Spa is a Sundance Aspen portable, about 1999, with ozone. There is a circulation pump which runs continuously. While this pump is running the spa makes a steady "knocking" sound, at about 80 or 90 beats per minute. If you obstruct or momentarily block the circulation pump outlet with your finger, the sounds slows or stops, then resumes when you remove the obstruction.

The sound appears to come from the corner of the spa near the circulation pump outlet and the main corner seat with five jets. The sound does not come from the equipment bay.

The spa makes this sound all the time, except that a couple of times after the spa was cleaned and refilled it ran silently - until the next time it was emptied, then it started up again.

I've now dug my way through the foam to the hoses behind the circulation jet outlet. There is one hose (about 1 inch external diameter) which I think is feeding the circulation outlet, and another much larger hose passing in front, which I think is the main pump feed to the other jets. By holding on to these hoses while the circulation pump is running I can feel that it is the larger hose which is "twitching" in time to the knocking sound - the smaller hose is not.

My best guess is that this is some sort of airlock effect. Does anyone have experience of this ? Alternatively can I get a diagram or photographs of the hoses under my spa, without having to pull out all the foam to see them ?

Any help appreciated!

It would be my guess that the circ pump is faulty and will need replacing.

Thanks for your feedback. I've had a technician look at the problem, and conclude that the pump is ok. Could you give me more information on why you think the pump could cause this noise ? The frequency of the knock is much slower than the rpm of the circ pump motor.

Hi Brianm,

It sounds as though you have a "Swing Valve" in line of the circ pump.
This valve serves to isolate the suction of the circ pump flow, usually so it can share the same suction as the booster. When booster is off, the valve closes with gravity and opens when the jets come on.

http://spapartsnet.com/Reference-Catalo ... 7_0_1.html

The valve is supposed to be plumbed so that at rest, the valve has a positive closure. I fear that the valve is sitting slightly offset from horizontal, and the valve paddle is porpoising. The fix is to replace the valve, or re position the plumbing.

Thanks for that - this sounds like a likely explanation. So I guess if I keep digging, following the larger pipe, I should find this valve ? Is this likely to be a PVC component, cemented into the pipe in-line ?

When it comes to replacing all the foam I've removed, I'm thinking of using an aerosol expanding foam, sold as a gap-filler for round the house. Would that be suitable for the job ?

Is it a mag-drive pump? Did it make the sound while the tech was there?

I believe it is a mag-drive pump, and yes, it did make the noise while the technician was there. It was he who said to investigate further would require digging into the foam to check out the plumbing.

The reason I suggested the circ pump is that I have a Sundance Cameo (circa 94) and my old circ pump occasonally made a thump-thump sorta noise... the mag drive impeller was worn and would sometimes hit the housing off balance. I don't have a check valve in my setup. In fact, even if I were to have one, it wouldn't matter in my case, as the circ pump intake is from the filter housing, and returns through it's own jets. Doesn't share any pluming with the 2" mains. BTW the circ pump in my setup uses 3/4".

I know this is an old thread but I was wondering if you ever got a definite resolution to this. I'm having similar noise coming from my spa. It's not that loud...but I'd like to make it go away if possible.

No definite resolution. I believe the swing-valve explanation is the most likely to be correct in my case. I dug out the foam from under that corner of the SPA, but did not get to that valve and have never completed the job. At the moment it has stopped of it's own accord (which it has always done from time to time)!