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Poor Man's Variable Speed Circulator

For anyone who is so inclined, here are directions for modifying a Grundfos UPS15-58FC 3 speed circulator for computerized control. Note: following these directions is at your own risk. It may void your warranty, curdle your milk, and cause spontaneous defenestration. You've been warned.

This approach is designed to be 'fail safe': If no power is applied to any of the relays, the pump will be under the control of the original controller (the EKO in my case).

My control system is designed to drive 12vdc relays. Three relays are needed: two DPDT and one SPDT. Almost any will work - I used Potter Brumfield relays from Digi-Key, part number PB968-ND at $1.90. These are rated at 8 amps. At that price, you can use these for the SPDT relay as well.

One relay - the 'Assert' relay - switches control from the EKO to my controller when power is applied. The other two relays replace the function of the speed selector switch on the pump, and add a fourth 'Off' position.

  1. Take the cover off and remove the screws holding in the switch / contact assembly. Remove this assembly by pulling it up gently. It is basically an 8 prong plug.
  2. Pry / cut the cover off of the switch / contact assembly. Discard the rotating switch component.
  3. Reassemble. Install relays and wire according to the schematic below. The pins on the connector are numbered 1-8, with 8 being at the end where the power connectors are. Pins 3-4 and 5-6 are connected in the housing - you will have to provide a jumper for pins 1-2. Make sure you reconnect the capacitor.
  4. When you're done, it might look something like the photo below. The neutral and ground wires can plug directly to the original spring clips. The hot (line) goes to relay A.

You'll need to run 12v wires to the pump to control relays A and B, the 'Assert' relay should probably be in the housing on top of the boiler next to the boiler's controller. The table at the bottom of the schematic above shows the pump operation as determined by the state of relays A and B.

If the 'Assert' relay is not energized, then A and B should not both be energized - that would prevent the boiler from turning on the pump.

Tested and works great. If you don't care about failsafe operation, then the 'Assert' relay is not required, and the state table would make more sense if you swap the normally closed and normally open contacts on the A and B relays. That would mean that the pump would be off if neither relay were energized, and on / high if both were energized.