Small pool leak - hydraulics question

pools_kalavo

Member
Jan 2, 2016
23
UK
We've got a deck level indoor pool. It's got a leak that's currently about 90 litres per day. One aspect of the leak is that it seems slightly worse during periods when the pump is not running.

I appreciate this could mean a suction side leak (although I'm not seeing any air coming into the pump basket) and need to do further tests to check that.

I initially eliminated any possibility of the leak being in a return. This is down to the theory that a pipe under pressure would leak more water and therefore it would lose more water when the pump was running. However, the more I think about it, the more I question whether that is correct. The returns start out from the plant room as flexi pipes, but there is a joint underground where they change to solid pipe via a "collar" type connector. If the leak was between a flexi pipe and the collar then is that part of the return pipe actually under less pressure when the pump is running?
i.e. because the water would have to go round a corner and reverse direction to get through the hole.

Thanks very much,
Kalavo
 

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
"K", you have the right concepts, however I'm not sure I follow you about the last sentence of your question. Once the return pipes leave the plant room as flexi pipes, they eventually are joined to a "collar"as you call it (perhaps coupler/union). Either way, at that point they convert to solid pipes. The pressure in those pipes (solid or flex) should be the same.

Now this sentence confuses me:
because the water would have to go round a corner and reverse direction to get through the hole.
Can you clarify please?

Also, since this is an indoor pool, and you mention it being "deck-level", I'm assuming that is a concrete decking all around the pool correct? Would it be difficult to see a pressure side leak? Was french draining or some other form of protection installed around the pool to protect the house or structure in the case a leak was anticipated? Wondering if that would make it more difficult to find a moist area/leak.

Yes, you would think that if it were a suction side leak, not only would there be water loss when the system is off, but you would also see air getting "sucked-in" effecting pump prime. But much could depend on the size of the defect, location, and whether any amount of air was significant enough to disrupt priming anyways. So I wouldn't rule that out just yet.

Do you have a light? If so, have you inspected the light niche? And last, what about a drain? If you have a drain, it may have a hydrostatic valve which can go bad over time. Anyways, those are some initial thoughts. Others will chime in later I'm sure to assist.
 

pools_kalavo

Member
Jan 2, 2016
23
UK
Thanks Texas,
The joint between flexi and solid is a socket with two female ends. So the flexipipe goes inside the socket. I was always a bit dubious about the joints between flexi and solid and wish I changed to all solid pipes after I sacked the pool builder. The joints, however, pressure tested ok and the leak only started in June this year, over a year after we finished the pool. What I mean is, when the pump is running, the force of the water going down the returns is towards the pool. If the leak was where the flexi fits into the socket, then a water droplet going on it's journey down the return would have to go round the end of the flexipipe and back through the gap between the outside of the flexi and the inside of the socket. So against it's direction of travel down the pipe.

It's tiled concrete around the pool. There's no french drain but the soil at pool depth is a kind of shale which would probably quite easily take 100 litres a day without showing any signs at ground level outside of the pool building. One thing I have noticed is that the leak does appear to be affected by the ground water level outside. We had lots of rain in October and that seemed to reduce the leak by about 10 litres a day. Please note, the pool was consistently using 12 litres a day approx. for the months leading up to the leak starting and the weather outside had little effect on that.

The pool has 16 lights, 4 return fittings and 2 main drains which go to the surge tank by gravity. I tried using food dye in a syringe at all the fittings to see if that showed anything but without success. 100 litres a day is like a tap dripping once every 1/2 a second (I tested it!) so not easy to find.

Kalavo
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,771
Northern NJ
Flexible PVC underground can be damaged by termites chewing into it.

How old is the pool?

Have you pressure tested the lines?
 

Jimrahbe

Mod Squad
TFP Expert
LifeTime Supporter
Jul 7, 2014
14,623
Bedford, TX
Kalavo,

Sounds to me like you are loosing about 1/8th in of water a day.. I know this is an indoor pool, but why do you not think this is just evaporation? Has it been doing this since it was new or is this a sudden change?

