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It is quite simple and inexpensive to build the buckets and recirculation. Its everything else that starts to cost money. The lights, the IGS environment controller computer, the 10,000 BTU air conditioner, the many osscilating fans, the air pumps, the 5 gallon things of nutrients, the water chiller, etc etc etc... Thats what costs money, so this system can be taken to whatever level you want.
I have done this system as small as 4 plants in a 5x5 closet with wonderful results. The # of buckets is completely up to you, and this will detirmine your size of reservoir and the type of water pump you use.
The parts necessary for recirculation are 1/2" hose, 3/4" hose, 1/4" hose, 1/4" 90 degree barbs, 1/4" straight barbs, 1/4" air hose (I use clear as it differentiates between water and air), 3/4" Ebb and Flow (Flood and Drain) bulkhead fittings (for the buckets and reservoir, 1/2" E & F bulkhead fittings, as many 3/4" and 1/2" 90 degree elbows, on/off valves, and T fittings as you need. Finally you need your water pump and your reservoir(s). For a much more detailed list of construction and parts, search my past threads in the Search function. The E & F fittings work great on the buckets and have never leaked. You just have to be careful when you drill the holes, keep the drill very straight.
For 4 plants, I used a 350gallon per hour pump, which is excessive but I had it laying around. For only 4 buckets, youd be fine with a 200gph but its nice to get a bigger one incase you expand in the future (since magnetic drive pumps dont wear out any faster even if they arent putting out their maximum output). Its also nice because it fully recirculates the water in a matter of minutes which is great for pHing and setting nutrients. Fast moving water also has even less chance to form pathogens, but more on that in a second.
I would use a 20 gallon reservoir for 4 plants or less (a Rubbermaid tote works great, I double them up because otherwise they can sag out from the weight of the water). The size fits easily between or amongst the 4 buckets or somewhere completely different. If the pump is strong enough you can have your reservoir in a completely different room or closet. The pump sits in the reservoir, pumping the water into the top of the buckets via the 1/2" main line that has1/4" line going into the buckets. The water sprays against the 6" Netpot, keeps the medium and rootball wet, creates water droplets, and adds water. The 3/4" return line recirculates the water back to the reservoir via gravity (the weight of added water in the bucket forces it to go somewhere, and your just giving it a place to go back too). You must have your reservoir the same height as the buckets because the level of water in the reservoir determines the level of water in the buckets. This takes a few tries to get right.
The absolutely most important part In My Opinion about DWC and Recirculating DWC is your water temperature. You can have everything else absolutely perfect but if your water temp is not controlled, its a good chance you will have problems or not experience the explosive growth you should be. DWC provides some of the best combinations of nutrients, Dissolved Oxygen, etc for some of the craziest growth you have seen. That is why 68 degrees F is your golden number. This is the best combination of available dissolved oxygen in the water that will keep any pathogens from forming and keep everything very clean. Anything cooler and you will stunt growth, anything hotter and you will most likely run into root rot. For this reason, I use a 1/4 horsepower water chiller for my big room. This can lower 150 gallons of water by about 15 degrees easily, meaning it can lower 75 gallons over 30 degrees. It works absolutely amazing and is the single best purchase I have made. Keeps it at a solid 68 all the time.
That being said, I have completed more then one recirculating DWC grow without a chiller and with great awesome white roots and good yields. However, I kept the temperatures very very low in the closets (usually between 60-70 degrees F), I used an 8" fan to exhaust (thats a lot of CFM for a small closet), and I put ice in the reservoir anytime it got warmer then 68 degrees. Sure, the water got as high as 75 degrees sometimes but if you keep things clean, use a supplemental cleaner like Hygrozhyme, you should be fine.
I also wanted to mention another thing about CO2 and room temperature and dehumidifiers. I have actually STOPPED using dehumidifiers as I found they created more problems then helping. I have tried a variety of methods of using them, including building an exhaust system for the hot air or isolating it or whatever. The problem is that when I need a dehumidifier the most is running a Sealed Room with CO2, but running a Dehumid exhaust the CO2 out the room. So if you have it exhaust the air back into the room to not lose the CO2, the room will get to hot.