Grundfos Scala2 and external pressure tank, short cycling

[Assquatch]

Hello,

I added a Scala2 and a moen Flo to my system. I had a highly elevated home (3-4 stories from main to living space). Once I added the pump, pressure was great, but I observed fluctuating and creeping up of pressure through the flo. I just added an expansion tank to the cold line after the pump and moen flo, but my pump is short cycling. It seems to pump to its upper limit, and then ride off the pressure accumulated in the tank, and then cycle back on when it reaches the lower limit. I understand there is a 10psi threshold that trigger it on and off.

I think I may have a problem with my tank pre-charges but I can't figure it out. Scala2 is configured to run at 2nd highest setting of 5 bar = 72.52 psi. I set the Scala2 interior tank to 49psi (70% of the outlet pressure, per the manual) I also set the external expansion tank to the same.

Please let me know if you can think of any reason for this, things to check etc.

 

[Valveman]

The air charge in the tank will not cause the pressure to bleed off. If you have a ball valve after the pressure tank, close it. If the pressure still bleeds down the check valve is not working. I don't see a check valve in the drawing? Counting on the check valve built into the Scala is just the first place it will give you trouble. A constant 72 PSI is great as long as the pump is working. But you can use a regular jet pump and get that strong constant pressure from a simple Cycle Stop Valve. It would cost less, last longer, give fewer or no problems. The Cycle Stop Valve is considered a disruptive product because it makes pumps last longer and eliminates the need for expensive and problematic variable speed type pumps.

 

[Sponsor]

 

[Assquatch]

To clarify, the original problem is that pressure was increasing to unsafe levels during non-use (100psi+), so I suspected thermal expansion (and the moen flo detected this and told me so). I do have a ball valve after the expansion tank. And yep there is a check valve in the pump which is why I needed the tank.

The bleeding off of pressure was just my observation as the pump would kick on, send high pressure for 10-20 seconds, then turn off, pressure would die down a bit (though still good, maybe only by 10psi less), and the cycle would repeat. This seems new after I added the tank, so to my eyes it seems like some imbalance between pump outlet pressure and tank pressure causing a swing between the pump, and the diaphragm doing the work.

I would consider a CSV if I can return the thing but I'd like to make it work as it worked fine without the tank in the picture just days ago.

 

[Assquatch]

I had a chance today to drain and triple-check my pre-charges were set to exactly the recommended pressure. Pump outlet pressure set and measured @ 72.5psi. Internal pressure tank, and external pressure tanks set to 70% of that = 50.75psi.

With 2 taps open wide, the pump clicks on for a mere 5-10 seconds, and then stays off for around 30, before cycling back on. My measured pressure seems to ping-pong from 60-70psi as the pump goes on and off... which is... fine for my use. Maybe it's unnecessarily stressing me out, but I don't like the sound of the pump turning on and off. I thought the speed was supposed to be variable and expected it to hold a steady pressure at whatever speed was needed. It clearly has speeds, as the pressure settings on the pump create audibly different frequencies. I just spoke with their distributor and he told me that this seems normal, but my understanding was that the pump should "slow down" to maintain a constant pressure.

It seemed to cycle a lot less before I introduced the external pressure tank. My theory is now that there is more "elasticity" in the system that allows pressure to be maintained within the 10psi target band of the pump for longer so it takes breaks. I imagine if I had much higher flow, or a lower set pressure, it might stay on longer or continuously. I might give a smaller pressure tank a try. I went with a larger Amtrol ST-12 thinking it would help maintain a bigger buffer of pressure and smooth out the pump engagement, but maybe it is taking a little too much work off the pump slowing the feedback loop that the pump depends on.

This video shows what I am experiencing, but I have longer cycles:

 

[Valveman]

I am so sorry for your problem. And sorry for the delayed response. Just slipped down the list on me. You are correct that a variable speed pump should slow down to supply the demand and never shut off as long as there is a demand. The little tank built into the pump is after the check valve that is also built into the pump. If expansion was what caused the pressure to spike above 100 PSI, it would show you the tank in the pump is so small it is useless. Expansion would also only cause spike pressure sometime after the pump shut off. It is also very common for the pressure transducer or VFD itself to malfunction, which will also cause the pressure to spike above 100 PSI. I have a video of this happening and will try to post it.

I cannot believe the distributor told you that is the way they are supposed to work. But I guess it is possible. That is also the way the Pentair PID system works. Long explanation but it has to do with the pump not knowing how slow it can go and when to shut off. So, they just bump the pressure and turn off the pump every few seconds. If the pressure drops, the pump knows you are still using water and comes back on. I guess that is one way to handle one of many very technical problems with VFD controls, but it doesn't go along with what they say the pump will do.

SCALA2 has integrated speed control which allows maintaining perfect pressure in the taps. I guess not?

