Please consolidate info on Grundfos MQ3-45

[Alternety]

I have spent hours trying to figure out the issues involved. I am confused. I am working on this for a friend. He has a Grundfos MQ3-45 pressure pump. The pump is cycling at a short interval when water is being used. It is not clear to me, from much reading, how that pump actually functions. But I believe that action is wrong.

I have read a whole bunch of stuff about this configuration. Many conflicting views. As a help to me, and others with similar problems, can you guys provide some concrete information on how the whole thing works and the failure points. Let us not go to discussions of CSV. I am trying to work with the existing equipment.

As I understand it:

• There is a flow switch on the pump input that must operate (i.e., sense flow) before the pump will begin to operate properly.
• There is a pressure switch which controls the output of the pump (but no control mechanism I can find).
• There is a very small pressure tank that supposedly deals with output fluctuations.
  
• There is no mechanism I have identified to manage these two parameters

I am trying to help a friend. He has public water at about 25 PSI delivered to his house. So far, I have not been able to get a measurement of delivered pressure vs flow volume of the public water source under load.

The public water is fed to a Grundfos MQ3-45. Storage for the output of the MQ consists of the small tank included in the MQ and a very small additional pressure tank installed by a wandering plumber. I have no information on what people have done with the internal back flow components of the MQ.

I have not found anything I understand relating to how the pump manages to provide an appropriate pressure to the output. This information is consistently glossed over.

If the MQ sensors are not performing in a useful manor, how do I fix that.

I have a number of thoughts/questions about the system.

• How is the system output pressure controlled. I have seen no way to actually set the output pressure. The documented max MQ pressure seems to be way to high. The pressure switch can be changed if it is removed and adjusted. But it is rather a trial and error adjustment. Nothing on flow control.
  
• How does one control the threshold for the input flow.

What I am thinking is disabling the controls on the MQ and simply depending on a 40/60 pressure switch on the output, with a fairly large external pressure tank. That seems to be a relatively simple replacement for a whole bunch of firmware in the MQ. The output pressure sensor is easy to interdict with an external pressure switch. Flow control is an issue I do not understand. It is a three wire connection from the flow sensing turbine, with no documentation I have been able to find. Is there a way to disable the flow sensor and the input it has on the controller.

So, how do I deal with an MQ that may or may not be operating properly? Trouble shooting suggestions welcomed.​

 

[Valveman]

Well it sounds like you may not want my help? But I am very familiar with how those pumps work, or don't work. There are reasons why they no longer make the MQ, and you have listed most of them.

The MQ works very similar to the masscontrol/presscontrol/burcam type devices. It has low pressure and/or flow on controls, but low flow is the only control for pump off. When the flow sensor senses low flow, it times out for a few seconds and shuts off. Then when a tap is opened the little tiny built in pressure tank is suppose to supply enough flow to make the flow sensor turn and start the pump. But just in case the flow demanded is so low (dripping faucet) that the flow sensor will not start the pump, they also have a low pressure start that bypasses the flow switch. When pressure drops to a certain pressure, the pump is started. But it only stops when the flow sensor shows low flow, not on high pressure. The built in pressure tank only holds a teaspoon of water, so low pressure is the most common mode for start.

If you can bypass the electronic brain all together and just get to the two wires that feed the motor, you can use a regular pressure switch for control. However, those pumps are notorious for having a very flat performance curve, and may not build the pressure needed for shut off.

The Scala2 is Grundfos' idea as a replacement for the MQ. However, it has even more electronics, sensors, etc., than the MQ, and so far are not lasting any better either. You just have to remember that any pump with electronic controls are made to sell and not to last. They can make all those electronic features sound attractive, which is the bait. But the hook is that they don't last very long and are not repairable.

Get an old style dumb pump like a Goulds J series if you want something that will last 20-30 years. Then if you want it to act like a smart pump and deliver constant pressure, you know what I would suggest.

 

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[Alternety]

Valveman, I have no problem working with your expertise. I just don't want to evolve into a CSV discussion.

I have talked to my friend some more and a few details have manifested.

The pump stops for about 10 seconds when it is running to support a flow of water.
When running, the system pressure seems to be around 50 psi. That would be reasonable for house pressure. They would like a bit more pressure and stability. I don't think (from conversations) that they are getting sufficient flow. But it is currently a guess. The pump specs say it can provide a reasonable flow.

My preferred solution is the same as yours. Buy a pump and put in a nice tank and pressure switch. I am trying to avoid his cost of that approach. But remains my favorite.

Poking about online I have seen the insides of the MQ. It is relatively simple to replace the internal pressure switch with an external one. The internal switch can be adjusted by taking it out and putting a small wrench in the butt end. It is not calibrated. One of the things that has confused me is the lack of anything I have found that discusses setting this pressure in the MQ. A 50 psi system from a 25 psi source (not knowing what source pressure is under load). The plumbing around the pump has not left a lot of opportunities to test things. So, except for the 10 second cycling, the pump output might be OK. There is a small external storage tank attached to the pump output (couple of gallons).

