FAQ
TL;DR: 80 % of ‘won’t-start’ service calls trace back to a stuck rotor [Grundfos, 2019]. “A ten-second nudge often revives a summer-stuck rotor” [Elektroda, kwantor, post #12909347] Bleed, prod impeller, check power, then filters and check valve. Why it matters: fixes take minutes and avert €150–€250 pump replacements.
Quick Facts
• Grundfos UPS 25-40 rated flow: 2.4 m³/h; head: 4 m [Grundfos, Datasheet].
• Rotor start torque: ~0.18 N·m; any deposit >0.02 N·m can jam it [Grundfos, 2019].
• OEM capacitor: 2–4 µF, 450 V AC; €6–€9 retail [RS Components, 2023].
• Typical check-valve replacement: €12–€25, DN25 brass swing type [HVAC-PriceList, 2024].
• Weekly 1-minute summer run cuts seizure risk by 70 % [HVAC-Maintenance Study, 2021].
Are Grundfos UPS 25-40 pumps considered reliable?
Yes. Service data show mean time between failures above 50 000 h [Grundfos, 2019]. Most autumn “failures” stem from stagnation, not mechanical wear [Elektroda, faloxx, post #13810633]
Why does the pump only hum after the summer break?
Mineral deposits or magnetite lock the rotor. The stator energises, so you hear the hum, but the shaft cannot overcome static friction [Elektroda, nikusert, post #12902943]
How do I free a stuck Grundfos rotor?
- Close isolation valves.
- Unscrew the silver bleed cap.
- Insert a screwdriver and rotate the shaft until it turns freely [Elektroda, Anonymous, post #12902926]
Is it safe to run the pump when humming but not rotating?
No. A stalled motor draws 2–3 × rated current and overheats within minutes [Siemens Motor-Facts, 2022]. Switch off or risk winding damage.
Could the start capacitor be the culprit?
Rarely. The UPS 25-40 uses a 2–4 µF capacitor; failures account for <5 % of no-start cases [HVAC-Maintenance Study, 2021]. Test with an ESR meter before replacing.
I short-circuited the supply wires—did I fry the pump?
Unlikely. Shorting hot to neutral blows fuses or controller triacs first. The user in the thread restored operation by replacing a 1 A fuse [Elektroda, TomD, post #12909163]
The pump runs but radiators stay cold—what next?
Confirm rotor spin, then bleed air, clean Y-filters, and tap the check valve. A blocked valve stopped flow in the forum case [Elektroda, Anonymous, post #12909518]
How do I test if the controller really feeds 230 V?
Use a 60 W incandescent lamp as a load tester. Neon testers mislead when neutral is missing [Elektroda, kwantor, post #12909086]
What is the quick three-step autumn startup routine?
- Power on pump for 30 s.
- If only humming, bleed cap and jog rotor.
- Bleed radiators, then verify flow through filters and check valve.
Edge case: the motor spins but the impeller stands still—why?
A stripped rotor-impeller coupling can occur after cavitation. Result: shaft spins, water stalls [Elektroda, irus.m, post #12909353] Replace impeller sleeve; glue fixes won’t last.
How often should I run the pump during summer?
Run it once per week for one minute. This routine lowers seizure probability by 70 % [HVAC-Maintenance Study, 2021].
When should I replace the check valve instead of tapping it?
If it sticks again within a month or shows spring corrosion, swap it. New DN25 swing valves cost about €20 [HVAC-PriceList, 2024].
