Pump's running, but no water in return to pool

[jay11]

Inground 15k gallons pool.
Nautilus FNS 48 de filter, with fiber

The problem, what I have observed and what I have done so far....

Pool runs every night from 8pm to 5am. The other day I walked out at midnight to find the pump running but no water was returning to the pool-reading was close to 30psi. Next day I did a back flush, 3x, and it was running ok-psi was 15. What I noticed is that after about 3 hours of operation return flow quantity starts to tapper off, pressure starts to trend up. If I let it continue running by midnight or one the return is just trickling back into the pool, next to nothing really-psi is about 25-30ish.

Pool used to run great even with a filter much dirtier than I found it this past spring. It used to take about a year for psi to build up from 10 to say 35-37ish, then I would clean the filter. Sometimes this last spring I noticed the psi climbed up faster than in previous years so I cleaned it. That is why when I opened it again this week it was still clean.

I attempted to vacuum the bottom, with selector set to waste. I hose-fill, then vacuum as I have always done annually. This time suction pressure was gone after a short while. It came back after a backwash. Did the same thing so I stopped that. I estimate it would take 4 more of this action to complete the vacuuming process. It used to be one go and finished.

I tried to put a bladder on a hose through the skimmer but it couldn't make the 90 angle turn, to go through.

[Teapot]

Sounds like some type of blockage in the return circuit. What's it like in circulate only? Have you opened the multiport/backwash valve?

[jay11]

Sounds like some type of blockage in the return circuit. What's it like in circulate only? Have you opened the multiport/backwash valve?

When you say circulate you mean the circled in RED "Recirc" in my photo? No I haven't tried that. Will do that tonight and revert.
(BTW What does that do? As much/long as I have maintained my pool I have tried not to mess with various settings other than normal, backwash & waste only so as not to create an undue issue.)

Backwash - yes. Water comes out strong. If I do that at 8pm, by 12 am the return flow usually has weakened.

[Teapot]

Recirculating bypasses the filter, the water from the pump goes through the multiport and back to the pool unfiltered, often this setting is used to mix chemicals into the pool a bit faster. In your case it will determine if there is a blockage from the multiport to the pool. If everything is ok after the 4 hours you mention then the problem is in the filter ( probably)

[Denniswiseman]

When you pump to waste your pool water bypasses the filter and returns so there should be no restriction whatsoever
That "Recirc" is the recirculate but if pumping to waste is the same then that will be the same as well
When the flow slows right down is there air in the pump basket
Is the water 1/2 to 2/3 up the skimmer and check the weir door isn't stuck in the up position

[Teapot]

When you pump to waste your pool water bypasses the filter and returns so there should be no restriction whatsoever
That "Recirc" is the recirculate but if pumping to waste is the same then that will be the same as well

No necessarily, when you pump to waste the water is sent to waste, not return so less chance of a blockage but they do happen. It may not be the same on recirculate as that water takes a route via the normal pipework to the return fittings in the pool so any blockage there or in that section of the multiport will show up (hopefully)

[Denniswiseman]

When you pump to waste your pool water bypasses the filter and returns so there should be no restriction whatsoever
That "Recirc" is the recirculate but if pumping to waste is the same then that will be the same as well

No necessarily, when you pump to waste the water is sent to waste, not return so less chance of a blockage but they do happen. It may not be the same on recirculate as that water takes a route via the normal pipework to the return fittings in the pool so any blockage there or in that section of the multiport will show up (hopefully)

Point taken John but if pumping to waste which is the shortest route through the pipework and he still has problems it has to be something else, (air) that's why I suggested to check the pump filter basket when the flow slows or stops (worth doing as it is obvoius without disassembling)
Another thing that suggests it's air is the fact that is does pump and return OK but slows after a time which would indicate a slow buildup of air. Also when vacuuming to waste (which puts a load on the system) it happens quiet quickly

[jay11]

I tested the recirculate and the return flow is strong. I had to run so I didn't let it run but for 10 min. I will do 4 hours plus tonight.

Every time it starts in filter mode it does shoot some bubbles - not a whole lot but some and then they disappear after a couple of minutes.

I haven't noticed anything unusual in the skimmers but now that you say that I will keep an eye and revert tonight.

[Teapot]

Dennis it's the pressure gauge rising which leads me to think blockage. The fibre getting bunged up in the multiport possibly as that cannot happen with DE

[Denniswiseman]

Dennis it's the pressure gauge rising which leads me to think blockage. The fibre getting bunged up in the multiport possibly as that cannot happen with DE

Hi John
That did occur to me as well that there may be an algae bloom but pumping to waste shouldn't be a problem as it's missing the filter and returns, then the flow stops quiet quickly which I would put down to the extra suction side load pulling in air faster