631 DHW recirculation pump – insufficient flow

What the control is telling you

With dedicated recirculation mode selected, the recirculation pump is running but the boiler does not see at least 1.3 GPM of flow.

Applies to: I-Series Plus Combi boilers — 150K BTU and smaller and 175K BTU and higher

Likely causes

• Recirculation parameters (9, 12, 13) set incorrectly
• Pump not wired to Pump 4 terminal
• Blown Pump 4 fuse
• Hot/cold lines reversed or cross-connected
• Dirty inlet filter, faulty check valve or air in the recirculation loop
• Flow turbine/sensor or water flow control problem
• Undersized or failed recirculation pump

Test and repair steps

1. Check parameters P9 = b (DHW recirculation on via Pump 4); P12 = A for crossover valve only, b for dedicated recirculation (needs over 1.3 GPM); P13 = A (default; enables external timer control).
2. If the pump does not run, confirm it is wired to the Pump 4 terminal on the PC Board (external pumps: 120 VAC, max 2 A).
3. With power off, check the Pump 4 fuse for continuity and replace it if open.
4. With all fixtures closed, run the recirculation pump and read flow in performance data #7.
5. If flow reads 0 isolate DHW and open the PRV or service port. If still no flow, check that hot/cold lines are not reversed or cross-connected.
6. If water flows at the port but data #7 reads 0: confirm the flow turbine spins freely and flow sensor wiring is sound and dry.
7. Check performance data #13 reads 0 (mid) or 1 (open) while the pump runs; see code 651 for testing the water flow control.
8. If flow is below 13 (1.3 GPM): clean the inlet water filter, confirm the dedicated return line and its check valve are installed and working, and confirm the flow turbine spins freely.
9. Bleed all air from the recirculation loop and confirm the pump is sized for the system and the boiler's piping (over 1.3 GPM needed).
10. Replace the recirculation pump if it is not working.