# Forum Home Renovation Pools, Spa & Water Features  Pool Filter Pump problem

## DIYGeoff

I have a conventional sand filter/salt chlorinator setup. All was well until the original filter pump failed and I simply replaced it with a Davey 2HP unit. The new pump only occasionally wants to pump to its full capacity. It will only ever seem to work properly when circulating water through the sand filter and then not all the time. At other times the pump filter bowl is half empty and the pump output is less than half of what it should be. Interestingly if I switch the pump off in that state the filter bowl instantly fills. Turn the pump back on and the bowl half empties. Could this be cavitation and if so why?
In desperation I replaced the pump with a cheapy from Ebay but it gave the same results. 
Given I can only occasionally get it to pump correctly when pumping through the sand filter its as though the pump needs some back pressure to work (but not all the time!!)
I considered a partially blocked inlet line but this problem was evident as soon as the new pump was installed. The old (failed) pump worked perfectly. 
I am at a complete loss and would appreciate any ideas.
Thanks
Geoff

----------


## r3nov8or

It sort of sounds like it's sucking air and can't prime properly, but as you say, not all the time... Are all the O rings in place properly and in good order?  Does putting pressure on the pipes and fittings close to the pump make any difference? (checking for looseness/air issues

----------


## DIYGeoff

Yes I thought the same thing for a while but I have triple checked and redid all the plumbing when I tried the second pump. New O rings etc. I have just watched a couple of videos on youtube re cavitation and it is demonstrated by restricting input flow. Could the pump be oversized? Its a std 40mm PVC line from the pool about 10m away with a rise of about 0.5m. I keep wondering if there is some sort of restriction in the line but apart from putting a camera down the line (if that's possible) I dont know how to check it. Could rig up some flexible hose to the pool I guess as a test (that just occured to me!)
Thanks for your thoughts

----------


## Tealman

Hi, right up my ally pumps are. Question 
1    what size was the previus pump
2   is the pump elevated above the water surface. 
I suspect that the pump is too large for the size of pipe and distance it has to suck.
Basic rule of thumb is to keep the suction side as short as possible and as big as possible

----------


## Tealman

A lot of people dont understand what cavitation actually is, most think it is the O2 being sucked out of the water, if you want to see it for real then as an experiment, get a syringe some light engine oil (works in water, but best in light oil) put the syringe in the fluid and draw the plunger back quickly.
You should see what looks like a lot of air bubbles, but are actually bubbles of vacum, once the presure has stabilized the bubbles will disapear.
These bubbles can cause a lot of damage to metal pump components when the colapse.
That is basicly what your pump is trying to do, suck to har through a pipe that is too small and too long, 10 metres is a very long run.
The other thing that can happen when people put in a larger pump is the sand filter splits 6 months or so down the track.
you are better off having a small pump, big sand filter and run it as long as you can, 24 hours a day is great. Cheaper to run, filters better and you use less chemical, or if you are on a salt water chlorinator, you can turn it down and it will last longer. 
Of course you may just have an air leak, but I dont think so.

----------


## Bros

The way to reduce pump capacity and output is to throttle the discharge.

----------


## DIYGeoff

Thanks for your comments.
I cannot remember what the original pump size was, and I am not sure there was any spec visible for me to replace like for like,  but the replacement I put in was a Davey SLS200 which is 200 l/s capacity. From what I have read in the last 24h this seems about right for a 40mm pipe system although perhaps this needs to be down graded for a 10m run?. I now realize that the pump I used to replace the SLS200 is too large but that does not explain why the SLS200 did not work correctly.
The pump is elevated above the pool water level by about half a meter.
I am now confident that the problem is cavitation. The reason is that the instant I turn the pump off the filter bowl fills completely. I have read that this is a sure sign of cavitation. So it seems either I have some sort of restriction in the inlet to the pump or even a 200 l/s pump is oversized. (I mentioned earlier that the only time there is no cavitation is when the pump outlet is circulating through the sand filter (effectively throttling the discharge to some extent?), which to me further supports the cavitation theory I think.
I have no idea how to check the piping between the skimmer box and the pump but I am tempted to put the SLS200 back in and run a temporary 40mm line above ground as a test?
Or I guess I could bite the bullet and say put in an SLS100 or equivalent? Perhaps I have been chasing my tail from day one as the very original pump may have been much smaller than I thought. 
Really appreciate all your comments.

----------


## r3nov8or

What's the reason you don't always run with the sand filter in line?

----------


## DIYGeoff

When I vacuum to waste or to drain. But it will not always pump properly even when the sand filter is in line. Sometimes I have found that when it seems to be cavitiating (in line with sand filter) I flick the pump on and off a couple of times it will "snap" back into proper pumping!!

----------


## r3nov8or

> When I vacuum to waste or to drain. ...

   Well, yeah, of course, silly me.  :Smilie:  
When it gives you trouble even through the sand filter, have you tried creating more pressure by restricting the output flow at the pool outlets? e.g. blocking one if you have two, and see if that refills the bowl...

