Marcus_in_the-Darkus
Active Member
There are several threads here on ventilation. I started one last week asking if anyone had experience with DC blowers and Electronic Speed Controls (ESCs) and, other than answering my own question in post #2, there were zero replies. I therefore assume that not many people have taken this path so I'm going to share my experiences, successes and failures with you all here on this thread.
First, a statement of the problem, the technical basis for my opinions on the matter, and this disclaimer: I'm a first-time grower, a tinkerer, sustainability freak and not a trained engineer, so I may not know what the fuck I'm talking about. I am, however, a scientist who works in geothermal HVAC and thermal energy storage so I know enough to be dangerous.
It is my considered opinion that most people operate their tents at far too high a flow rate and waste a lot of electricity in the process. It's also wasteful of granular activated carbon (GAC). Where do these "rules of thumb” for air exchange rates come from? At the risk of sounding like a conspiracy theorist, these myths are perpetuated by the people who manufacture and sell tent HVAC equipment, because it’s in their interest to sell you bigger, more expensive equipment than you need. They will also look forward to selling you more activated carbon in the future. They don’t care about how much energy or GAC you use.
Let's start with the objectives of ventilating a tent or grow room:
Odor control is accomplished by using a ventilation system to put the tent under a negative pressure. I’ve seen pictures of tents (with obvious high airflow and a small intake) where you can see the poles bowed inward like an hourglass. All you need for good odor control is a slightly lower pressure inside the tent than outside the tent. This can be achieved with a low airflow and properly sealed tent with a properly located and sized intake. Depending who’s within smell of the outlet vent system, you will probably also want to control odor with GAC. How much airflow does my 4x4x6.7 tent need to achieve odor control? I don’t know yet, but it’s probably less than the airflow I’ll need for temperature and humidity control.
Temperature control is accomplished by removing heat from the lights and grow space via ventilation, or by adding supplemental heat inside the tent, like I’ve had to do for my legal grow in my unheated Massachusetts basement during the winter. How much airflow does my grow tent need for temperature control? I built my own 2 x 300W COB lights because (a) I’m into energy efficiency and (b) I wanted to generate as little waste heat in the tent as possible. We all know that LEDs run much cooler than HPS or MH or even fluorescent lamps, so it’s reasonable to assume that these ventilation “rules of thumb” can be reduced in my case because we’re introducing much less heat that needs to be vented.
Humidity control is accomplished by either adding a small humidifier to the tent during early veg cycle when the plants aren’t transpiring much water, or by venting humid air from the tent later in the grow cycle and exchanging it with less humid air outside the tent. How much airflow does my tent need for humidity control? I didn’t need any the first few weeks, but now that my plants are bigger it’s starting to be an issue, and I suspect the ventilation required for humidity control in my tent will be higher than for odor or temperature control.
GAC Consumption and Ventilation Airflow
Based on past professional experience using GAC for environmental remediation projects, the life expectancy of a GAC canister depends on the influent concentration and on the airflow rate through the GAC canister. Lower airflow rates mean longer residence time of air passing through the GAC, more effective use of the GAC, and longer time before breakthrough occurs and replacement is needed. The adsorption isotherm models can get complex but for a given influent concentration the life of the canister should be inversely proportional to airflow. If you cut your airflow in half, your GAC canister will last 2X longer. If you reduce airflow by half again your carbon canisters will last 4X longer. Clearly, there’s more than an energy benefit to minimizing ventilation airflow.
Energy Use and Ventilation Airflow
Let’s begin an analysis of ventilation-related energy use by defining the “status quo,” or how most new indoor growers will set up their tent or grow room ventilation system based on common recommendations of equipment manufacturers and recommendations of retailers and the practices of growers. I present Exhibit A, which states that a “common goal” is to achieve 1 volume exchange per minute. It is recommended that I calculate the volume of my tent (107 ft3 for my 4x4 tent) and multiply by a factor of 1.25 to account for head losses in the ductwork. So according to the status quo way of doing things I should look for at least 134 cfm.
The closest I could find to this were 4” centrifugal (squirrel cage) blowers in the 170-190 cfm range, so I bought an iPower 4-inch 190 cfm blower and carbon filter on Amazon. When I fired this thing up I knew it was moving way too much air so I bought an iPower fan speed controller. With the blower set to full slow it still made more air than I think I’m going to need. It was also noisy, and these kind of AC motors make an annoying hum at low speed, they’re really not well suited to variable fan speeds. Here’s how I’m going to define the status quo for my current situation:
Status Quo
Blower: 190 cfm centrifugal blower, brushless motor, 84 W (0.7A @120V AC)
Blower sound level: 78 dB full speed, 62 dB slow speed
Ventilation system configuration: discharges from tent in basement to outside air
Temperatures: 75°F tent temperature, 35°F outside air temperature (OAT)
First, a statement of the problem, the technical basis for my opinions on the matter, and this disclaimer: I'm a first-time grower, a tinkerer, sustainability freak and not a trained engineer, so I may not know what the fuck I'm talking about. I am, however, a scientist who works in geothermal HVAC and thermal energy storage so I know enough to be dangerous.
