any temporary equipment like a shopvac largely has to do with the way HVAC systems work and use cases. You're most likely to be inside the room doing work at the beginning or end of the day, right? So, it will most likely be cooler outside, below 105°F, I'm sure. And because of that, the lower environmental heat gains mean you have room to spare in terms of the loading characteristics of your HVAC system. On top of all that, your HVAC system is always going to be oversized for your cooling basic cooling requirements. Small considerations like you mentioned will almost always fall within the buffer that the equipment will provide.

Now the things I wanted to bring up but ran out of time earlier are as follows:

The reason I used the conditions for the un-ventilated attic have to do with use cases that are common in grow rooms. No ventilation. It is very common for people to want to control the smell, and a lack of ventilation in the room is the first thing that happens when a room is setup. Instead of laughing at why someone would have an unventilated attic, consider the use cases that it might relate to.

And that brings me to HVAC equipment selection and insulation.

Yes, that room, in Palm Springs, without insulation, will require 82,143 BTU/H of cooling, or 6,845BTU/H per light. Now, you can't buy an HVAC unit that does 82,143 BTU/H of cooling. But you can buy them that do 90,000, or around there.

And this brings us to another thing that not many people know about when it comes to HVAC system sizing. That rating is at 95°F. When that temperature goes up to 105°F, it will perform worse. Let's look at the DX11SC from Daikin. That link is to the data sheet. If you look at the expanded data on the DX11SA0903, which is a 7.5 ton nominally rated unit, with an 80°F Indoor Dry Bulb temperature (IDB) and the rated temperature of 95°F Outdoor Dry Bulb, performance of that condensing unit is 88,000 BTU/H. This might look like it's enough to do the job. But if you look at the performance numbers for 105°F you will see it drops to 83,600 BTU/H and at 115°F it drops to 77,400 BTU/H. Doing the right interpolation, at 111.3°F, the unit will perform at 79,700 BTU/H. This is below the design conditions. So, I have to make a decision in regards to whether or not this is acceptable. It's pretty close after all. So, I may ask for additional information from the grower. What relative humidity do you like? Is 50% necessary? Can it rise to 60%? Does it stay at 60% all the time? Can the indoor temperature rise a little during those hot days? Lets say you tell me that you like it at 60% and you try to keep right around there.

All of those previous performance numbers were at an Indoor Wet Bulb (IWB) temperature of 67°F, this corresponds to relative humidity of 51%. But your room is wetter, as you might have said. Looking at the chart again, at a relative humidity of 62% (71°F IWB), which is still just fine for growing cannabis, you will see the performance of the HVAC unit changes a little. That 115°F performance number just rose to 82,800 BTU/H, which means the unit will now perform above the design conditions at almost all times, even above the 0.4% design temperature. So, letting it run a little more humid would mean the unit would perform as designed. Running dryer means it would not.

That is why 6900 BTU/H per light is a bullshit number.

So, when the unit you bought maybe isn't cooling as well as you thought. Instead of getting pissed off at the engineer, you should probably first consider all the things you told the engineer, the importance you put on them, and how your current conditions vary from what you originally mentioned, and then consider improvements you could make. There is nothing bad about being wrong about something, as long as you can accept it and learn from it. Accepting it is the first step to self improvement.

Even with a top notch build HVAC systems are really pushed hard in a growing enviorment and that is why I believe 4900BTU/1KW is not sufficient. I live in San Diego and have seen numerous houses have 10 tons of cooling.

#### mclovin707

Hey guys getting ready to start flowering in my new room. I am currently running 6k double ended ac DE hoods. Running a 100pint dehuey in room. All ballasts are outside the room.

Here's my issue have a brand new mitsubishi 48000 btu mini split with 2 24000 btu indoor units 1 on each side of room. This should be plenty of cooling power. I did a test run today because it was about 90 degrees out and wanted to make sure I could get the room ice cold if need be with lights on. This is the first time I have fired up the ac with the lights on. Room spiked up to about 92 degrees. The room is fully sealed and insulated as well as double rock and sound proofed. Has a concrete floor too. I'm beginning to wonder if my HVAC guys screwed up on the install. Not enough refrigerant??? Any thoughts from an HVAC guy in here???

