MCB tripping intermittently

[Buck Taylor]

My reading of your description was that it the MCB only cuts out when the kitchen hot tap is opened on its own. Also, that it was the motor humming prior to the MCB tripping. Did I get these wrong?

No your correct, the motor/pump hums and the MCB only trips (intermittently) when opening the tap in the kitchen.

 

[Mark K]

My reading of your description was that it the MCB only cuts out when the kitchen hot tap is opened on its own. Also, that it was the motor humming prior to the MCB tripping. Did I get these wrong?

My understanding also Chuck, is that it only trips when (only?) the kitchen tap is used, which is only on the hot side of the pump.

He hasn't said if it's the mcb or the motor that hums. As you say, if it's the motor then that could be faulty, but I'm barking up the "that would be the same whichever outlet was opened" tree.

I wonder if the other outlets in the house are mixers, so everything except the kitchen causes flow on the cold and hot.

Need more info from the OP I feel.

 

[Buck Taylor]

My understanding also Chuck, is that it only trips when (only?) the kitchen tap is used, which is only on the hot side of the pump.

He hasn't said if it's the mcb or the motor that hums. As you say, if it's the motor then that could be faulty, but I'm barking up the "that would be the same whichever outlet was opened" tree.

I wonder if the other outlets in the house are mixers, so everything except the kitchen causes flow on the cold and hot.

Need more info from the OP I feel.

Hi Mark,
It is the pump that hums as the consumer unit is in the garage and I would not be able to hear that whilst in the house. I have a few other mixer taps in the house and single taps and tried many times to trip the pump and been unsuccessful.
Your diagnosis is spot on Mark. it is something to do with the mains pressure / hot tap. I have also bled the system to ensure their are no air pockets working from the bottom of the house up.

 

[Mark K]

Hi Mark,
It is the pump that hums as the consumer unit is in the garage and I would not be able to hear that whilst in the house. I have a few other mixer taps in the house and single taps and tried many times to trip the pump and been unsuccessful.
Your diagnosis is spot on Mark. it is something to do with the mains pressure / hot tap. I have also bled the system to ensure their are no air pockets working from the bottom of the house up.

OK. Chuck was right about the humming and I was wrong. Without seeing it, I am going to say the resistance (water) at the kitchen tap is higher than any other tap. This is causing the motor to work harder and draw more current.

What I would want to hear, this will take teamwork, is, while the pump is humming, another tap is opened and see if the motor stops humming and starts pumping.

 

[Buck Taylor]

OK. Chuck was right about the humming and I was wrong. Without seeing it, I am going to say the resistance (water) at the kitchen tap is higher than any other tap. This is causing the motor to work harder and draw more current.

What I would want to hear, this will take teamwork, is, while the pump is humming, another tap is opened and see if the motor stops humming and starts pumping.

Yeah understand, I'll give it a go ( in house myself at moment) have a toilet downstairs I can get to tap, if I run fast !!!
If not I can try when the missus comes home.
Thanks guys really appreciate the help.

 

[Chuck]

I'm barking up the "that would be the same whichever outlet was opened" tree.

I suspect it's a motor problem but it's borderline-symptomatic at the moment and the kitchen tap, for the reasons you listed, is the worst case. I.e. starting against near-closed outlets.

There's another clue buried in the symptoms, which is that the system has been in for 3 years but this started as a problem only 6 months ago. Of course, it's difficult to decide whether the pump is dying of natural causes or whether something about the installation is overstressing it.

My next move would be a forensic comparison of the installation with the Grundfos instructions. IIRC those pumps have a minimum displacement rate of 5 l/min, for example. Grundfos also have a faultfinding page:

Technical Troubleshooter - Grundfos

and a contact page for 'faults not covered', which is the case here:

Contact - Grundfos

 

[Buck Taylor]

I suspect it's a motor problem but it's borderline-symptomatic at the moment and the kitchen tap, for the reasons you listed, is the worst case. I.e. starting against near-closed outlets.

There's another clue buried in the symptoms, which is that the system has been in for 3 years but this started as a problem only 6 months ago. Of course, it's difficult to decide whether the pump is dying of natural causes or whether something about the installation is overstressing it.

My next move would be a forensic comparison of the installation with the Grundfos instructions. IIRC those pumps have a minimum displacement of 5 l/min. Grundfos also have a faultfinding page:

Technical Troubleshooter - Grundfos

and a contact page for 'faults not covered', which is the case here:

Contact - Grundfos

Thanks Chuck I've been in contact with Grundfos numerous times. The advise they gave was fitting a non return valve on the hot inlet to prevent any back pressure from the mains cold feed. It done diddly squat. They also give 5 year warranty but only 2 years for domestic customers? What's that all about.

 

[Mark K]

Thanks Chuck I've been in contact with Grundfos numerous times. The advise they gave was fitting a non return valve on the hot inlet to prevent any back pressure from the mains cold feed. It done diddly squat. They also give 5 year warranty but only 2 years for domestic customers? What's that all about.

I think that's your answer...they are worried about back pressure from the mains cold water onto the hot. What that would do is restrict flow. But, so would having no opening on the cold tap, or a very restrictive tower tap on the hot.

As Chuck says, this has been going on for 2.5 years, giving the motor and it's capacitor a hard time, and now it's showing up.

I suspect they would say it's an incorrect installation causing flow below the minimum if you went for a warranty claim.

 

[Buck Taylor]

Yeah understand, I'll give it a go ( in house myself at moment) have a toilet downstairs I can get to tap, if I run fast !!!
If not I can try when the missus comes home.
Thanks guys really appreciate the help.

OK. Chuck was right about the humming and I was wrong. Without seeing it, I am going to say the resistance (water) at the kitchen tap is higher than any other tap. This is causing the motor to work harder and draw more current.

What I would want to hear, this will take teamwork, is, while the pump is humming, another tap is opened and see if the motor stops humming and starts pumping.

Hi Mark/Chuck,
With the pump in the hum state I opened up another tap immediately and the MCB still tripped.

 

[Mark K]

Hi Mark/Chuck,
With the pump in the hum state I opened up another tap immediately and the MCB still tripped.

Definitely points to a failed/failing motor then.

What happens if you reset it and open a different tap?