Pump/Filter Sizing Mytery

[kingbud]

We bought a place with a gunnite or concrete pool 20 x 40 x 12’ deep, knowing that we were going to have to work on it. 45,000 gallons. Call it Lake Woebegon. The 1.5 hp pump is shot, it cavitates, it leaks, it is loud. The DE filter is also shot – amazingly, it is a 36 square foot Sta-Rite unit which I discovered at startup had two grid ports permanently plugged, reducing capacity by something like 15%. When you pick the grid assembly up it tinkles like Christmas ornaments, there is so much broken plastic rattling around. No kidding. And the grid’s fabric is punctured in quite a few places. Another delightful surprise we found is that the main drain is plugged with screw-in plugs. To recapitulate: the pool in its present state is being serviced by a severely crippled filter, an oversized pump that is slamming it with air, and one 1-1/2” suction line coming from the skimmer. By the way, there are three return lines, two at each end opposite the skimmer (which is located near the corner of the deep end), and one on the same side as the skimmer in the shallow end.

Those who have read my other recent posts might recall my tale of a massive algae bloom, which with patience and a lot of chlorine we managed to kill off. Well, here’s the kicker: the existing filter/motor setup cleared up the water after the bloom, if not overnight then over two nights. The water is crystal clear.

What am I not getting here? A quick glance at filter sizing charts recommends 72 square feet, a 2 hp motor, and three 2” suctions for 45K gallons. How is my crazy system doing such a good job of clearing the water? Is this a testament to the intrinsic superiority of the DE filtering concept?

Another question: assuming that I can get the main drain opened up and repair or replace the broken sections of plastic pipe, thus giving us two 1-1/2” suction lines, then what minimum filter/motor combination can I “reasonably” get by with when I go to buy new equipment? (I must confess, I am squeamish about punching holes in the pool walls to add suction lines.) Incidentally, I am seriously thinking about converting to a vinyl liner – one big advantage (I assume) would be that I can fill the bottom 4 feet of the deep end with sand and shrink the pool volume to roughly 36K gallons.

Talk about your fun in the sun!!

Let me say this, I am grateful for having the support of the many kind and knowledgeable people that participate in this forum. Where would we be without the internet?

[Vinyl_Guy]

Well you've got your hands full..

How much do you run your system? I've seen inadequate pumps and filters keep pools crystal, but they ran up quite the electric bill. .

I know I'm not much help in this area, but I bet some one here will crunch the numbers. With a single 1-1/2" suction line a 2hp pump is a waste, but if your planning on doing some major overhauls then you'll have to plan for that. 72sqft sounds like overkill to me, a nice Nautilus at 48 should do the trick, don't you think? Or one of those new amazing cartridge filters. I was putting a lot of them in towards the end of my pool days (4 years ago). I didn't do much spec on the pools I built the salesmen and owners always did that. I just built the darn things.

Here's a thought though. Have you considered just converting one of the 3 returns to a "side wall" suction? Rather the trying to fix the main drain? Which tends to be costly and sometimes impossible. Or coring through the wall (not fun either). If you had 1 suction and 2 returns it's just robbing Peter to pay Paul. But with 1 suction and 3 returns, the numbers might work a lot more efficiently with a set up of 2 and 2. And the line already exists.

Is your pool free form, or is it fairly symmetrical with some good straight lines and well defined corners and angles?

I've done a couple vinyl conversions though. They're not fun and if the pool isn't totally uniform I suggest having the liner company measure it also. It costs extra but sometimes they will guarantee the fit when they do. I did a concrete wedge pool conversion once. Shear walls from top to bottom (deep end too). It looked like a car ramp turned upside down. The steps were free form and I had to supply cardboard templates of the treads (tops). I also had to take plumb bob measurements to make sure areas of the bottom didn't bulge too much. It was 8hrs just to measure the damn thing.Then tapconning the extrusion into the wall, was a ball breaker. Needed pipe staging for the deep end. Fit beautiful and the owner loved it. I believe we spray tacked foam on 100% of the pool, if I remember correctly. The liner company couldn't or wouldn't guarantee the liner with out some buffer in between , because of the roughness of the concrete. The only other option would have been to diamond grind the whole bottom and walls.

Just tossing some thoughts out there.

[kingbud]

hmmm... well first, thanks for weighing in. (They don't offer a Smiley icon with a furrowed brow, do they?)