#### MrBlah

Hey guys getting ready to start flowering in my new room. I am currently running 6k double ended ac DE hoods. Running a 100pint dehuey in room. All ballasts are outside the room.

Here's my issue have a brand new mitsubishi 48000 btu mini split with 2 24000 btu indoor units 1 on each side of room. This should be plenty of cooling power. I did a test run today because it was about 90 degrees out and wanted to make sure I could get the room ice cold if need be with lights on. This is the first time I have fired up the ac with the lights on. Room spiked up to about 92 degrees. The room is fully sealed and insulated as well as double rock and sound proofed. Has a concrete floor too. I'm beginning to wonder if my HVAC guys screwed up on the install. Not enough refrigerant??? Any thoughts from an HVAC guy in here???

Possible that it's not enough refrigerant.

More likely that one of your indoor units is not turning on to let refrigerant in.

Your indoor units might be running off a solenoid valve that opens to let refrigerant flow. If that isn't turning on, you'll experience what you are right now. So, it may not be the fault of the HVAC tech and it may be an equipment fault.

Among a myriad of other things.

#### mclovin707

Possible that it's not enough refrigerant.

More likely that one of your indoor units is not turning on to let refrigerant in.

Your indoor units might be running off a solenoid valve that opens to let refrigerant flow. If that isn't turning on, you'll experience what you are right now. So, it may not be the fault of the HVAC tech and it may be an equipment fault.

Among a myriad of other things.

Thanks for the quick reply. It does have a 3 port switch box to run the indoor units. I guess it's there to control the refrigerant going to the indoor units. I'm going to call the guys back this week to check it out. Hope to get it figured out quickly the ladies are going to be ready to go in soon.

#### MrBlah

Even with a top notch build HVAC systems are really pushed hard in a growing enviorment and that is why I believe 4900BTU/1KW is not sufficient. I live in San Diego and have seen numerous houses have 10 tons of cooling.

Having 10 ton HVAC systems doesn't mean a whole lot. You could have a need for 8 tons. But an 8 ton unit doesn't exist. So, you have to buy the next largest available size, which is 10 tons.

In addition, you need to increase your maintenance schedule when your equipment is used for 10-20x more hours throughout the year than it is designed for. In doing so, you'll maintain performance instead of seeing it diminish below design conditions. This means cleaning the outdoor condensing units far more often than a manual suggests. Probably once every two weeks to a month even.

#### mancdank

Awesome thread I'm in the UK so never really had much need for ac but we have a place in spain so this has been a much needed bit of education thanks guys :)

R

#### Ronbo

I wanted to start this thread to answer some basic questions about sizing cooling systems. Feel free to ask for help here for sizing your garden but first check out this interactive BTU calculator specifically for cooling gardens that I helped create. I personally tested everything 3x's and am very confident with my numbers. I did this by absorbing the heat in to water and then measuring the BTU's in the water. I put up some round numbers to make it easy, so these are not my exact findings.

http://www.hydroinnovations.com/faq/14032011-CoolingSystemChart R2.pdf

Basically this is how it breaks down...

Cooling BTU's needed before any equipment is added. This is for warm climates with 3 1/2" insulation, approx 40 BTU sq ft

1000 watt lamp inc radiant heat 4,000 BTU
1000 watt magnetic ballast 3,500 BTU
1000 watt digital ballast 2,500 BTU

600 watt lamp inc radiant heat 2,400 BTU
600 watt magnetic ballast 2,100 BTU
600 watt digital ballast 1,500 BTU

Co2

http://www.atlantishydroponics.com/co2calculator.asp

Dehumidification
This is even trickier to calculate. BTU's created of course are based on run time. It's effected by outdoor humidity levels, how well the room is sealed, what growing system you are using (soil is highest IME), amount of plant material, and room temperature (the warmer the room the more the plants transpire), cooling system used, and humidity setpoint. Far to many variables for me to make a solid suggestion on BTU's. But to help size them I will tell you that a 70 pint dehu produces about 2k BTU per hour. If you think that you need a 70pint I would figure worst case scenario and add 2k BTU to your total. I use a 70 pint dehu for my garden (12' x 24') and it runs 50-70% of time depending on my plant cycle, but its my only form of dehumidification. With a standard a/c you will get some dehumidification out of it making the dehu run time less.