Also, when you are not using any water the pump should stay off. It doesn't matter how much air is in what tank or how much elasticity the system has, the pressure will not drop unless water is leaking somewhere. If it is not leaking in the house plumbing somewhere, the check valve in the pump is leaking back.

If you want a tank to work like it is smaller, reduce the tank air charge. With 75 PSI air in the external tank, it just acts like a plug in the end of the line when there is less than 75 PSI pressure in the system. You are not putting a drop in the tank until it gets to 76 PSI. Same with the internal tank at 60 PSI. Below 60 is is doing nothing. I would try reducing the air charge in both tank to like 40 PSI, AND see if the pressure holds when no water is being used.

We replace a lot of those with Cycle Stop Valve systems. It is just a matter of how long a person wants to put up with the problems and how many times they can get them to shell out another 800 bucks before they try something different. The stuff in the drawing I posted earlier can be much less than 800 bucks, will last a long time, and does deliver strong constant pressure without the pump cycling.

Pressure gauge on the VFD system is at about 40 seconds.

 

[Assquatch]

Thanks for your comment. I think I have you confused. There is no problem with the system holding pressure while closed. No leaks or backflow from the pump. I meant that the pump stops while the tap is open, leading to a slight pressure decrease from say 70 to 60psi in 20 seconds before the pump decides to turn back on.

I thought the distributor was wrong too about the pump. I can def try playing with the air charge. I think there is something to this, because the troubleshooting guide notes

If there is an expansion tank or vessel on the water system it needs to be set to 70% of the working pressure of the pump. If this is not done, the pressure vessel can cause the SCALA to cycle on and off. Your plumber can check this.

 

[Assquatch]

What you say about the expansion tank not getting a drop until the pressure is above the air charge gets me thinking. Why am I charging the tank to 70% of the pump pressure? Shouldn't I charge it to 100%? This way the tank only takes "additional pressure" coming from expansion when the system is not in use.

From what I can see, my pressure tank is acting like a balloon that fills up with water til 70psi, then the pump shuts off, the balloon deflates pushes water until it reaches 60psi, at which point the pump turns back on.

Don't I want elasticity to handle my thermal expansion and nothing below that?

 

[Valveman]

It does work like a balloon. But you put 75 PSI air in it. So, it can only absorb anything above 75 PSI. Until the pressure gets up to 76 PSI, you might ss well have a plug in the pipe as the pressure tank. With only 40 PSI air the tank can cushion anything above 40 PSI. A water heater won't cause that much expansion. The high pressure is coming from the pump not shutting down fast enough and no tank for the extra water to go into.

Those pumps don't last long anyway. Cycling like that it will last even less time. If you can't get it to quit cycling, I would send it back for warranty and get something reliable.

 

[Assquatch]

This cycling problem only began when I added the pressure tank. Before that, I had no pressure tank, and no cycling. When I first added the pump, I believe the internal check valve must have closed the system from the city main and this is when I saw pressure creeping up toward 100psi during non-use, so I added the pressure tank. This was not due to the pump running above its configured pressure.

I think I do want the pressure tank to act like a plug under ~72psi (my set working pressure of the pump). The only need for the tank is to catch thermal expansion occurring when the pump is stopped, and taps and system are "closed" and static pressure is at the baseline 72 already. Tank should catch anything over that due to thermal and give me a buffer.

I am confused why the grundfos manual says charge it to 70% to avoid cycling when it seems logical to charge it to the system's configured pressure. It sounds like charging to 100% of system pressure gives me the plug behavior, which is essentially what I had when there was no tank on the system.

 

[Assquatch]

Ok I just drained the system and charged my external tank to 72psi to match the pump setting. That seemed to fix the problem. The pump is no longer cycling, and the variable speed aspect is now working. I can open more taps and the pump works harder, less taps and it quiets down.

I guess the scala2 instructions were just wrong about the 70% precharge in the external tank, and in fact this causes the cycling they were suggesting it corrects. Very strange.

I left the internal tank charged to 51psi, assuming at least that part of the guide was correct. Seems to work. Thanks for your help!

 

[Valveman]

The internal tank doesn't hold a teaspoon of water. Your idea of putting 72 PSI air in the external tank will make it work like the line is plugged. But should help of you really have that much thermal expansion. What the pressure tank is supposed to do is give a gallon or three to the taps or leaks before the pump comes on. With 72 PSI air charge the external tank cannot help with that. Problems occur when the check valve or any taps in the house start leaking. The pump will come on and off continuously until something fails. Like I said, those pumps are not designed to last very long. When you get tired of the problems and expense of replacing the Scala, the Cycle Stop Valve and a standard jet pump will last a lot longer, give better pressure, and so little maintenance you will forget you even have a pump.