There are several things that can cause a flow switch error. The first, and easiest to remedy, is just a bad flow switch. Replace it. But if that doesn't fix it, and you have to go further, read on!
Flow switches are directional, mechanical devices (usually) and are either open (no flow) or closed (sufficient flow). They require 2 conditions be satisfied before they will close. The first is quantity of water; Flow switches are usually labeled with a GPM rating. If a switch is rated at .5gpm it means .5 gallons of water per minute will be required to close the switch. But, along with the amount of water, there is one other condition that's just as critical, and that's water pressure. All the flow switches we use not only require the rated water flow in GPM's but also require a 40psi differential between the input and output, before the switch will close. In other words, if you are supplying .5gpm at a forward pressure of 60psi and have a back pressure of 15psi the differential will be 45psi and the flow switch will close. If, however, you have .5gpm of water flowing at a forward pressure of 60psi and a back pressure of 25psi the differential will be 35psi and the flow switch will not close (even though you are supplying the proper amount of water). Back pressure, at differing levels, is present in all systems, especially systems where the return is being sent vertically through pipes to empty somewhere else in a manufacturing facility.
Therefore its critical to know 4 things before you determine where the problem really is:
what amount of water are you supplying (does it meet the GPM spec's of your flow switch?)
what is your forward pressure in psi's?
what is your back pressure in psi's
does the differential between these 2 meet the criteria to close the flow switch (remember 40psi differential is required for the switch to close).
Since these are mechanical devices they can also just plain "stick" open or closed. The metal body flow switches can be disassembled, cleaned out and "unstuck". Sometimes reversing water through them will un-stick them. Also, we have had several instances of water being connected "backwards". Since flow switches are directional its very important thet the water being supplied is in the right direction. Most of our switches have an arrow on them to indicate direction. Make sure this arrow is in the right direction. Also make sure the inlet and outlet are connected properly. It's absolutely critical that your supply be connected to our inlet and our outlet go back to your return. If you reverse these, the flow switch will not close because you are running water backward through the system.