Yeah, I think 48 should be plenty with the properly sized pump. I hadn't thought about cartridge filters, I will check into that. I have had tunnel vision about DE since that is what I had in my old place, and it did such a good job... I have since learned that my old system was oversized for the pool, I guess that had a lot to do with it. I'm gonna investigate sand as well, since I've learned in this forum that you can enhance with DE.

I never thought about converting a jet to a suction, I think that is a great idea! Thanks! I think the one diagonally across from the skimmer would give the best circulation.

On the subject of vinyl... you gave me a lot of food for thought regarding fit that I hadn't recognized. I had realized how much work would be involved to tapcon or otherwise anchor some kind of rail or hanger for the liner - scaffolding or some kind of bosun chair would definitely be necessary - but there are other problems. Even though the pool is rectangular, there are steps to contend with; there is also about a 8 - 10" shelf that runs all around the pool at the elevation of the shallow end, or 42"+/- below the deck. It is kind of a cool detail, but now I don't know how you could put in a liner that would accomodate that, unless the liner terminated where the bottom starts its slope toward the deep end, leaving the shallow end un-lined... maybe that would work. Another consideration is that the walls have a slight taper going down, maybe 3 or 4 inches in the deep end.

Of course if I did manage to deal with all these issues, then in my scheme to "shrink" the pool by partially filling the deep end I would have to figure out a way to extend the main drain up to the new floor, assuming I could get it opened up and working (one advantage of my day gig is I have free access to master plumbers... )

sigh... I guess I'll just have to deal with the water bill and go ahead and fix the cracks in the concrete - I think I can do that myself. I would grind them out to a uniform width and then -- caulk or grout them back. Would you mind giving your opinion on which method would be better?

Hey man, thanks a lot for your time!


PS: What I meant by "shelf" was that I have vertical side wall 42" tall, then the pool wall cuts in 10 - 12", and then it drops, slightly tapered, to the bottom. So the measurement at the top would be 20' across, and then 42" down, it would measure a little over 18' across. Incidentally the top side walls appear to be some kind of plastic or fiberglass panel with caulked (leaking) seams, and lovely blue patterned glazed tiles topping it off, with a central 4" main tile and 2" border tiles top and bottom. It's really pretty, although it's missing one section and it looks like somebody with a tooth knocked out... I don't know how the top side panels mount or tie in on their bottom edge.... yet

[Vinyl_Guy]

No problem KB,

I know how you feel, it's hard to translate this type of thing into words that make sense.

The "shelf" is generically called a safety ledge. At some point designers had an idea that if some one fell in the deep end (that couldn't swim), the ledge would give them the ability to "stand", and orientate themselves.

What happened more often was, adventurous children used it for venturing out from the shallow end, using it like a sort of cat walk. They ended up being more trouble then they were worth.

Vinyl comes in rolls that contain a wall pattern, tile at the top, and a pattern below it, in several common wall heights. 42", 40" and so on. What happens when there is a safety ledge (shelf), is the additional volume of the the triangle that makes up this area is added to the calculation of what the deep end would be with out it. They figure out the bottom as if it wasn't there, then add extra liner, minus a bit for a tight fit. That's the simple version of how they do it. The ever decreasing deep end angles is all done on CAD machines, and some how they add the volume of the this shelf into the those numbers. The CAD program spits out a sort of parts list of triangles and other shapes. It's all cut out of vinyl of the the "floor pattern" you've chosen and "seamed" together. It's actually more like a triple stitched weld since the vinyl actually melts a bit. It's done with low intensity lasers.

As far as the main drain goes. If (and that's a big if), it can be repaired, theoretically hammering it out to the point where you can attach a pipe to it and extend it through the added sand is not an impossibility. I've done a few like that, but it was during builds in which the original dig was too far into a water table and the pool had to be "shortened". Also this entire process would have to be done before the liner is measured by any outside parties. Allowing for the variations in how it might be done would be far too complicated.

The liner can't really stop at the deep end or terminate anywhere underneath the water for that matter. Unless something has been specifically designed to receive it that way. At one point in history a pool was created opposite of how you describe. With a liner in the deep and shallow ends but terminating about 6" up the wall. The wall was designed with a water proof "track" to receive a normal liner in it. The theory being that most liners fail above the waterline due to UV and chemicals accumulating on the surface of the water. I believe the company was named Polynesian or something to that effect. They were a miserable failure and plagued with problems. It was a short lived design. I've only seen two, one is still hanging in there, as I said plagued with problems. The other was torn out completely and the hole was reused for a new pool.

Liners were intended to "line" the pool's entire volume of water, that's how they work best, IMO anyway .

