logo elektroda
logo elektroda
X
logo elektroda
Advanced Search
logo elektroda

Grundfos UPS 25-40 Circulation Pump Fails to Start After Summer, Only Hums

faloxx 61797 23
Best answers

Why does a circulation pump only hum and fail to start after being idle over the summer?

The pump is likely stuck or seized from inactivity, so first unscrew the bleed screw/cap and gently turn the impeller or shaft with a screwdriver, making sure the pump is isolated so water does not leak [#12902926][#12902943][#13810633] If the motor hums but the water still does not reach the radiators, check the check valve (one-way valve) and any filters for blockage; in the thread, tapping the valve restored flow [#12909518][#12909536] One reply also noted that the rotor can turn while the impeller stays still if the mechanical connection is loose [#12909353] To help prevent it happening again after summer, run the pump briefly once a week [#12909716]
Generated by the language model.
ADVERTISEMENT
Treść została przetłumaczona polish » english Zobacz oryginalną wersję tematu
Report a violation of the law
| New topic
  • #1 12902918
    faloxx
    Level 10  
    Posts: 22
    Rate: 14
    Hello
    Today I turned on the central heating furnace and immediately noticed that there was no water flow in the radiators.
    I quickly figured out the reason for this. The circulation pump does not pump water. Throughout the summer, the stove operated in "summer" mode and only heated water for the boiler. The underfloor pump started without any problems, but the radiator pump did not.

    I also noticed that when the stove turns on the pump, it makes a quiet humming sound, like the sound of a transformer. And that`s it, nothing else happens. For now, I have switched the stove back to "summer" mode so that it doesn`t get worse.

    Are these pumps very reliable? What could be the cause of this failure??
  • ADVERTISEMENT
  • #2 12902926
    Anonymous
    Anonymous  
  • #3 12902943
    nikusert
    Level 41  
    Posts: 5428
    Help: 699
    Rate: 1752
    Hello
    Disassemble the pump, maybe the impeller is caught. Unscrew the screw at the back of the pump and try to move the impeller with a screwdriver, turn off the water before and after the pump if you have valves, because water may leak out.
  • #4 12909053
    TomD
    Level 11  
    Posts: 35
    Rate: 39
    Sorry to hijack this topic, but I had a similar problem. But now an additional one has appeared, which I caused myself. I have two Grundfos pumps that work with the fireplace. After the summer period, one of the pumps, the one behind the exchanger, did not work or worked too inefficiently to push water to the radiators. But not about that now. While checking the pumps, I found a short circuit on the second one. I simply inadvertently touched the cable from minus to plus, and the pump does not work. What could burn after such a short circuit? Capacitor or something in the engine?
  • #5 12909057
    Anonymous
    Anonymous  
  • #6 12909071
    kwantor
    Level 38  
    Posts: 4263
    Help: 352
    Rate: 1225
    By short-circuiting the power supply, you damaged the controller or some protection - the fault lies there far beyond the pump ,.
    My friend beat me to it, so he`s deleting it.
  • ADVERTISEMENT
  • #7 12909073
    TomD
    Level 11  
    Posts: 35
    Rate: 39
    Jerzykowski wrote:
    The pump was not damaged by the short circuit. I would look for the fault in the controller, check the fuses.


    I checked the fireplace controller and there is electricity at the output. The fuses are all OK. The pump receives electricity, so theoretically it should work.
  • #8 12909082
    Anonymous
    Anonymous  
  • ADVERTISEMENT
  • #9 12909085
    TomD
    Level 11  
    Posts: 35
    Rate: 39
    kwantor wrote:
    By short-circuiting the power supply, you damaged the controller or some protection - the fault lies there far beyond the pump ,.
    My friend beat me to it, so he`s deleting it.


    But where? As I wrote above, the pump receives electricity, so if the controller was damaged, it seems to me that there would be no power to the pump.
  • #10 12909086
    kwantor
    Level 38  
    Posts: 4263
    Help: 352
    Rate: 1225
    What is this current? connect a test tube made of a 230V bulb, e.g. 60W.
  • ADVERTISEMENT
  • #11 12909088
    TomD
    Level 11  
    Posts: 35
    Rate: 39
    Jerzykowski wrote:
    How did you check that the current was coming? It`s best to connect a light bulb instead of the pump because neon indicators are misleading.


