I just learned the hard way why not use custom-built control systems.
I am currently on a business assignment overseas and will not make it home for at least one more month. My wife reported there isn't hot water in the house. The logic in my PLC disables hot water heater anytime the geo is running and re-enables it when it stops. That way the geo keeps the water heater charged preferably by the heat pump. The problem started when PLC requested start of the heat pump but it didn't start for at this point unknown reason and thus kept the water heater disabled. Then I spent few hours of figuring out how to override the PLC and instructing my wife how to reprogram the HP to stand alone mode, latch primary pump relay to always on, set heat accumulator SP in PLC cabinet to low temp so the logic would think that it is hot and didn’t try to start the heat pump thus disabling the hot water heater etc.
So now the house is warm and hot water is available.
Lesson learned:
The emergency PLC bypass should be built in the system, tested and detailed override procedures written. In other words I have to redesign the whole thing. It will be fun project. Well, until something unexpected happens again.
Last edited by Ladia; 02/28/09 11:24 AM.