A liner can be measured to fit the "shelf" and your steps, and just about anything. How well, and with how few or no wrinkles is the question. A liner company might guarantee the fit if they themselves measure it, but it costs to have a team of them come onto the site and do that. They might not even guarantee it if the pool is far too irregular.

Tapconning or whatever method you use to attach the extrusion (track), is another story. With a rigid metal track there's more strength to it, and they require less fasteners. With something more durable with the water a plastic for example, they need them every 6". The track is usually the same as any other one for a vinyl liner. But it's only the slot itself and no coping or decorative edging to finish it off, or receive a concrete deck. It's basically a rail with the extrusion as part of it. Except it's mounted vertically (the slot) and the liner goes up and over the forward edge of the rail, and then down and into the "slot" itself. The few I've done were "spec'd" out this way by the vinyl people. I think it allows for the rail itself to be more narrow as it now would not have to accommodate the size of the liner bead horizontally. A wider or thicker rail with the liner bead inserted horizontally would have a lot of down force or pull on it. In close and tight to the wall or "thin" is better when exterting downward forces.

Whether the pool is gunite or concrete makes a difference too. Gunite is not really all that substantial of an material to drill into. Since it has no aggregate (stone in the case of concrete), it's more like mortar or grout when drilling into it. Gunite tends to deteriorate from drilling and holds fasterns not nearly as well as concrete. Plus if it's gunite there's a layer of plaster that dulls drill heads quickly since most pool plaster contains a an element of marble dust as part of its make up.

I've not done a lot of gunite or concrete work and sealing anything like cracks in either of these materials is a whole nother discipline. The times I have done it, a "V" groove is carved out of the crack itself to widen it. Then a water proofing like mortar is added. Thorite or some kind of water plug. Caulking below the water is rarely used since it always remains "gummy" after water is added. I suppose a two part deck sealing epoxy might do the job, but I don't know about it be submersed for long periods. And if the cracks are on anything other then a flat surface deck sealing products don't work well, they are very "runny". Unless your referring to cracks in the deck, then disregard this last part .

If any of this doesn't' make sense.......I'm not surprised. Something always gets lost in the translation when things get this complicated.

[kingbud]

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wow, dude, you are really generous with your time!! thanks. I'll be back later... processing... processing... processing...

[Vinyl_Guy]

Thanks

My time is sort of relative at the moment.

I'm unemployed (as of now) and I type like the wind.

It's good for me to stay busy and feel useful .

[kingbud]

Hey VG- just a bit of an update. Good news, or at least an advance, concerning structural issues: I found the outfit that built my pool 30+ years ago, and I've got an appointment for a walk-around and consult with them, although they can't get to me before next week. They remembered the pool, a little vaguely to be sure, but they remembered. At least found out the pool is definitely gunite, since the name of their company is "Black Gunite Corp.", and they have been doing pools for a long time. So I'll be able to pick their brains about repairing cracks and caulking the fiberglass side walls.

One bummer, I was hoping they had built in sub-drains. Alas no, that was not SOP unless they hit a spring when they were digging the hole. So that has an important impact on another one of my questions: The South (long) side of the pool is built up against about a 25 - 30 degree hillside, and I'm quite nervous about draining the pool unless I'm pretty certain that the wall is not going to collapse. We've had a rainy year and had 4" last Friday... I've been thinking about putting a drainage tile along the length, if there would be any good in it. Of course that would only take away the runoff. I might not get to do any work IN the pool til next year, and that would suck. Can't investigate the main drain, for example, or do any masonry work. So. By the way, that was a swell idea about changing one of my returns into a suction!! That one is on the books for sure.

Take it easy chief. Any further thoughts will be greatly appreciated.

[Vinyl_Guy]

That's actually a heck of a stroke of good luck the installer is still around.

Let me get the picture straight in my head. The vertical side walls, meaning anything above the shallow end bottom , the deep end bottom, and any "angled" areas is actually a fiberglass? Then it transitions to gunite for the other areas I've mentioned?

If so, it sounds like hybrid I've never seen, having focused most of my pool career on vinyl. Most gunite pools I've seen are entirely gunite. Actually it's plaster that is the last layer giving it the white appearance you see. The gunite is sprayed onto a metal framework then the plaster is applied to that. But generically they are refered to as a Gunite Pool.

Because they're called Black Gunite doesn't automatically mean your pool is a gunite pool. It could very well be concrete, but I highly doubt that at this point.