    Well, I checked it with an ordinary current tester.

    Added after 40 [seconds]:

    kwantor wrote:
    What is this current? connect a test tube made of a 230V bulb, e.g. 60W.


    Okay, since you say it will be better, I have to check it out.
  • #12 12909097
    Anonymous
    Anonymous  
  • #13 12909100
    TomD
    Level 11  
    Posts: 35
    Rate: 39
    Jerzykowski wrote:
    What kind of sampler is this and how do you interpret the measurement results?


    The most ordinary one for PLN 2 :)
    Interpretation, it shines or it doesn`t shine :)

    Okay, I`ll test it with a light bulb and then I`ll be sure if there is normal voltage there.
  • #14 12909130
    kwantor
    Level 38  
    Posts: 4263
    Help: 352
    Rate: 1225
    There certainly isn`t. No neutral.
  • #15 12909163
    TomD
    Level 11  
    Posts: 35
    Rate: 39
    kwantor wrote:
    There certainly isn`t. No neutral.


    Thanks for the help. I checked the bulb and it didn`t actually light up.
    I replaced the fuse in the fireplace controller (luckily I had one) and now the pumps work.
    Unfortunately, there is still the same problem as the friend who started this topic, i.e. the pumps work, but they do not push water into the system.
  • #16 12909196
    Anonymous
    Anonymous  
  • #17 12909337
    TomD
    Level 11  
    Posts: 35
    Rate: 39
    Jerzykowski wrote:
    Have you checked the filters?


    Which filters do you mean? The ones on the installation pipes?
  • #18 12909347
    kwantor
    Level 38  
    Posts: 4263
    Help: 352
    Rate: 1225
    You need to determine whether the rotors are spinning, if so, there is air in the system, or it is a matter of filters, as a colleague wrote, if there are any.
  • #19 12909353
    irus.m
    Heating systems specialist  
    Posts: 1248
    Help: 122
    Rate: 453
    Yesterday I had a breakdown in the UPS - the mechanical connection between the rotor and the pump axis was loose. The pump is turning, the impeller is standing. Check it out.
    Regards
  • #20 12909434
    TomD
    Level 11  
    Posts: 35
    Rate: 39
    It currently looks like this:


    There is circulation everywhere, but it ends in the place marked in yellow, i.e. where there is (I don`t know if I`m calling it correctly) a one-way valve.
    However, when I manually turn on the underfloor pump, the water immediately continues to flow, as if the underfloor pump was sucking in the water further.
    Grundfos UPS 25-40 Circulation Pump Fails to Start After Summer, Only Hums
  • #21 12909518
    Anonymous
    Anonymous  
  • #22 12909536
    TomD
    Level 11  
    Posts: 35
    Rate: 39
    I guess that was it. I tapped the valve and it also went to the radiators. We`ll see what happens next, but it seems that everything should be OK. The only question is whether I shouldn`t replace this valve now to make sure it doesn`t get blocked again.
  • #23 12909716
    Anonymous
    Anonymous  
  • #24 13810633
    faloxx
    Level 10  
    Posts: 22
    Rate: 14
    Topic to be closed. The pump stopped after the summer season. It was enough to unscrew the cap, lightly move the screwdriver and go. Thank you all for your comments.
| New topic
Report a violation of the law

Topic summary

✨ The discussion revolves around a Grundfos UPS 25-40 circulation pump that fails to start after the summer season, producing only a humming sound. Users suggest troubleshooting steps, including unscrewing the pump bleed screw and manually turning the impeller to check for blockages. One user shares a related experience with another Grundfos pump that had a short circuit, leading to discussions about potential damage to the controller or fuses. The conversation highlights the importance of checking filters and ensuring the mechanical connection between the rotor and pump axis is intact. Ultimately, a user resolves their issue by tapping a blocked check valve, restoring water flow to the radiators. Recommendations include regular maintenance during the off-season to prevent similar problems.
Generated by the language model.

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?

  1. Close isolation valves.
  2. Unscrew the silver bleed cap.
  3. 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?

  1. Power on pump for 30 s.
  2. If only humming, bleed cap and jog rotor.
  3. 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].
Generated by the language model.
ADVERTISEMENT