Cavitation

[bfinl]

I have just fitted a new pump to my shower and it has now started suffering from the dreaded cavitation.
I think need to fit a flange to the top of my tank and reconfigure things.
However at the moment the tank outlet to the shower is on the side and not the top so i'm not sure why I have started to suffer this problem.
I moved the hot outlet to the side and the cavitation went away a while ago
and now it has come back.

My tank has a male fitting on the top and I'm struggling to find a flange for it.
I have one from screwfix, a salamander one but it does not fit.
Someone posted a link on the forum to a company that supply the correct type but I can't find the link.
Any help would be appreciated.
Merry Christmas. Bry

 

[ChrisR]

SIde (Essex) is better than the top - leave it there.

Is your water too hot?
Above about 60 can cause cavitation.

 

[bfinl]

No temp is about 55.
I used to get cavitaton until I moved the pipe down onto the side.
About a month ago the shower pump died so I fitted a new one and
have suffered cavitation ever since.

 

[bfinl]

The mixer valve on the shower maybe on its way out as it seems to need turning further than it used to to achieve the same temp but I can't see why this would cause the problem.
There is definately air in the hot water side as I've watched all the little bubbles come out of the hose when its submerged in the bath.

 

[ChrisR]

Then you must have a restricted supply.
22mm from cistern to tank is not big enough for a big pump.
They sometimes used restrictive stop cocks on cistern outlets too - replace with full flow ball valves (or gate valves if you must).

Try holding a pot of water up to the vent over the cistern in the loft.
It might draw some water up into the pipe, when the pump comes on, but it should stop and hold at a certain level.


You could also have a leak allowing air ion but not water out.

 

[bfinl]

I think I'll order a new mixer set then try that first.

 

[bfinl]

Anyone else help on this.
I still can't understand where the air/bubbles are coming from
especially as my supply to the shower is from the side of the tank.

 

[bfinl]

Anyone?

 

[Hugh Jaleak]

I can only think the pump is drawing water from the cylinder faster than it can be replaced. Possible restriction in cold feed as Chris R has already stated. This will result in the water being drawn back down the vent and letting air in, did you try the tests Chris R suggested?

 

[bfinl]

The pump is built into the shower which is a Mira Event non thermostatic.
Should the vent pipe be filled with water up to a certain level anyway?
I will try the test tomorrow when I'm off work and post my findings.

 

[bfinl]

I have tried the vent test and there is no suction down through the vent pipe
at all.

 

[Onetap]

Cavitation is not the same as the problem you have had with entrained air bubbles.

You'd usually get air problems if you take the HW from the top of the cylinder without using an Essex/Warix/Surrey/whatever flange.

Cavitation is caused by low pressure in the pump inlet. The hot water boils at the reduced pressure and the bubbles implode as they pass from the low pressure at the inlet eye to the high pressure at the pump outlet. It involves bubbles of water vapour, not air bubbles.

The usual causes are inlet pipes too small (usually 22mm is used), reduced bore isolating valve (should be full-bore), inadequate head from the CW storage cistern, pump inlet filter clogged, water too hot, cold feed to cylinder too small, cheap reduced-bore flexible connectors etc..

Read the manufacturer's installation instructions; if you notice something where your installation deviates from the recommendations, that'll probably be your problem.

 

[Onetap]

The mixer valve on the shower maybe on its way out as it seems to need turning further than it used to to achieve the same temp but I can't see why this would cause the problem.

Inlet strainer/filter on the shower valve hot connection clogged with limescale? Check the manual to see if strainers are fitted.

 

[bfinl]

I have just won a complete replacement shower on ebay
so will give that a go when it arrives.
Hopefully that will solve the problem.
None of this was a problem until the pump died and I replaced it

 

[bfinl]

The feed to the shower runs up and over then back down to the shower.
Does it need to be vented at the top ?