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g/plumbing • Posted by u/pex_pioneer •

Recirculation pump on a demand loop keeps failing after 8-10 months — third one Service Call

Custom home, 4200 sq ft, dedicated return recirc with a Grundfos UP15-10SU7P on a demand switch. Third pump in under three years. Each one runs great then starts making noise and eventually seizes.

Water is on a well, treated through a softener. Hardness after softener is 2 gpg. pH 7.4.

Pump is not running constantly — it is demand-triggered, maybe forty cycles a day, thirty to ninety seconds each.

I am replacing pumps under warranty and eating the labor. What am I not seeing?

💬 6 replies

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u/admin_jake ADMIN •
✓ Accepted Solution

Both of the above are pointing at the same thing from different sides, and the reply above has it — but I would test for it directly before you buy a fourth pump.

The test: run a demand cycle and listen to the pump in the first three seconds. A pump that is passing water immediately sounds different from one that is spinning in air and then catching. If it makes a brief rattle or gravel noise at start and then smooths out, that is cavitation on start and it is destroying the bearing forty times a day.

The fix is a proper air separator at the loop high point plus checking that the return actually rises continuously back to the water heater. On a lot of custom homes the return line gets routed around a beam and creates an unvented high point that the plans never showed. That pocket fills with air, the pump pulls it every cycle, and no pump survives it.

Trace the actual return route in the field. Do not trust the drawing.

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✓ Solution
u/pex_pioneer OP •

Traced it this afternoon. There is a hump over an HVAC trunk in the mechanical room ceiling that is not on any drawing. About eighteen inches higher than the water heater inlet. No vent, no separator, nothing.

It gravel-rattles for about two seconds on every start. I have heard that noise on all three pumps and I filed it under "small pump noise."

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u/boiler_bill •

There it is. Put an automatic air vent at the top of that hump and a separator in the loop and your fourth pump will outlive you.

Also worth telling the customer what happened, in writing. Three pumps in three years is the kind of thing that turns into "my plumber cannot fix it" and now you have a specific, documented cause.

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u/boiler_bill •

Check for air. A demand loop that sits idle between calls can accumulate air at the high point, and a circ that runs dry for even a few seconds every cycle will kill a bearing in under a year. Forty cycles a day times 300 days is 12,000 dry starts.

Where is the air separator, and is there one?

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u/drain_dave •

Forty starts a day is a lot of starts. What is the pump rated for? Some of the small circs are not happy with high cycle counts even if total runtime is low. Starting is where the wear is.

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u/pipefitter_pat •

The general lesson — when the same component fails repeatedly in the same installation, the component is not the problem. Sounds obvious written down. Takes most of us three pumps to internalize.

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