Grundfos MQ3-35 booster pump short cycling

[semipro]

Our MQ3-35 that has operated reliably for several years recently started short cycling, resulting in a very distinctive pressure fluctuation at the use points. It was most notable in the shower. I installed a pressure gauge on the outlet side of the MQ and noted that the unit shut off every time the pressure reached the boosted pressure. I read in the Groundfos literature that the pump should continue to operate even after that pressure is reached as long as their is sufficient flow. This made me suspect that the either the flow sensor or control board was bad.
I removed the flow meter (a 1o minute job) to make sure it wasn't physically obstructed or bound up somehow. It wasn't. I looked for diagnostic data on the sensor but could find none. I'm pretty sure its uses a hall-effect sensor so I checked with a multi-meter to see that it was receiving about 5 volts to the sensor when the pump was energized - it was. I don't have an oscilloscope or I would have tried to measure the sensor output while the pump was operating (and the flow sensor impeller turning).
Since the control board is pretty expensive I took a shot and ordered a new flow meter for $80. I installed it and the pump seems to be working properly as before.
I thought I'd share as I could not find any related information for this particular problem on the web.

 

[Valveman]

Glad you got it fixed. But I will bet it won't be long until you have to work on it again. I wouldn't say a pump has been reliable unless it goes 15-20-30 years without having to be repaired. Grundfos makes some really good pumps. It is the ones with electronic circuits that are the least reliable. If you had purchased one of the regular JP series jet pumps it would not be giving you problems, because it has no electronics to cause problems. It is either the electronic boards or the sensors that go to them that cause the problems. A regular "dumb" pump controlled by a regular mechanical pressure switch is about the most reliable system you can find.

You have to remember that pump companies don't introduce new designs to save energy or save you money. A pump companies first objective is to make more money, and that is the number one priority when designing new products. The most reliable ones have not been changed in 30 years and that is why those kind are still running.

 

[Sponsor]

 

[kivfer]

Our MQ3-35 that has operated reliably for several years recently started short cycling, resulting in a very distinctive pressure fluctuation at the use points. It was most notable in the shower. I installed a pressure gauge on the outlet side of the MQ and noted that the unit shut off every time the pressure reached the boosted pressure. I read in the Groundfos literature that the pump should continue to operate even after that pressure is reached as long as their is sufficient flow. This made me suspect that the either the flow sensor or control board was bad.
I removed the flow meter (a 1o minute job) to make sure it wasn't physically obstructed or bound up somehow. It wasn't. I looked for diagnostic data on the sensor but could find none. I'm pretty sure its uses a hall-effect sensor so I checked with a multi-meter to see that it was receiving about 5 volts to the sensor when the pump was energized - it was. I don't have an oscilloscope or I would have tried to measure the sensor output while the pump was operating (and the flow sensor impeller turning).
Since the control board is pretty expensive I took a shot and ordered a new flow meter for $80. I installed it and the pump seems to be working properly as before.
I thought I'd share as I could not find any related information for this particular problem on the web.

I have same problem for three days. I just ordered flow sensor kit. Do I have to shut off main water line and drain water in house pipe lines when I install the flow sensor kit?

 

[Reach4]

I just ordered flower sensor kit.

I suspect you meant flow sensor.

 

[kivfer]

I suspect you meant flow sensor.

sorry, just edited

 

[semipro]

I thought I'd report back that the repaired pump is still going strong with no further issues.

 

[Brian K]

Semipro and Kivfer, having the same issue now, curious if the flow sensor kit was the answer and your systems are still working properly. Also, where did you buy the kit? Thx!!

 

[pacificdune]

I have exactly the same issue. I am hopeful that this is the problem. Just ordered the flow sensor, if not, will likely just get a new pump.

Update: After replacing the flow sensor the pump stopped short cycling. The only other issue was that the pressure would get too low before the pump would turn on, which I was able to resolve by adjusting the pressure switch 3/4 turn clockwise. Note, I am running it with an external expansion tank. I really appreciate this thread! It gave me the confidence to order the flow sensor instead of ordering a new pump.

Saved $550.

 

[pacificdune]

Semipro and Kivfer, having the same issue now, curious if the flow sensor kit was the answer and your systems are still working properly. Also, where did you buy the kit? Thx!!

I noticed your question and I had the same problem. See above post. Bought the flow sensor kit online from Supply House. Note there is a very small nylon gasket that goes over the impeller which is easy to misplace or lose when you first disassemble the sensor. I believe this is important.

 

[Valveman]

Glad you got it working! The external expansion tank will help. But save your money, those type pumps don't usually last very long and most of them end up here.

 

[curio124]

Hi guys. Really appreciate this thread.
I had an issue that the pump turned off every 10-20 seconds for just a brief moment. I solved this by increasing tension on the pressure sensor

(

).

But now I have different issue - pump turn off after 20-30sec with the red light. When I unplug the flow sensor from the circuit board, pump runs for 5minutes and then turn off what is a good behavior as I know that pump must be without flow for 5minutes to turn off. I'd buy a new flow sensor but it's rather expensive and the issue may be with the circuit board and not with the sensor. I've got 2 questions:
1. do you know how to measure the flow sensor? There are 3 pins. What can I measure and what are good values without and with the flow?
2. is it possible to somehow trick the pump that there is always a flow even without any signals from the flow sensor? I want to unplug the flow sensor (because without it I don't have the current issue) but don't want to pump stop after 5 minutes. Can I shortcut something? Give some voltage and/or resistance to some pins? (and I don't mind if the pump will run dry a) it will not - I have a good source and b) even if it burns - it's ready for trash anyway)

thanks a lot...

 

[Valveman]

The flow switch is the problem a lot of the time. Just block it in the up position and the pump cannot shut off. Figure out which wires make when the flow switch it up and you can probably just jumper them together instead of blocking the flow switch in the up position. I think it is shutting off after 5 minutes because of the thermal overload or low amperage condition, which isn't good. You could add a regular 40/60 pressure switch to turn the pump off instead of using the flow switch. Doing so, you will have just about eliminated all the electronics in that MQ pump. It would be better to start with a regular jet pump that doesn't have all that electronics. As you can see the electronics is what makes a water pump unreliable.