----------


## Tealman

Ok, if you want to check the pipe for restrictions, this is how you get a pull through from the skimmer box to the pump.
1 you will need a string, in your case 12 to 15 mtrs 
2.  tie a hair ribbon or light peice of rag to the string
3.  remove the skimmer basket 
4.  turn the pump on, making shure that it is pumping water through
5.  feed the ribbon and string in through the skimmer box and let it get sucked through to the pump
6  have some one stand at the pump and when they can see the ribbon (use a bright colour) turn the pump off.
7.  using the string,  pull through a stronger string and then a light rope and then tie a rag (that will fit down the pipe easily) pull it through
8.  make sure you tie another peice of rope to the rag, ie   (rope rag rope) so that you can pull it back if it gets stuck. 
That should give you an idea wether or not your pipe is clear.
Q.   afiter tthe pump has sat for 24hrs, and at the point of turning on does 
a/   it prime up easily
b/  have a muddy look to the water 
If it primes up easily, it is likely that your pipe work has no leaks (espialy as the pump is so far away and above the pool suface)
If the water has a muddy apperance to it on start up, you prob have a cracked pipe, and as it is below ground level the dirt seals the air from entering (to a degree)
Q.  how much of the pipe coud you dig up if required?

----------


## Tealman

At 200 litre a min that  would equate to 1hp pump, give or take, not that big.
Having worked with pumps all my working life, some too many years he hehe. 
If you want to reduce pump out put short term by all means restrict out flow, but it does put a presure load on the system, not good for sand filters, glue joints, pipe work and rubber couplings.
If you want to reduce pump output permantly, reduce the size or diameter of the impeller , or buy aa smaller pump.
Bros, I appoligise for dissagreeing with you, espicialy as I am a new member.

----------


## Bros

> Bros, I appoligise for dissagreeing with you, espicialy as I am a new member.

  I don't know what you mean as you admitted that to reduce the pump output was to throttle the discharge. I never said it was a permanent solution as to do it properly would be by means of an orifice plate in the pump discharge. 
My pump experience is from pump with impeller diameter of 2.5 M with a discharge pressure of 50 kp up to multistage pumps with outputs of 20Mpa but as for knowing the exact cause of the op problem I would be only guessing and my best guess is to big a pump as you have said or a restriction in the suction line or too long a suction line causing to low a suction pressure but that come back to an oversize pump.

----------


## DIYGeoff

Many thanks for your details on how to check the pipe. I wont get to it until the weekend but I will get back to you with my findings.
Re your other questions:
a/ It primes up easily say 20 seconds if nothing has been disturbed. eg the pump filter has not been opened. If it has been opened it could take say 2 min for flow to start.
b/ Have never seen muddy water 
Digging up pipe a major issue!! The pool surround is concrete with pavers. There would be 7 meters of the pipe under the concrete..... 
Re Question from r3nov8or; Good idea. I will try what you suggest and let you know the result. 
Again, many thanks for help

----------


## DIYGeoff

Ok, Update!!
I followed Tealman's instructions and got to the point where I had my wife pulling the rope at the pump end and me feeding in the rag at the skimmer box. I wasn't sure how much rag to use so I tied an old sock to the rope and ran it through a bit of 40mm PVC with a couple of elbows in it and it went through quite easily. So my albeit limited instruction to my wife was to pull the rope while I fed in the rag. What I didn't realize was that when I said pull the rope she dutifully pulled with all her might. Yes you guessed it....stuck rag. Simply pull it back you say.... Well my little wife was stronger than I thought. I could not pull it back and in hindsight a sock was a poor choice as it is too long so I am trying to pull it back over its self. For half an hour I pulled back with all my might noticing that the rope is starting to fray at the bottom edge in the skimmer box. Needless to say a bit panic is now setting in but just as I was about ask for divine help it gave way and the sock came out!! With my confidence rattled I wasn't prepared to try any more rag. I did tie a larger knot in the two ropes, I guess about 20mm in size and it ran through but there did seem to be some "rough" points immediately near the skimmer box. I have no doubt an experienced person could have worked up larger sizes and formed and opinion re pipe obstruction. (A camera might be interesting one day)
I then decided it wouldn't take much to run a a separate feed line to the pump and I rigged up my vacuum hose. It just happened to be the right length so with a bit of silicon and tape and after a few self priming issues the pump sprang to life. Results? Not a sign of cavitation in any of the scenarios described previously. This temporary setup is I guess ideal in that it was a straight run, no 90 degree bends so I guess some doubt remains as to whether there is an obstruction in my pipework.
Where to from here? If there is an obstruction fixing it is a massive exercise and I decided not justified. So I have installed my Solar pump (0.45kW 1/2 HP I guess) and it works as expected. Basically I have taken your advice...small pump run longer.
My plan is now to get rid of the salt chlorinator and replace it with an Ioniser and use a Robotic cleaner. Comments would be appreciated on models etc
Again may thanks for all the help.

----------

