Air getting into filter system

[NigelW]

My pond has a Hozelock Bioforce Revolutions filter with a matching Ciprio pump installed about 8 years ago. The water from the underwater pump goes out through 40mm hose, exits the pond up a rise to where the filter is mostly buried. The top of the filter is about 2' above the water level. The 40mm hose from the filter re-enters the pond on the plant shelf about 6" under the surface, generating a visible current that the fish can be seen swimming against. Over the past few months there have been quite large amounts of air periodically coming out of the outflow pipe, and bubbling to the surface. There are no water leaks, and no loss of water from the pond. Both ends are underwater, so the air must be getting in via the filter. I can't work out why, or how, the air is getting drawn in. I would have thought that the water pressure would prevent this. Plus, if air can get in, why is no water getting out? I normally just use the Revolutions handle and waste pipe the keep the system clean, but I have just taken the filter apart for a more thorough clean, though it was surpringly clean anyway. Can anyone explain this to me please?

 

[JRS]

Welcome to the forum. No experience with that filter but some type of venturi effect comes to mind. Perhaps some air trapped at the high point in the filter box above the water level and the water flowing past sucks some it along as it returns to the pond. Or or perhaps a small crack or hole letting air get sucked in to the system.

 

[j.w]

and welcome @NigelW
Can't help ya on that filter problem.

 

[NigelW]

Thank you both for the welcome. To clarify, the Revolutions filters have a handle on top that rotates paddles in the filter medium, whilst the outflow is temporarily switched to a waste pipe instead of the normal outflow to the pond. I normally just use this rather than opening up the filter. Given that the filter medium was quite clean, it seems to work well. The eruptions of air from the outflow pipe are quite volcanic, so I am hoping that someone will know why this has started happening!

 

[mrsclem]

It sounds like the gasket on the lid to your filter may be letting air get sucked in. Have you opened the filter lately? Check to see that the gasket is clean and properly seated.

 

[NigelW]

Thanks for the reply mrsclem. I did take it apart a couple of days ago to give it a proper clean. The lid has a rim that fits into a seal slot all the way around, and all secured with a substantial clamp. The water must come above that level to force the water out of the outlet, which is in the lid alongside the inlet. I am perplexed as to how air can get in without water getting out. The whole system is pressurised by the pump, and it is quite powerful as demonstrated by the current in the pond where the outlet returns. I am a bit suspicious of the valve that switches the flow to the waste pipe for cleaning, but I am reluctant to try to dismantle the lid and all the mechanisms in there!

 

[mrsclem]

Air must be getting in to the filter somewhere. Is it possible there is a hole in one of the hoses?

 

[NigelW]

I did replace the hoses last year. Given the water pressure, I think that a hole would be leaking water out, not letting air in. This is what perplexes me. When I changed the hoses, one of the unions with the filter was leaking. I did eventually fix that with PTFE tape. It is very strange - any sort of plumbing leak normally results in water getting out, not air getting in!

 

[mrsclem]

When exactly did this start? After you opened the filter?

 

[NigelW]

No, I have never opened it before - I have just used the revolving paddles to stir it up to get rid of the gunge. It has been fine for years, and the bubbling started in January. The pond was all netted over for winter, and I was only able to take the net off last week after the magnolia petals stopped dropping. I was hoping that a proper clean would fix it, but it didn't. It really didn't need cleaning anyway, but I do need to replace the UV bulb. Thanks for the suggestions. It is time for bed - 11.30 pm here in the UK!

 

[poconojoe]

Very strange that nothing is leaking but air is getting in.

I would take a look at that union that you patched up, maybe change it. I wouldnt trust any type of tape considering all that water pressure.

This is a long shot....
Did you try turning it off and then looking for a water leak? Maybe without the pressure of the pump the leak will be more evident?
Just a wild guess...

 

[combatwombat]

Is it possible your pump is cavitation and discharging air bubbles into the hoses?

 

[NigelW]

Thanks to everyone for the suggestions. The problem is that there is no way of seeing what is happening inside the system in operation! All the hoses connect to the lid. The water coming in through the inlet is supposed to sprinkle down through holes in the plastic baseplate of the lid, go down through the filter mediums, back up through a central tube past the UV lamp, and back out through the outlet. The outlet to the pond can be switched to a waste outlet to flush out dirt whilst a handle on top turns internal paddles to agitate the filter sponges. It seems very unlikely that there would be no sediment in the filter when I opened it up last week, and I suspect that something has gone wrong inside the lid, such that the water is not actually going down through the filter medium. This could also explain where air is getting in.
I have put in a support request enquiry to the manufacturer, but they may take a week to reply, and may not have the answer. I hope I don't have to buy a new filter as they cost £250/$300.
In the meantime I am open to ideas!

 

[poconojoe]

When I changed the hoses, one of the unions with the filter was leaking. I did eventually fix that with PTFE tape.

Are you sure this patch with tape is not the culprit?
For the low cost of a union, I wouldn't chance it. I would replace it.

 

[NigelW]

Are you sure this patch with tape is not the culprit?
For the low cost of a union, I wouldn't chance it. I would replace it.

The flexible hose is the type with the spiral wire, and it slides directly on the the filter inlet. It was dripping all the time, no matter how tight I clamped it. I wrapped the inlet with PTFE tape before sliding on the hose, and then tightened the clamp. That stopped the drip. PTFE is plumber's tape, and used in the trade to help fill the threads on screw joints.

This was the inlet to the filter, and under the highest pressure. I am confident that there is no air getting in there.