Ventilating lights
This varies depending on temp of the air being used and the amount of airflow (CFM). I've never cooled lights using outside air because in my location the air is too hot to use so I don't have any suggestions for sizing for this. I can tell you without a doubt if the air used to cool the lights is above the room temp is far less effective, in fact it can heat your room. Reflectors have about 8 sq ft of metal and 4 sq ft of glass and along with all the ducting they become heaters. Lets say your room is 80 degrees and the air you are using to cool the lights is 90 degrees. After the air flows through the reflector(s) its heated up well beyond this. All of the reflectors and ducting heats up to this temp and radiates heat in to the room. Something else to consider is that HID lighting produces radiation that even if using ice cold air radiation still passes out of the reflector. A great example is to hold your hand 1/4" away from the metal on the top of the reflector near the bulb and you can actually feel the radiant heat. This is approximately 500 BTU per hour per 1000 watt. A simple fix for this is to use Reflector Heat Shields.They will block 99% of the radiant heat on anything they cover and are a must for air cooling IMO. The radiant barrier fabric traps more heat in the reflectors allowing it to be removed with the forced air. For anyone using above room temp air to cool their lights I would insist they use these covers, I guarantee they will make a significant difference.

Lastly I always design my cooling systems 10-20% over the BTU's needed so that the cooling equipment can cycle on and off properly. These figures above have some cushion built in to them so 10% is probably fine.

If you need help sizing beyond this or have any other questions feel free to ask :)
Well the general rule is 3.41 btu per watt of light i run 4 btu per watt. I use a gree minisplit system with automatic heat and air so i dont have to switch back and forth due to ambient conditions...believe me i went thru all the inexpensive ways to grow this stuff and u have to invest to do it properly. I have 2 12x12 rooms with 10000 watts each and 2 4 ton minisplits. I also pull outside air for some co2 and clean air...good filtration each room has 2 12 inch carbon filters for odor and air cooled hoods 8 inch. We tried 6 inch but they sucked also all lights are hard piped its a pain but well worth it. The flex duct blows, leaks and tears over time...hortilux bulbs 1000w hps. Some digital ballasts but i prefer magnetic for reliability. I use heavy 16 nutes with aptus facilator and great white root enhancer, terpine plus with bat guano tea. Be4 all the upgrades we got about 7oz per plant now our average is 14oz. As far as room temps in summer on a 90 degree day i can maintain 72 degrees but i set it at 74. In winter well even easier. Ac sizing is key 3.41 btu per watt of light but 4 btu per watt is better. And the unit that switches from ac to heat automatically is a dream in the winter but the settings in auto is 77 degrees for cool and 68 for heat in auto mode. Np with outside air in winter it still holds 74 degrees easily. I tried all those so called portable ac's amd window shakers nothing but hard work trying to maintain temps....also each room has a dehumidifier maintaining 40-45 % humidity at all times. Trust me i know it sucks to spend alot up front but eventually ur gonna spend it anyway....for the minisplit get the Gree Vireo (whatever rating u need due to lighting) it will run in ac mode down to 5 degrees ambient twmperature and in heat -20 degrees....good luck keep growing

Ronbo

O

#### Ogbullriderguy

I'm just wondering how this figure was calculated as it is higher that I have previously been informed from other cooling specialist. I was under the asumption that a 1K bulb puts off around 3410 of btu.

1000W x 3.41 BTU's = 3410 BTU (this is the formula I was told works)

thanks
For 7000 to 8000 watts i needed 32000 btu plus i had to add a 12000 btu ac to keep it under 75.

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