The "sub drain" is something that's only installed when ground water exists, they are correct about that. It's an additional cost that would only be done, if the home owner requested it. For instance if they knew there was an extreme between the dry season and the wet season of the water table.

Just how much water does the pool lose? They have automated filling systems that are quite simple. A garden hose, hooked up to a float type system. When the water drops to a certain level the a valve is open and water is let into the pool. Sort of like the way a typical toilet tank is filled. It whacks out your chemicals to a certain extent, but that can be compensated for. Also they're good through the winter assuring the pool doesn't empty it self. Could be a hit on your water bill though.

Your "hill" is a tough call without seeing it, but it's obviously a potential drainage problem. My only thought is if the deck is shedding the water properly now a drainage line at the perimeter might actually end up putting more water behind that "long" wall. If anything make sure the drainage "kicks" the water away from either side of the pool (if possible). Get it out and away, rather then just off the deck and straight down underneath it. A "pipe" that's only preforated on the top and not the bottom and pitched properly.

In your first post you said the maindrain seemed sealed with a "screw in" plug. If your mean a threaded plug and not the black rubber "stopper" type then there's a good chance it was permanently sealed at some point for a reason. If the plug is properly glued in a pressure test might be able to be performed on the line. This involves cutting it away from the system (assuming it's still plumped into the suction end of your pump . A pressure test kit essentially is whatever can be rigged up to keep a garden hose sealed to the pipe (jacketed clamp usually) a cut of off or pet cock, and a pressure gauge. The pool is filled with water, and then pressure is applied with the PSI of your hose (8-12 pounds should do it), then the pet cock is closed. The gauge is after the pet cock, so it will register the pressure you've applied. If the gauge falls after a few minutes, then there's a leak, if you can't get pressure into the line, then there's a huge leak. A pressure test isn't to see how much pressure a line can take, but only if it can hold pressure. The problem with doing this on a main drain with an existing plug, is it's not known if the plug is leaking (when a small leak does occur). When this happens you have to dive with a bottle of "dye" to the main drain. Wait for the waters to calm and then squirt the dye near the plug to see if water is escaping from the plug. All the while someone has to be the top side making sure that a constant pressure is being put into the pipe to replace the escaping water, sort of regulating it.

You could do a dive, remove the plug and "see what happens". If the pump draws air when it previously didn't then there's a smaller sized leak in it. If it won't prime at all then there's a large leak. This is assuming the line is still plumbed into the suction end of the pump. If the line isn't plumbed into the main drain then you can still "see what happens". Does the pool lose more water then before the taking the plug out? Same? Less? (doubtful).

There are options even when the pool has water in it. They just take more experience, an ability to hold your breath, or scuba equipment and certification.

As far as the "cracks" go, if they are deep enough so as to make it through the plaster (assuming it is a gunite with plaster pool), any failure in a fix will result in the pool leaking again. The gunite itself is not watertight, it's the plaster that's the final and watertight seal. Are the cracks "hair line" meaning very very narrow like a hair on a white sink? Or are they cracks one could slide a piece of paper or cardboard into (1/8th to 1/4 inch)? Or larger ?Hairlines don't always mean water is escaping from the pool. Although the gunite is not totally "water tight" is pretty darn close and will retain some water that makes it through, just not large amounts.

The next question would be in a trouble shoot for a leak is: Does the pool lose more water when running? Or does it lose no water when not running? Losing more water when running vs. sitting idle would point to a two fold problem. One in the shell and one in the system. A return line (water going back to the pool) leak will not manifest itself in any kind of pump or filter problem (not priming, air in the lines etc etc). But the pool will lose water when the pump is running because the line is under a certain amount of pressure. With out that pressure the leak might not be large enough for water to escape. If it only loses water when the pump is on, then I would highly suspect a return line leak.

If this is the case then eliminating (at the pump) each line individually and either pressure testing them, or running the pool to see what happens. Just plugging them at the wall of the pool would only worse the problem because it would apply more pressure to an already leaking line. If you have a way of regulating the return line with valves at the system. Try running the pool off of each line indivdually and monitoring the water loss. More? Less? The same?, with each individual return line.

You could probably take a few of these basic and relativity easy (other then the dive and pressure test), methods in the remaining days of having the pool running, and possibly narrow done your areas of concern for next season.

[kingbud]

"possibly narrow down your areas of concern for next season."

Thass what I'm talkin' bout! This whole morass of ignorance and anxiety about figuring out exactly what's "out there".... unfortunately it's not swimming pool hypochondria... actually, more like a bad dream. But you are being a really big help. (btw, I'm going to try to get a couple of snapshots for you to look at.)

