Low pressure after moving filter

[EricD]

A few updates:
I took the pump apart. The impeller was clean. As I reassembled everything, I lubed all the O-rings and replaced the Teflon tape on the threaded connections. Started the pump, and it ran great... for a few hours.
Then I opened the pump cover to clean the basket and started vacuuming the pool. I noticed the vacuum was a little weak, disconnected it, and noticed the pressure was low again.
So I moved to Plan B and lowered the pump about 8". It didn't change a thing.
Finally, i broke down and moved to plan C. I dug a new trench following a straighter path, and put in 2" pipes. Two days of work and, unfortunately, no improvement.

So I ruled out obstructions in the line. The planets aligned twice and it ran perfectly, but I cant' figure out what's wrong when it's not working.

Here's my setup (pending photos):
Skimmer > 3' flex 1.5" tube > 35' underground 2" Pipe > 3' flex 1.5" tube > Pump + Filter > 3' flex 1.5" tube > 35' underground 2" Pipe > 3' flex 1.5" tube > Return Jet.

TIA, -- Eric

[Poconos]

I'm a little confused. You say the planets aligned twice. Are you saying the pressure and flow were fine twice, and then the pressure and flow dropped? If so then that tells me the system is capable of working fine and something is changing dynamically. What are the pressure readings working and not working if you have them? Actual numbers will help determine the amount of change. Does shutting the pump off for a few minutes, then back on cause it to repeat the working, not working cycle? Are there any air bubbles visible at the returns or in the pump basket chamber? I'm kind of thinking small suction side air leak. With that much of a suction side length the suction vacuum at the pump will be higher than normal so maybe this is causing something funny. It's puzzling that you said it ran for a few hours then went south. I don't know what it would show but can you try running without the cartridges just to see what happens? There is an explanation for everything and just a matter of digging to find it.
Al

[EricD]

Last summer, when the filter was about 10-12' from the pool, the pressure was consistently 10psi, the pump basket was full of water and the return jet pushed my hand away.

The 2 times it worked properly, I had the same results except a slightly higher pressure (11-12psi).

When it's "not working", the pressure is around 15psi, the pump basket is 3/4 full of water, the water flow fills about 3/4 of the inlet pipe, fairly consistent, and the return jet has some flow but I can easily block it with my hand.

Both time it worked were after a full, thorough cleaning. I forget the details of the first time, but the second time it ran well, I stopped the pump, cleaned out the pump basket, and hooked the vacuum. Then I wasn't able to get it working well afterwards.

I can stop the pump for hours and restart it with no problem. There does not appear to be any air leaks when the system is off. Of course, when the pump is on the dynamics change and it could be pumping in some air. I'll test it again without the cartridge and report back.

Eric

[mas985]

First, your pump is a AG pump which requires a flooded suction. In other words, the pump must be below water level otherwise it may not prme properly and air will remain in the pump basket affecting flow rates. If the pump is below water level, it should prevent most of air problems although if there is still too much suction head, the pump could continue to draw in some air. You mentioned lowering the pump 8" but if the pump is 18" above water level, you need to drop it 20" or so. 18" lift is not that much and probably explains why it is working somewhat but the performance of the pump would most likely suffer.

Second, most of the head loss in a plumbing system is in the equipment and the fittings and the straight pieces of pipe add the least amount of head loss unless they are really long. So just changing the pipe won't yield all that much improvement.

[EricD]

mas985 and Poconos, you've both helped me identify the problem.

I removed the cartridge (which was dirtier than I expected — pollen) and ran the pump without the cartridge. It ran beautifully. Tank pressure around 8-9psi, no air in pump basket, very strong return jet.

I cleaned the cartridge and put it back in. The pump primed quickly, no air in pump basket, tank pressure around 14-15psi and noticeable drop in return jet. I ran it for a few hours.

I added the vacuum (Lil Shark). Not enough pressure for it to move on its own. Barely enough to slowly vacuum manually.

I'm upset as the salesman, as I explained what I was planning and he said the pump would work fine as long as it was primed. It worked last year 10' away with 8" lift, but struggles 30' away with 10" lift. And a little dirt on the filter is enough to stop it. I'll see if they accept a trade-in for an inground pump. What pump do you recommend for this pool (24' round, approx 14,000 gallons)?

BTW, I haven't been able to find an explanation on the mechanical difference between IG and AG pumps.

[mas985]

The primary difference between an AG and IG pump is that AG pumps requires a flooded suction as I mentioned before. This is because the design of the wet end of the pump has a higher NPSHr (net positive suction head required) than IG pumps and will cavitate easier than IG pumps if there is not enough NPSHa (net positive suction head available). You can learn more about this here.

So when you moved the pump away from the pool, NPSHa decrease and could have gotten lower than NPSHr which would start to impact pump performance and if it had gotten low enough it would cause cavitation although I don't think that is the problem here.

Another difference between AG and IG pumps is that most AG pumps have a lower maximum head (i.e. smaller impeller diameter) so they don't work well for long plumbing runs or for solar on roof tops.

For all the above reasons, AG pumps can be manufactured cheaper than IG pumps but because of that have more restrictions on the operation such as flooded suction and low head loss plumbing.

[EricD]

The discussion has moved on to this thread. I'm writing the results here for future visitors with the same problem.

The consensus is that a small inground pump would have been a better fit for this setup, but the original pump works fine. The real problem was a clogged filter cartridge that clogged very fast with pollen.