A couple of people have fiddled with the system. Adding/removing what are either back flow preventers or a pressure reducer and adding the small tank. I need more detailed info on what the things are.

The flow sensor has me concerned. I don't see an obvious way to deactivate it and I don't want it fooling around with sizable external tank. My thinking is that with the external tank and a pressure switch (either a new external or the internal one), and a backflow preventer between the pump and tank; it may work.

With the new tank full, pressure switch off, the flow switch sees no flow. Fine. Nothing is going on. As the tank runs down to the low pressure set point, the flow switch is still seeing no flow. Fine. Nothing to see here.

Now the tricky part I am not sure of. When the tank pressure trips the pressure switch, the pump should turn on and there will be immediate flow. But my understanding of the pump behavior is that there needs to be flow to be allowed to start. I believe it is flow driven to start and pressure driven to stop. This is a part where input would be gratefully accepted. The static pressure on the source will not be enough supply flow into the tank to start the pump when the external pressure switch goes to say 40 psi. My conclusion is that this would not work. If the flow switch allows delay time to start a flow with the pressure switch and then watches until flow stops; it may work.

Valveman, your suggestion of the external pressure switch going directly to the motor is a good one. I am concerned about the potential issues with the motor. I believe it is a variable speed pump. Which makes sense in the application it is designed for. If the motor is a regular old 120 VAC one, it is good. If the motor is a multiphase motor specific to variable frequency drives; no joy. I have found no information on the motor.

I am trying to help the guy out and avoid a big bill. That 10 seconds is a concern. Could be an issue with either the internal or external small tanks. Trashing the whole thing and putting in a regular old pump and it's accouterments would get fairly expensive. But stable and long lived.

 

[Valveman]

The MQ has a water cooled motor, but it is not variable speed. It just has a really flat curve. Meaning it can only build maybe 50 PSI total. You can add to that the incoming pressure, so if you have 20 coming in, that pump might be able to build to 70 on the output. I think you may have trouble wiring around the controller because of the start circuit. I don't know if it has a start cap and relay.

It has a low pressure and/or flow start. But it should only stop on loss of flow. This is why it has such a flat curve. Whatever pressure it can build will be seen on the output. It can't build enough pressure to reliably use a pressure switch for turning off. It just runs at the pumps deadhead pressure until the flow switch stops turning plus a certain amount of time delay. I think the 10 seconds it is running is just the delay time. The flow switch most likely isn't delivering a signal.

Either way it is a piece of junk. A cheap harbor freight pump with a pressure switch and little tank would be a more reliable source of water, and that is not saying much.

 

[Alternety]

I wrote a long post but it disappeared. New version.

Still trying to work out a reasonable solution. I have not been able to work on the system myself.

The input from the owner has been a bit less than needed.

Reported (latest version) with variable reliability, owner is going to try better measurements by dissembling some piping:

1. Pump starts on water demand flow in house.
   
2. Pump cycles on more or less 20 second intervals. No actual testing done.
   1. I don't think that should be happening.
3. Pressure gauge on source input (public water system) remains steady at about 25-30 psi when drawing water.
   1. That implies that the source can keep up with the flow as it currently exists.
4. System output pressure fluctuates over a significant range based on discernible effects on showers and other uses. I believe this is a pump/tank issue that does not maintain a reasonable range of pressure. It may be the design of the pump.
   
   1. I believe that at some situations the house pressure approaches 80 psi with lows in the 40 psi range (insert a grain of salt here).
5. Small external pressure tank was added to output of the pump (a few gallons) to try to mitigate pressure variation. Unsuccessful.
   1. No readings available on pressures in the built-in or external tanks.
   2. Owner not aware of the issues of air pressure in tanks vs system pressure.
   3. If I can get things set up, I will empty the system and check tank pressures.
6. My gut is telling me (but that area has failed me before) that the pump should be replaced by a simple pump, pressure switch, pressure tank.
   1. If the source can deal with the flow and a reasonable pressure tank, that fixes the issue. If the source flow is below expected requirements, I am thinking a second pressure tank in front of the supply pressure tank. Running at a higher pressure than the house feed, isolated by a check valve and pressure reducer, controlling the pump. This could allow the one tank to try to keep the second tank in the desired pressure range while accumulating water. This may be excessive. And a good chance of not being useful. Simply paralleling tanks would provide essentially the same performance in terms of stored water.

All of which is pretty much is what valveman said.

 

[Valveman]

That pump system has no control over the pressure. That pump will add 62 PSI to whatever pressure is coming in. So if you have 18-20 PSI coming in, you will have 80 going out of the pump. A second pressure tank is not going to help. You can put a pressure reducing valve on the discharge of the pump, which will keep the outlet pressure at a set amount, say 50 PSI. But it won't have any effect on the pump cycling on and off. The pressure will still drop until the pump starts, then it will shut off after the time delay of about 20 seconds. So the pressure will fluctuate from inlet pressure to the PRV set pressure every 20 seconds as the pump cycles.

Either the flow switch is not workking or the electric brain is bad, which is very common on these type pumps.