"Fiberglass side walls" was a bit misleading. What these are are about 10' long sections that go from the elevation of the shallow end up to the tile, about 4" below the water line, and they are the same height all around pool: in the deepend and the slope they go to the top of the "safety ledge" we have already discussed. The safety ledge itself is masonry of some kind, as well as all the remaining surfaces. The vertical joints in my pool have been messily, and without doubt ineffectively, caulked by DIY people who had the pool for around 5 years before me, or even before that.

This leads me to the question of how much water am I losing. For an illustration of rate of flow, the pool was filled by recent heavy rain (Thursday/Friday - 4") to a mark I had previously made on the side wall. Convenient, yes? Today is Tuesday and at noon the water was down 2-1/2 - 3" from that mark, no equipment running...

I have a hypothesis that at the lowest the water will only drop to the level of the shallow end, which was dry when we first looked at the pool - the rest of it was full. The pool had not been touched in probably 2 years. We walked in the shallow end and launched massive chlorine bombs from the water's edge, which was actually where the drop to the deep end took off.

So my suspicion is those vertical seams, although as I mentioned I don't have a clue how they are anchored to the frame or whatever, i.e. some other unseen defect might be possible. I will find out about that from the installer. You'll see if I can get you a pic, they are a pathetic, ugly mess. (I know a little bit about caulking since I ran a good sized waterproofing job once).

The hill: I'm pretty sure water doesn't run into the pool. But dig this: This pool is 40' long. There are two serious cracks in one deck - the "hill" side - that both run on a slight diagonal outward, from the pool rim to the edge of the deck: imagine a shape with one side 18' long, the other side 20' long, with symmetrical ends angling out from the short side to the long side (the "hill" side.) They both start about 11' from the ends, measuring to the "short" side if you follow me. And the concrete between these cracks is dropped about 3/4", maybe less- I haven't actually measured any of this precisely.

So basically I have one solid section of deck dropped uniformly from these two cracks, well over 100 square feet. My first thought was that there was already a drainage system in place that had failed, but probing with a rod out to about 3' past the deck (up the hill) I didn't find any evidence, like filter fabric, gravel, solid pipe. I'm counting heavily on my meeting with the installer to shed light on this and help me plan a solution. I've thought about mud jacking, although I'm not very familiar with that... and of course the rehabilitation of the deck is not as important as figuring out the reason for the failure and fixing it.

My idea for a lateral drain tile would be to install it about 3 feet uphill from the deck, except to "peak" it in the center a couple of feet so I get my fall naturally from the hill, without having to grade my pipe - like a peaked roof, falling gradually from the center to both ends. Trouble is, that's great for future rainfall, but what about saturation?

let's take a short break........................... ah, there!

Still with me? Concerning the main drain, assuming I can prove the integrity of my return pipes, your idea about converting one of them into a suction makes any worry about the main drain pointless.

Cracks appear to be from about an 1/8th anywhere up to 3/8", maybe even 1/2", but I don't think that bad. I have two cracks in the bottom I have spotted that run the whole width of the pool (expansion joints?), plus a few other, lesser faults. You aren't going to believe this, but there was one that had a weed or a root hanging out of it. My wife thinks that the water loss started after she pulled it out, like pulling Johnny's finger out of the dike. Who knows? But I'm not that worried about my cracks, I feel that if I can just have a dry pool and decent weather to work in I can get them cut out, cleaned out, and patched. Thorite is most likely the right stuff, although I've also heard of "hydraulic cement" -- possibly just a generic description of the same stuff.

Testing procedure is straightforward and doable, although I have no idea if the pool loses more when the pump is running. I do know a few things however: the pump leaks when operating. The chlorinator leaks. The filter leaks/sucks air from the vent cock -- I should have fixed that already, but to what end?

On the subject of prime, well, you can't get the suction pot to hold water. You fill it up and it drains back to the level of the main suction pipe, which has a little cleanout plug you can stick a hose into, but you can't fill the line. I've got my shovels honed and oiled... Owners before me "replaced" all the pipes because they were "old". I'm not too worried about finding and repairing the broken pipe on the suction side, if not myself I have plumbers I can commandeer, good tough men. I will do all the excavation myself. Finally, concerning the returns, I have slightly fizzy, bubbly flow from the jets. Hopefully they are all okay because they are certainly deep.

Kah Flooey!!

Like I said, we knew we (I) were getting into a bit of work. But we got the house very right - national housing crisis, you know - and our deal included a certain amount of "warranty" on the pump and filter - not enough to completely cover, but it takes a bit of the sting out of it.

