I'm sure we've all had a few 'gulp' moments messing with automated water change systems and I thought it would be useful to share the problems we've had. My hope is that we can all improve our designs and minimize any trouble these systems can cause - because the know the benefits are fantastic!
1. I used these float switches from autotopoff.com, after a year and a half one finally gummed up and stuck. All that happens with my system when that occurs is that the tank will refuse to fill. I covered my sensors with loose tea filters that look something like this:http://baldmountaincoffee.com/Mercha...L/00002131.jpg
. I'm not 100% certain whether its a good idea or not.
2. I used a diaphram style drain solenoid like this: http://www.madhocontrols.com/solenoid_valve_lar.jpg
. The problem I had with it was under unusual circumstances but basically I was messing around backflushing my filter and a tiny piece of carbon stopped the diaphram from sealing completely.
The result was the tank kept draining, thus it kept filling. At that point I didn't have a thermostatic valve in place and the inlet water was cold (small volumes didn't matter). Consequentially some fish got very cold, some didn't make it. Two design changes were required. First a thermostatic valve, the best I could find was like you found, limited to 90 deg F or so. A bit warm and still an issue if the tank were unattended for long periods but better than freezing the fish out! And the second I did not implement: limiting the length of a fill cycle with a timer program. I really think this is an important one to avoid lots of potential damage.