How do you size a Mastercool?

First off, just to be clear, I am not a professional in hvac. I am just an over zealous homeowner!!

To answer your question about my home. Yes, a 5500 cfm single inlet mastercool type evap cooler keeps my home cool in the summer. The evap in our home, given the low humidity conditions typically found in Phoenix, consistently is able to supply the home with air that is 35 degrees lower than the outside air, up until about the end of July. At the end of July, the humidity in Phoenix is high enough that an evap cooler will not be able to produce cool air. As an example, in the prime evap cooling months, before the end of july, if the outdoor temperature is 90 degrees with 10 to 15 percent humidity, our evap is pumping 55 degree air into the house. As to overall humidity in the house, I have never tracked the relationship of outdoor humidity to indoor humidity, but I do know that the indoor humidity, typically, does not go above 40 percent, unless we run the system when the dew point is above 50 degrees.

as I stated before, we added a second duct system to our home, just for the evap. This was because the AC duct system was too small for the amount of air the evap was trying to push into the home. It is quite common for homes in phoenix to have an evap piggy backed into the AC duct work. The "too small" AC duct system caused tremendous static pressures in the ducts and seriously limited the air flow of the evap. The small ducts also caused the humidity to be high in our home because the air could not move through the house and was stagnated in the ducts where it picked up a very high humidity load and the air flow was noisy as the highly pressurized air in the ducts had a very high velocity at the registers. so the question to you is how big was your old evap?? some of older four sided systems were as low as 2000 cfm and only rarely were they larger that about 4000 cfm. additionally, how big are the distribution ducts in your home.

You didn't state if your home has an ac system also. It is quite common for homes in phoenix to have an evap piggy backed into the AC duct work. but it is also not uncommon for home to have evap only. Is your home an evap only home? I was assuming that you had a piggy back system.

Your idea of adding a water pump shut off based on humidity in the house being too high is interesting, but I am not sure how well it would work because without water running to the pad(s) of the evap, the cooling effect of the water evaporation at the pad(s) would stop working. At that point, you would indeed would be moving dry air into the home, but you would also be moving Hot air into the home. you didn't state when this high humidity occurs, so I am guessing here, but if your humidity is high in the prime evap cooling months (april, may, june, july) I would suggest that the air flow from the evap or the air flow of air out of the house is low. Stagnate air flow in the home will cause high humidity. Of course if you are trying to run the evap cooler in august when the monsoons can raise the humidity to 90 percent, the home will naturally be very humid, even with good air flow.

If you are just trying to control the pump with the thermostat so that the pump is not left on, that's fairly easy to do. In my case, I put a delay relay on the evap motor. Here is how mine works. When the thermostat calls for evap cooling, power is supplied to the pump and the fan delay relay. The fan delay is set to about 2 minutes. when the delay is over, the fan starts to run. so basically, the water pump runs for 2 minutes before the fan turns on, this allows the pad to get nice and wet before the air starts moving over the pad. When the thermostat temperature is satisfied, power to the fan and the pump are removed, thus stopping the pump.

I have no experience in dump pumps. We use a bleed off system here. We run the bleed off into a small plastic hose that we move around the yard to water the plants with. Last year we added a rain barrel and we now just put the bleed water there. The rain barrel is 55 gallons and it takes about a week to fill up the barrel when the system is running most of the day.

Motor size, as well as drive pulley adjustment are very important. in order to keep the motor from burning up, keep your utility bill low and insure good air flow, you need to check the amp load of the motor in relationship to the air flow and adjust the drive pulley accordingly. Most of the mastercool single inlet systems I have seen have an adjustable drive pulley. This adjustment is sort of and art and a science too. Bottom line is that you do not want to run the motor at full amp load 100 percent of the time, so you adjust the drive pulley for maximum air flow, below the full amperage of the motor. I try to keep mine below 90 percent of max amps. note that the amperage is also dependent on how much your have your windows (or upducts, or what ever you use as an air exhaust venue) open. amp reading are effected by house pressurization. That being said, 3/4 horse motors will give you more air flow, because you can adjust the pulley for more air and still keep the amps below maximum, but again, if your ducts will not handle the air flow, 3/4 horse will not run efficiently. If it was me I would try your existing 1/2 horse and see if it gets the job done!!!

BTW what part of Phoenix are you in?