Low speed bubbling issue

[LPCPool]

I have the timer set up on my 2 speed pump so that it only runs at high speed for about 8 hours. The rest of the time it's on the low speed setting. I've noticed that after it kicks down to low, the first inlet in line (there are 3 main and 1 for the aux pump) starts to bubble all the time. There are no bubbles showing when at high speed.

The bubbling doesn't start right away. It usually takes about 5 to 10 minutes. Since I have a SWG, I would expect it to do some bubbling whenever it cycles on. In my case, the bubbles don't stop unless I bleed air using the valve on top of DE tank.

Once I bleed the air, it will bubble when the SWG is active. After about 10 minutes or so, it'll start to bubble continuously again regardless of the SWG until I bleed the valve again.

I have 2 skimmers and a main drain. I've tried to play with the directional valve settings to see if I got any different results, but it all seems to end up with the same thing.

I've had someone check the system for leaks. He pressure tested the lines and didn't find any leaks there. He tightened down and resealed the fittings at the directional valves, pot, impeller and tank. He also replaced the pot cap o-ring since I had noticed that when it kicked down to low it would spit out a little bit of water from the top of the pot. All of this to no avail. Whenever it goes to low speed, I get a small bubble that forms in the pot and stays there (the water in pot lowers about an inch) and, after a few minutes, I get the bubbles coming out of the first inlet.

The person that did the work is blaming it on the fact that the pool equipment is higher than the pool (about 6-7 feet following the slope of the yard) and he claims that no matter what I do, I'm always going to have the low speed bubbling issue since it's got to work that much harder to pull the water uphill and can't prevent cavitation (almost his exact words).

Is what he's telling me in fact true? Since it helps to bleed the air out form the DE tank, is there some way to automate that?

I don't really want to go back to having the pump on high all the time and I'd rather have it on low than completely turn it off.

Thanks in advance for any feedback.

 

[mas985]

The person that did the work is blaming it on the fact that the pool equipment is higher than the pool (about 6-7 feet following the slope of the yard) and he claims that no matter what I do, I'm always going to have the low speed bubbling issue since it's got to work that much harder to pull the water uphill and can't prevent cavitation (almost his exact words).

Pull water? Really? Next time ask him if he ever tried to pull water. Sorry I get a kick out of comments like that.

It is also untrue that the pump works harder because of the elevation. Once primed, the pump does not experience anything different than if it was the same elevation as the pool. There is a difference in priming though. But it would not be cavitation causing the bubbles. It is not possible for the pump to cavitate on low speed. Plus, when a pump cavitates, the bubbles collapse well before leaving the impeller so they would never come out of the returns.

Whenever it goes to low speed, I get a small bubble that forms in the pot and stays there (the water in pot lowers about an inch) and, after a few minutes, I get the bubbles coming out of the first inlet.

There is obviously an air leak on the suction side and it is passing to the filter. When you purge the air out of the filter, it stops because there is no more air to pass to the returns. It must collect in the filter before it can then come out of the returns, probably through the air bypass tube.

My guess is that it is the pump basket lid or the pump drain plugs. Have you tried any pool lube on both of those?

 

[LPCPool]

There is obviously an air leak on the suction side and it is passing to the filter. When you purge the air out of the filter, it stops because there is no more air to pass to the returns. It must collect in the filter before it can then come out of the returns, probably through the air bypass tube.

My guess is that it is the pump basket lid or the pump drain plugs. Have you tried any pool lube on both of those?

Thanks for the explanation and your thoughts on the source. Since I've been taking the "let the expert handle it" route, I haven't checked either of those myself, but I'll take a closer look now.

I have noticed that even though it's been a couple of days since he pulled everything apart, and there hasn't been much rain, there seems to be a damp spot directly under the impeller housing. I'm not sure if that's trickling down from somewhere else or if it's coming from the impeller itself. As I mentioned, he did replace the o-ring on the pump basket lid, so I don't think it's that unless the lid itself (or basket) is somehow damaged.

Since I'm relatively new to some of this and with the understanding that when it comes to fluid dynamics, it's all a foreign language to me, could you explain why it's not possible for cavitation to occur at low speeds? Reason I ask, is that, after watching (and listening to) the video you posted on the subject, I do seem to hear what sounds like cavitation - meaning that I hear what I would describe as a coffee grinder type of noise right at the impeller when it's in low speed that I don't hear when it's on high speed.

 

[mas985]

When water cavitates, it is actually boiling because the pressure is so low it lowers the boiling point of water to the temperature of water. But this pressure drop is fairly high (~ -14 PSI or 28" hg or 32' of head) but there is not enough suction generated at low speed to get to those low pressures. Even if you were to completely close off the suction and measure the pump suction on low speed, it would probably be less than half of what is required.

 

[LPCPool]

OK - thank you for the added explanation.

Since it looks like the source of the air leak wasn't properly identified by the person that came to do the work, it's back to square one. What I may do is bring someone else in to take a look at the set up. When I bought the house 2 years ago, I sorta inherited the current service and they're the ones I called on.

In going through that exercise, I may want to make some other changes with my set up. Based on what I've read in other posts. The existing check valve is too close to the pump, correct?

As I mentioned earlier, the equipment is elevated above the pool by at least 6 feet (he claims 10 but I think it's no more than 8). Whenever I clean out the basket, I have a real hard time priming the pump (even though it's self priming). I would guess that having the check valve where it's at really doesn't help at all and, if anything, it does more to introduce a potential air leak point. Would it make any difference if there was one at each rise from the intakes or even just one at rise or 90 from the main drain and close the valve for the skimmers? Any place else and it would involve digging grass and/or concrete. In the picture the skimmer lines are joined by the top Jandy valve.

 

[mas985]

The location of check valve is not ideal but it isn't a big problem either. However, the age of the check valve makes me think that could be a source of the air leaks but check the drain plugs first.

The pad height makes it that much harder to fully seal off air leaks because it causes higher suction at the pump.