Jacob’s Blog

Air Exchange Installation
I finally dragged myself back under the house to run the exhaust duct for the air exchange system. The crawl space is not a nice place to work in. Prior to running the exhaust duct I had been running the air exchange for about a week. I have some PVC pipe in the area of the fan as well as some other odds and ends from the project, all of this stuff was covered in condensation. I’m glad I decided to run the exhaust vent when I did! So I ran about 25 feet of flexible ducting to the nearest crawl space vent where the moisture laden air is vented out of the house. My tank temperature is steady at 77.5 degrees F with all lights and pumps running.

Evaporative Cooling - An underrated method of maintaining the aquarium temperature.
Due to the air movement across the top of the tank (21 sq ft approx) I’m seeing 5-6 gallons a day of evaporation. The average water temperature is 77.5 degrees. At that temperature the latent heat of evaporation is 1049 BTU/lb. This heat remains with the water when it changes phases (liquid to gas), so it is removed from the body of water in the aquarium. In other words, evaporative cooling. For 6 gallons evaporated that amounts to 52,555 BTU’s per day (24 hous) removed.

To compare evaporative cooling to conventional (chiller)  we can break down the above heat removal to 2190 BTU/hr. A 1/5 horsepower chiller can typically remove between 1800 and 2400 BTU/hr. Let’s give the chiller the benefit of the doubt… At 2400 BTU/HR and 150w (1/5 hp) the efficiency is 16 BTU/hr/w - and this does NOT take into consideration a chiller feed pump! The fan I’m running draws a constant 50w. This works out to 44 BTU/hr/w. Hooray for evaporative cooling!!

Benefits of Air Exchange
There are numerous benefits to air exchange…

• Low cost method of maintaning aquarium temperature
• Increase working life of electrical and mechanical components in the aquarium stand/canopy
• Keep the aquarium oxygenated
• Reduce pH swings due to constant fresh air supply
• Reduces heat transfer into the home
• Chillers give off heat into the home, which is counter productive and uncomfortable, air exchange only removes heat (assuming the fan is outside the home)

The Downside of Evaporative Cooling
As wonderful as evaporative cooling is for aquaria, there are some pitfalls. The first, and possibly most obvious, is the fact that the evaporated water must be dealt with. 6 gallons of evaporation inside a home every day will lead to some serious humidity and probably mold growth. If an air exchange system cannot be used the air conditioner will need to be run often, which is wasteful. Another downside is the amount of water that must be replaced in the aquarium. On gut feel, forced evaproative cooling doubles the amount of evaporation realized. An auto top off system may be in order to keep salinity swings in check. Last but not least, the evaporated water is lost to the atmosphere. Water is a valuable resource, recovering the water vapor would be wonderful but this will likely reduce the efficiency of the system.

In Summary
I’m very excited about my air exchange. Overall it works great. One downside I did not mention because it is more of a tuning issue and that is the battle between air exchange and heaters in the tank. This is mostly when the lights are off late at night throughout the morning. In my case I need to reduce the volume of air being exchanged so the heaters are not adding heat back into a system which is removing it, all this serves to do is increase my electrical bill! That aside I highly recommend a simple air exchange system to anyone who has the luxury of making some holes in their floor, walls, and/or ceiling and running some ducting between the tank and outside of the home.