I suck at math, so I could be wrong about the amount.. With any type of auto fill off, how much does the water level drop per day?

Have you done a bucket test? Leak Detection - Trouble Free Pool

Thanks,

Jim R.
 

pools_kalavo

Member
Jan 2, 2016
23
UK
Hello Jim, I'm pretty sure it loses just 12 litres a day through evaporation/usage. Was consistently using that each day for Feb, March, April, May and most of June. Then end of June this year, it very quickly escalated to 60 litres per day. July was 60 litres avg per day, August 70 litres, September 80 litres, October 70 litres and then beginning of November it jumped up to 100 litres a day. I have a home made autorefill system which I didn't get going properly until Feb this year and it's only since then that I have been able to accurately measure.
I think it's even less than 1/8inch per day. A bucket test would be tricky. Would have to leave the pool cover off and pump off for at least a week before I could measure the difference. Leaving the cover off that long would not be a disaster but not great for the building.
 

pools_kalavo

Member
Jan 2, 2016
23
UK
We've got a climate control unit which gets the humidity down to 60% if it goes above. However, nearly all the time the cover is on the pool and I would say typically it's about 45% and the unit doesn't need to do anything with the humidity.
Pool temp is kept at 28degrees c.
Room temp is increased to 27degrees c if it goes below.
 

pools_kalavo

Member
Jan 2, 2016
23
UK
Cornwall, not too far from North Devon where those termites were found in the article! But I do think I would have to be incredibly unlucky for them to turn up here and there's plenty of other tasty snacks around they could eat.

We pressure tested 2 years ago. I can do more tests where I just use half the returns and bung up the others. Just takes time.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,771
Northern NJ
Cornwall, not too far from North Devon where those termites were found in the article! But I do think I would have to be incredibly unlucky for them to turn up here and there's plenty of other tasty snacks around they could eat.

It is not resistant to insects, in particular, termites have been known to eat it. Research has shown flexible PVC does not attract termites, however if flexible pvc is placed in between a termite nest and a food source, they may try to eat their way through it. (They are too dumb to go around it.) FlexPVC.com will not be responsible for failure due to insect damage, nor rocks rubbing against the pipe and wearing a hole in it. You can lace your backfill and trenches with boric acid to reduce the likelihood of termites, but it won't last forever. If you have termite infestation and want to use Flexible PVC pipe, you should retreat your trenches every few years to be safe, just like you do your house, garage, etc.
 

pools_kalavo

Member
Jan 2, 2016
23
UK
Thanks Allen, Will just have to hope we're ok regarding any termites. Not sure why anyone would actually choose to use flexible piping. I certainly regret that I didn't rip them out and replace them when I had the chance. Kalavo.
 

pools_kalavo

Member
Jan 2, 2016
23
UK
Hello Allen, I thought I was doing a kind of pressure test of the return lines by running the pump for extended periods of time and then not running it. The water loss was marginally greater when the pump wasn't running. Which leads back to my original question:- Is there a circumstance where a leak in a return pipe can actually be under less (or the same) pressure when the pump is running?
 

JamesW

TFP Expert
Mar 2, 2011
16,592
There are circumstances where the return can have a venturi effect where a leak is less while the flow is on, but its rare for it to happen.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
10,771
Northern NJ
Hello Allen, I thought I was doing a kind of pressure test of the return lines by running the pump for extended periods of time and then not running it.
I am suggesting a more formal pressure test. Plug a line and put a few PSI into it and see if pressure holds for 24 hours. Your flex PVC is a weak spot in your plumbing and you should confirm its integrity.

You could also have a badly glued PVC joint that came loose.

The water loss was marginally greater when the pump wasn't running. Which leads back to my original question:- Is there a circumstance where a leak in a return pipe can actually be under less (or the same) pressure when the pump is running?
Water flows in strange ways that are not always obvious. You can only infer so much by blind tests and trying to infer cause and effrect.