## Air conditioning, humidity, condensate and Summertime

Having to use the shop vac to get a few gallons of water out of the ductwork on a regular basis was an indication of a problem.

Where does it come from? I used Calculate Temperature, Dewpoint, or Relative Humidity to see how humidity changes with temperature. 90F at 10% humidity is a 26F dew point. Cool it to 75F at the same dew point and the humidity goes up to 17%. Another Clausius-Clapeyron calculator directly shows change in temperature effects on humidity. Add to this the impact of human breathing and sweat, showers, cooking, and other water vapor sources and the indoor humidity can get up to 40% even with a proper infiltration of outside fresh air.

How much of a problem is it? The Air Conditioning Condensate Calculator takes into consideration indoor and outdoor conditions, system size, and infiltration to calculate an estimate of how much condensate will be produced. With 90F at 10% outside (1.5 gr of water per cubic foot), 75F at 40% inside (3.8 gr/cubic foot) as typical here with a 4 ton AC and a 20% outside air rate, I could expect up to 8 gallons of condensate a day. Those conditions, of course, last only for the peak AC period of about 6 hours per day. That means probably less than half that or 3 to 4 gallons might be produced by my AC. That 3.8 grains of water per cubic foot means I have about 1 gallon’s worth of water in the household air. Common guidelines indicate that the AC should cycle the house air volume in 3 or 4 hours.

To calculate energy bills, the degree days of warming and cooling can be useful. The weatherdatadepot will provide that information with a comparison between selected years for a given location.

One of the interesting nuances with humidity is that it is often cited as how much water the air can hold. The physics stack exchange has a bit on this and notes that

“Actually the “can hold” can more accurately be understood as there is a rate for water vapor condensing, and another rate for droplets evaporating. The dew point is the temperature at which these rates are equal. Dew Point That means it does not actually depend on other gas, such as air, being present.” – Mike Dunlavey

It appears that ‘air conditioner condensate water recycle’ is a big item in some circles. Let’s see … if I collected a bit over three gallons per day for a month, I’d collect about 100 gallons. That might provide a percent or two of my water needs. Save the planet! Watch out for peak water! (OK, satire, but some folks sure get enthused about these things)