clvlnd- It all epends how warm you want it. I have a 10' X 15' single pane glass greenhouse in Zone 8. I heat my greenhouse with an electric, 900W/1500W oil-filled heater. It's got a thermostat and I keep it on the lowest setting so the temp stays above freezing. You can usually get a decent one for about $50 (e.g., from Amazon). If you get an electric heater that draws more than 1500W, or run more than one, you're going to blow the breaker on a standard 15 amp circuit, especially if you have anything else plugged in. If you want to use something bigger, you're going to have to seriously upgrade the wiring that supplies electricity to your GH. One of the reasons I can get by with a small heater is that I also have 3 seed-starting mats that measure 1.5' X 4', and each one only draws 100W. I just set my sensitive plants and seedlings on that and everything seems to be happy. Since I usually have a power outage a couple of times during the winter, I have a small propane heater for backup.

BigOleDude, I did a quick Google search of NO relative humidity. Average is over 84% in the morning, but the average is in the mid-60% range in the afternoon, every month of the year. Relative humidity of the ratio of the partial pressure of water vapor to the partial pressure of water when air at that given temperature is saturated (ie, the dew point)...Nathan, correct me if I'm wrong about this...so when the temps go up during the day, the dew point goes up and the relative humidity goes down. So you check this chart and see that at 90*F and at 65% relative humidity, you still get 7*F drop in temp with evaporative cooling methods. I use the same methods Cactusfreak uses...shade cloth, misters, exhaust fans...earlier this summer with humidity in the 60% range, I was 87*F inside my GH when the outside temp was 91*F. Here is a link that might be useful: Evap cooling chart

Cindy, follow the link below to view my pads. Mine is bit more complicated than needed since I have it in two sections around the door. One pump with the supply line split to feed both sides. The two supply tanks are connected and being on the same level, they still filled with one automatice float valve. The pump is a 300 GPH@5feet of lift. You can see in the photo it is a small pump. Sits right on the bottom of the tank. The pads are each one foot wide and three feet high. I have eight for my house. Yours wouldn't need that many being a smaller house. You should be able to get by with half that. They are Glacier-Cor pads from IGC:http://www.greenhousemegastore.com/ The frame is simple as you can see in the photo. A vinyl gutter from Lowes rest on the bottom of the frame. A piece of benchtop in the gutter keeps the pads out of the return flow of water. The water distribution line is adjustable with the valve at the top. Small holes drilled about 2 inches apart down the PVC pipe provide the flow. Holes at the top so the holes will be less likely to clog. The union on the pipe allows you to rotate the pipe to center the flow over the pads. I have mine barely squirting up out the holes. You want the pads moist but not a flowing wall of water seen cascading down. The sides and top of the frame are sealed with plastic to keep the air from slipping around the pads. Once every three weeks, I add Physan 20 at their recommended rate to keep algae from developing. A small greenhouse is hard to cool that way since the short space the moist air is moving across does not give it much time to evaporate and provide the needed cooling. Someone else may have experience as to what to expect from your size. Another consideration is the fan size you have. Assuming it now provides the recommended exchange of air once every 60-90 seconds, it should work ok. Hope I've given you some ideas on how to design a simple pad system. Good Luck! Jerry Here is a link that might be useful: My cooling system

some points to consider are: 1.if you move the pad a foot downstream from the fan it will help prevent moisture on the rather exposed windings in the motor 2. a box fan is a propeller which when loaded or restricted in air flow draws more current or loads up. A squirel cage fan is the opposite when restricting air flow it unloads or draws less current. Most comercial swampers are made with squirel cage fans. You may be able to find a junk fan from an old furnace for close to or nothing. 3. do you realy want to cool the nutrient?my understanding of plants is that if possible warmer nutrients produce faster growth. 4. beware of PPm of the nutrients as you stated you have added makeup water to offset this but most makeup water also contains minerals which you are now concentrating. Most comercial swampers also include a bit of overflow or wasting to help prevent mineral buildup which inhibits evaporation and clogs equipment. this is generally done by a unique balance in fill and overflow so that when it shuts off the remaining water in the lines and pads causes an overflow. when it starts the water leval drops to fill lines and pad causing fill valve to act. 5. The point has already been made and is correct evaporative cooling only works in dry air the effectiveness will be proportional to how dry the air is. I wonder if a better plan may be to run a swamp unit independant of your nutrient solution and if needed run the nutes through a loop in the pan of the cooler for some cooling without the addition of larger amounts of makeup water to the nutrients to allow for the evaporation. This would also allow some blowdown of mineral saturated swamp unit water without loosing nutrients or upsetting the mineral balance there.I would also question the need for cooling nutrients unless they are over 80 degrees. At any rate watch your ppm close while using until you get the kinks worked out. Sounds like a good idea in general as swamp cooling is about the only method feasable in greenhouses and most plants deal with higher than ambient humudity fairly well. Another option is shading in hot and summer climates most plants get more sun than they can use so partial shading may also reduce heat without loss in growth.