Thank you, muh bruh-thuh!!

Edit by Watermom: Kingbud -- a family-friendly forum, please. I deleted the questionable references to the previous owners.

[Vinyl_Guy]

We need a picture.

The higher the resolution and more detailed the better. If you are not able to upload it to a location, I can provide one for you if need be.

I'm thinking this is not a standard gunite pool, and if it is, it's just been through so much that it doesn't resemble one anymore.

I have a hypothesis that at the lowest the water will only drop to the level of the shallow end, which was dry when we first looked at the pool - the rest of it was full. The pool had not been touched in probably 2 years. We walked in the shallow end and launched massive chlorine bombs from the water's edge, which was actually where the drop to the deep end took off.

That's very telling. Sounds like this fiberglass type thing I've yet to wrap my head around, could be the cause of a majority of the problems, when talking water loss.

Cracks appear to be from about an 1/8th anywhere up to 3/8", maybe even 1/2", but I don't think that bad. I have two cracks in the bottom I have spotted that run the whole width of the pool (expansion joints?), plus a few other, lesser faults.

Purposeful "expansion" type joints would only occur in a concrete pool and not a plastered gunite pool (maybe rarely).

Is the pool "painted" that you can tell? That's a dead give away of "concrete construction", of some type. Or is the "bottom" (shallow end floor,drop off, and deep end), a white (possible not pure white) smooth substance?


Your idea for the landscaping drains sounds about right but remember there's also a large volume of water that is shed from that section of deck too. Getting at the water as it comes down your hill is good, but the water that falls onto the deck and then is shed back to that area can cause just as many problems behind the walls. Although it's probably the much lesser amount of water compared to the hill it could still be enough to cause problems.

But dig this: This pool is 40' long. There are two serious cracks in one deck - the "hill" side - that both run on a slight diagonal outward, from the pool rim to the edge of the deck: imagine a shape with one side 18' long, the other side 20' long, with symmetrical ends angling out from the short side to the long side (the "hill" side.) They both start about 11' from the ends, measuring to the "short" side if you follow me. And the concrete between these cracks is dropped about 3/4", maybe less- I haven't actually measured any of this precisely.

Are these diagonal lines/cracks perfectly straight? If so the deck has dropped right at the point it was predicted it would happen after the pouring of the deck. This would have been a different kind of "expansion joint", a relief joint. It's a weakened indentation, cut, or "score" in the concrete put there to make any pressure that will eventually crack the deck occur in a uniform manner. Think of the sections of concrete side walk. Even though the lines in them might not extend all the way through the concrete that is where they will most likely crack when "heaving" happens. If they (the diaganol lines) are not perfectly straight then they probably left them out. Which is right were they are normally put (diagonally at the corners), and periodically in long stretches too.

Finally, concerning the returns, I have slightly fizzy, bubbly flow from the jets. Hopefully they are all okay because they are certainly deep.

That's most likely not a problem with the return line. Rarely will water being pushed through a return line, pick up air from that line and dispense it into the pool. A return line is more likely to push the water out of any "leak" while running. Air at some point is being introduced into the loop and thus being fed into the pool. Air most likely can't be introduced into the system with out it being "sucked" in. So water leaking from your filters, pumps or other systems topside are probably not the source (believe it or not), those are places water is being "pushed" out. Valves, threaded fittings, loose clamps form the suction of the pump, on out, is probably the source of the bubbles.

If you can see the bubbles in a your pump "chamber", try this: Get a watering can or garden hose that you can regulate. Start where the suction lines come up out of the ground and trickle (pour till the fitting has water all around it, and continue to do this) water on any fitting (elbow, coupling, bushing etc), and valve. See if the bubbles go away or are reduced as you trickle the water all around these "connections". What happens is, the constant flow of water effectively seals the air leak temporarily. It might be in several places, so it make take trickling water on two different ones. Work your way back to the where the suction lines ends up at the pump. Even this fitting might be the culprit. Or any lid, threaded plug or other thing in the pump body itself.

Like I said though at this point I think we need a picture. I still can't picture (no pun intended) this fiberglass type section your describing. I think I understand "where" it is, but why it's there and what its function is, I have no idea. Could it possibly be a section where the plaster/gunite/ or concrete has actually completely failed and dropped away?

If you need help figuring out how to get a picture here (on the forum), or to me, we can work it out. Emailing me is activated in this forum as well as several Instant Messenger services I use.