It is my considered opinion that most people operate their tents at far too high a flow rate and waste a lot of electricity in the process. It's also wasteful of granular activated carbon (GAC). Where do these "rules of thumb” for air exchange rates come from? At the risk of sounding like a conspiracy theorist, these myths are perpetuated by the people who manufacture and sell tent HVAC equipment, because it’s in their interest to sell you bigger, more expensive equipment than you need. They will also look forward to selling you more activated carbon in the future. They don’t care about how much energy or GAC you use.
Let's start with the objectives of ventilating a tent or grow room:
- Odor control (primarily during flowering)
- Temperature control (needs vary depending on heat output of lights and external temp)
- Humidity control (usually add humidity to tent during early veg, remove it later in the grow)
- Maintain good plant respiration (avoid CO2 deficiency)
Odor control is accomplished by using a ventilation system to put the tent under a negative pressure. I’ve seen pictures of tents (with obvious high airflow and a small intake) where you can see the poles bowed inward like an hourglass. All you need for good odor control is a slightly lower pressure inside the tent than outside the tent. This can be achieved with a low airflow and properly sealed tent with a properly located and sized intake. Depending who’s within smell of the outlet vent system, you will probably also want to control odor with GAC. How much airflow does my 4x4x6.7 tent need to achieve odor control? I don’t know yet, but it’s probably less than the airflow I’ll need for temperature and humidity control.
Temperature control is accomplished by removing heat from the lights and grow space via ventilation, or by adding supplemental heat inside the tent, like I’ve had to do for my legal grow in my unheated Massachusetts basement during the winter. How much airflow does my grow tent need for temperature control? I built my own 2 x 300W COB lights because (a) I’m into energy efficiency and (b) I wanted to generate as little waste heat in the tent as possible. We all know that LEDs run much cooler than HPS or MH or even fluorescent lamps, so it’s reasonable to assume that these ventilation “rules of thumb” can be reduced in my case because we’re introducing much less heat that needs to be vented.
Humidity control is accomplished by either adding a small humidifier to the tent during early veg cycle when the plants aren’t transpiring much water, or by venting humid air from the tent later in the grow cycle and exchanging it with less humid air outside the tent. How much airflow does my tent need for humidity control? I didn’t need any the first few weeks, but now that my plants are bigger it’s starting to be an issue, and I suspect the ventilation required for humidity control in my tent will be higher than for odor or temperature control.
GAC Consumption and Ventilation Airflow
Based on past professional experience using GAC for environmental remediation projects, the life expectancy of a GAC canister depends on the influent concentration and on the airflow rate through the GAC canister. Lower airflow rates mean longer residence time of air passing through the GAC, more effective use of the GAC, and longer time before breakthrough occurs and replacement is needed. The adsorption isotherm models can get complex but for a given influent concentration the life of the canister should be inversely proportional to airflow. If you cut your airflow in half, your GAC canister will last 2X longer. If you reduce airflow by half again your carbon canisters will last 4X longer. Clearly, there’s more than an energy benefit to minimizing ventilation airflow.
Energy Use and Ventilation Airflow
Let’s begin an analysis of ventilation-related energy use by defining the “status quo,” or how most new indoor growers will set up their tent or grow room ventilation system based on common recommendations of equipment manufacturers and recommendations of retailers and the practices of growers. I present Exhibit A, which states that a “common goal” is to achieve 1 volume exchange per minute. It is recommended that I calculate the volume of my tent (107 ft3 for my 4x4 tent) and multiply by a factor of 1.25 to account for head losses in the ductwork. So according to the status quo way of doing things I should look for at least 134 cfm.
The closest I could find to this were 4” centrifugal (squirrel cage) blowers in the 170-190 cfm range, so I bought an iPower 4-inch 190 cfm blower and carbon filter on Amazon. When I fired this thing up I knew it was moving way too much air so I bought an iPower fan speed controller. With the blower set to full slow it still made more air than I think I’m going to need. It was also noisy, and these kind of AC motors make an annoying hum at low speed, they’re really not well suited to variable fan speeds. Here’s how I’m going to define the status quo for my current situation:
Status Quo
Blower: 190 cfm centrifugal blower, brushless motor, 84 W (0.7A @120V AC)
Blower sound level: 78 dB full speed, 62 dB slow speed
Ventilation system configuration: discharges from tent in basement to outside air
Temperatures: 75°F tent temperature, 35°F outside air temperature (OAT)
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