Low flow at return jets / skimmer

I'm having trouble tracking down the problem causing my flow rate to be low and hope someone here can offer advice.

I have an inground pool with sand filter. With my multi-port valve set to filter or recirculate the flow is low, when set to backwash or waste the flow increases to normal. This seems to indicate that there is a blockage somewhere on the pressure side. Before I try to find a blockage from pump to return jets (which will be difficult as there is no easy access i.e. I will need to cut pipes) can anyone think of anything else that could cause the flow rate to drop so much between recirculate and backwash?

Thanks for any advice.

While it could happen, I've never known of an obstruction to occur on the pressure side, downstream of the filter. A pipe collapse could do it, but typically that would also involve a leak.

You're going at it correctly, however. Normal flow on backwash rules out a suction side problem. Abnormal (unchanged) flow on recirculate rules out problems with the filter.

I'd look at whatever's aboveground and downstream of the filter.

Thanks. I guess something (rodent?!) may have jammed itself inside - but I really do doubt it. There are no leaks (at least no noticeable drop in water above normal evaporation) so I have ruled out any joint or pipe breaks.

Is it possible that the pump or filter connections are sucking in air? We had to replace the floor under the sand filter and that involved a lot of brute force as we didn't remove the sand. The pump basket has a very small air bubble (about the size of a nickel) but I am fairly sure this is normal - it doesn't get any bigger during continuous operation. The return jets do bubble very slowly - maybe 2 or 3 tiny bubbles every 3-5 seconds. If the pump is sucking in air (maybe from a drain plug?) or air is coming into the filter from the pump pipe joints at the filter could this cause a loss of flow on both recirc and filter but not on backwash and waste? The water pressure does seem a little high (26 psi) - I think last year it was 20 psi or so after a good backwash. So I guess we have 30% higher pressure than normal and about 75% drop in flow. (On backwash pressure is 12-14 psi - which is the same as last year).

I have removed the multi-port (Hayward vari-flo) and replaced the seal - everything looked good but I didn't know about a "spider gasket" so didn't check this - could this be a possible cause?

A suction leak would lead to low flow on backwash, too.

It really sounds like you have an obstruction down stream of the filter. Could you have crimped a pipe when you "replaced the floor"?

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Originally Posted by PoolDoc

A suction leak would lead to low flow on backwash, too.

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Thanks - I just hoped my system didn't follow the laws of physics....

Quote:

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Originally Posted by PoolDoc

It really sounds like you have an obstruction down stream of the filter. Could you have crimped a pipe when you "replaced the floor"?

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Sadly, no. All the pipe work is above the floor and was disconnected during this process. Downstream of the filter I have a gas heater, solar panels and used to have a chlorinator. I can bypass the solar panels so I can rule them out as a cause. I'll check the gas heater (it isn't used and doesn't work any more) so maybe I should bypass it. I removed the chorinator because it started leaking and we always had too much CYA from it - maybe this wasn't done properly.

Many thanks for the advice - it's a great help to know where to look now.

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Originally Posted by Poddle

Thanks - I just hoped my system didn't follow the laws of physics.....

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:D :D

You, and quite a few others!

Bypassing your heater is a GOOD idea. Heater internals typically include components that, if they failed, could introduce a BIG obstruction.