Cavitation not Air in System

[hewie]

Thanks all that helped in my last thread, but I think my system is cavitating
To refresh its a fully pumped, open vent, 2 zone valve system
Boiler sited in garage on far wall. The length of pipe from boiler to the pump in airing cupboard is approx 30ft and is 22mm
Should I have piped in 28mm?
The pump is running on 1st speed and is delivering good temps to CH & HW
Is there a pump with a lower head than normal?
The boiler t/stat is on nearly 5 (max 6) for the winter months
I know the higher temp doesn't help matters

Any thoughts please

 

[jpm]

turn the pump up.cavitiation is not air?what else can it be then?????

 

[hewie]

gets worse on higher speed. Just thought with that length of pipe low pressure tothe pump inlet was boiling the water somewhere between pump & boiler

 

[paul H]

check the pump is the right way round and not pulling on the vent place a cup full of water under the vent pipe and see if the cup emptys when the pump is running
best of luck

 

[stevetheplumber]

sounds like it could be where the vent and cold feed are teed in may be wrong diagram or photo would help

 

[migoplumber]

or the pump bearings ?

 

[radswilldo]

What is the actual problem?, do you HAVE air in the system

 

[hewie]

hi, not being a plumber. air does emit from the auto air valve occasionally. I thought i might try a leak sealer just in case there's any micro leaks. I expanded an old system so I don't know whats happening under the floor boards
The whooshing noise at the pump occurs every now & then and I can hear it coming just like something has been suddenly injected into the system
I put a jar of water on the end of the vent pipe no problems there
hewie

 

[Bernie2]

Usual signs of cavitation is said to be noise like rocks in the system. There may be a case for cavitation if you have low pressure at the pump inlet and the pumps minimum inlet pressure isn't met.

With such a long run its possible the pipework resistance is knocking the head down, coupled with the rise in usual water temperature could be causing cavitation.

Try checking the pipe run?

Make sure all valves are fully open including pump valves and that you are not pulling air in anywhere.

You can also get discharge cavitation, which makes things worse if you speed up the pump.

 

[hewie]

 

[hewie]

sorry abouy the quality of diagram, i'm useless at this
I was by the airing cupboard when the system kicked in from cold, the whooshing noise was there first up so has to be air. Where's it coming from? Also where's the best place to place a auto vent valve. I cant see any evidence of leaking. I'll try the leak sealer the weekend

 

[Bernie2]

Going by your drawing I would say you where pulling air in, down the vent. Perhaps a close coupled feed and vent may help.

 

[hewie]

I've had a jar of water under the vent all week to monitor this,no probs there. I like the idea of the close coupled feed & vent. Do I just drop the feed down so its between the vent tee & pump. I noticed that when run on CH only it's pretty quiet, when I open HW valve manually it starts to whoosh before settling down to the occasional whoosh

 

[Bernie2]

Hi! hewie,

You would have to check your boiler allows it. But you join in behind the pump with two tees no more than 150mm apart.

I must admit some boilers do ask for a combined feed and vent pipe and it may be the one you have.

The thing is, you say you have had a cup of water over the vent all week and nothing has happened i.e no water sucked down the vent.

But has any air been driven out i.e water splashed out the glass?

The first tee behind the pump inlet is the cold feed, then the vent.

This puts the system under positive pressure with the neutral point being the cold feed tee.

I suppose as the water is driven around the system it then vents any air out up the vent before it goes back into the pump.

 

[hewie]

Thanks for advice. As I did the installation myself I just read circuits from the internet not knowing makers particular recommendations. The boiler is an Ideal w2000. Would you know what type of feed/vent type they require

 

[Bernie2]

Hi! Hewie,

Not sure, but would imagine if you can use a single feed and vent combined it should be okay. Wait and see if anybody else has anything to help or rephrase your question and start a new thread asking if it would be okay.

The problem is there are probably thousands of different boilers so there is no guarantee anybody has worked on that particular one. Usually you can download for free discontinued model information but I can't find any that relates to that particular model that is free.

Try e-mailing the manufacturer they may send you a free copy. Its the installation instructions you want.

 

[hewie]

I gave them a ring this morning, they said it was ok to use combined vent & feed, I will ask them for an installation guide. Thanks again for your efforts

 

[Bernie2]

Hi! Hewie,

The combined feed and vent, which you have now works, but it said tobe a hang over from solid fuel days and part of the air problem.

They are not as safe either.

The close coupled arrangement is a modern idea for high pressure boilers i.e. the boiler must have an over heat stat on, as well as other safety features. That is why I recommend asking the manufacturer.

You could also try a Myson Airjec. Google Myson "Aerjec"

 

[hewie]

Thanks Bernie2 for your efforts to help me. The story is that I need a new boiler as it started leaking really bad on Tuesday. Is the heat exchanger in 2 halves? It looked like the leak was coming thru on the centre joint. Also is it possible to suck in air without leaking water? If it is that would account for air in the system, the boiler only leaked as its warming up, the hotter the water the worse the leak
Thanks again
hewie

 

[Bernie2]

Yes unfortunately you can suck in air without leaking water.

Apparently the heat exchanger is in sections and you can renew the "O" rings.

Have a look at gas news.co.uk. Somebody seems to have been doing the same job, they even have spare parts number for the rings. Check it out in the manual.

 

[JWoolley]

Dear Hewie.
The problem you have described sounds exactly the same as on my system. The pump gets hit with air and speeds up causing the pipes to resonate around the house. Then it will settle down for few minutes before you get another whoosh and gurgle. Sometimes it loses prime altogether. I have installed several auto vents which work to a point, tried leak sealer (which just snotted everything up) and have generally fiddled with the system to no avail for the last 2-3 months. The worst part of course is that you can hear the air in the boiler cupboard all over the house due to the amplifying effect of the rads. The cavitation is caused by air but it's difficult to get the air out even with auto vents. It seems to keep coming back or just continually going round the primary circuit.

I also observe that on CH the system is reasonably quiet (comapared to DHW) but with DHW on it gets worse and the loop round the cyclinders also seems to be full of air even though an auto vent is installed on it. The autovents work as I've had them off to check they aren't blocked.

I have close couple feed and vent in the correct positions and no apparent leaks on the system (one pipe!). . I did have a problem with air being sucked down vent but have cured that by linking vent and feed so vent is always flooded. However , this appears to have made no difference after one week of operation with autovents in place.

I'm running out of ideas! I hope I don't have a leaky heat exchanger

 

[hewie]

Hi JW, sounds like you have got a leaky heat exchanger, same as mine. I've been told it can let air in without leaking water. All is not lost as the exchanger is in at least 2 halves bolted together with the waterways sealed by O rings these are prob damaged or perished. I'm having a new boiler fitted tomorrow, if I had known about the O rings earlier I would have drained down and repaired it myself as my boiler is in the garage.
Might be different if indoors. By the way mine eventually sprung a leak so fix it soon before you wake up to a flood
hewie