Power shower pressure

[PhiltheFrog]

Question: I have just installed a new power shower and showerhead complete with new HW tank and Warix valve. Pump and shower are both by Newteam - Easiboost and Tapmate Galaxy-T Thermostatic Mixer respectively. The pump is a twin impeller unit.

The cold water tank is in the attic above the HW cylinder with about 1.0m head (top of water to top of shower) and both tanks are of about 115litres capacity each. I have been working on this system for about 3 months now every other weekend (working away) and still no regular hot water flow out of the shower. The pump is at the base of the HWC and the shower approx 1.5m away.

The pump was until today protected by gate valves. The HW one was replaced today by a straight compression coupler to eliminate the possibility of air being drawn into the pipework. ALL pipework is new being 22mm and 15mm copper as required - 22mm feeds to the HWC and 15mm feeds to the pump from the CWT and HWC.

The problem is that air is getting into the hotwater side of the pump - not between the HW and the pump as the flexible pipework is rock hard being full of water but between the pump and the shower head the flexible pipe is soft and flabby. Cold water side is no problem both inlet and outlet flexible pipes are full of water.

If the union is loosened behind the shower mixer on the hotwater side water dribbles out with air mixed in and then the pump kicks in and a nice hot shower ensues. Switch it off and leave it half an hour and no hot water again and have to bleed the system AGAIN! Something that might be worthy of note - if the pump is run long enough it suddenly starts to screech with a \\\'kerplop\\\' and the hot water starts up.

Are we talking of a faulty reed switch here causing the air lock? Or is air getting into the system elsewhere or what? Management is not at all happy having laid out a fair amount of money for a new suite and shower plus tiling and lighting and I\\\'m going grey as this has got me beat at the moment. The gate valve can go back on as I think I can say it\\\'s not that!

The only thing that I could not do as per manufacturers instructions was to install the 15mm CW feed to the pump from the CWT 60mm below the 22mm CW feed to the HWC as the HWT feed was already at the bottom of the CWT
- in any case as far as I can see its the head that matters not whether the 15mm CW pump feed is lower than the 22mm CW HWT feed.

Any ideas chums? Management is threatening electric. I do NOT want to install an electric shower - we had one before - not enough flow for my liking.

 

[ChrisR]

its the head that matters not whether the 15mm CW pump feed is lower than the 22mm CW HWT feed.

That's just so the HW runs out first and doesn't cook the user.

Could be your hot water is too hot - max about 60, or the pump will cavitate the hw.

 

[PhiltheFrog]

Someone in the office had mentioned cavitation. The cylinder thermostat is about all that is original - and I have a new one to replace it - because I did think it was sticking - but nevertheless the old one is set at 60. I thought there was a temperature cut-out on the pump.

It may explain why despite turning the pump around the same effect was occuring - not external air getting in but being created from within the impeller.

I shall change the cylinder thermostat tomorrow before changing the pump.

Thanks for your thoughts.

 

[Softus]

not external air getting in but being created from within the impeller.

Er, what?

 

[simond]

Where you have taken the Warix valve off the top of the cylinder you should have fitted a manual air vent at the highest point before the pump.

Then, before running the pump you should let 5-10 mins of gravity water pass through and vent it. Don't try and vent it with the pump on or you will suck through vast quantities of air.

You usually tell if the cyl stat has gone, because you scald yourselves regularly.

Finally, bear in mind that the pump creates negative pressure behind it; in the HW cylinder, for instance. Non standard pipework or other pumps elsewhere could be an issue.

 

[PhiltheFrog]

To simond

The reference to external air not getting and being created by the impeller is a cavitation issue.
To explain if I can: when a blade passes through water rapidly air bubbles are formed at the tip. This used to be a problem on ships propellers - the bubbles could erode the blades away - or cause problems to those caught up in them.

Hot water has a lower specific gravity than cold water, that is to say 'thinner' and less resistant to this phenomenon. The pump has an impeller inside which turns very quickly and therefore it is quite possible that if the water is hotter than 60 C then bubbles of air could be formed inside the pump and collect inside the pipework. Next time the pump is used as there is now air inside the reed switch is not going to operate and the pump will not start the hot water.

Hopefully that's right.

In terms of venting there was no mention of additional vents being needed in the instructions and the highest point above the pump is the HW vent to the CWT in the attic anyway. I was advised that additional vents are only usually recommended on long horizontal lengths of pipework.

I did all the manual venting before running the pump as per the instructions - this air is getting in or being created at every use. I don't think there's anywhere for a manual vent to be located certainly not on the HW inlet side of the pump - straight in from the Warix outlet at the top of the HWT and into the pump on the floor adjacent to it - can't get it closer. On the pumped HW outlet there is some space but nothing shown on any of the instructions - or did I miss something here?

Anyway thanks for your thoughts - I'll let you know how I get on.

So maybe the water is too hot in the HWT. So new thermostat in place this morning and I'll try that first.

 

[TicklyT]

Is the flow rate on the cold water feed to the hot water cylinder up to scratch?
If it isn't good enough the pump could draw air back down the vent pipe into the hot cylinder.

 

[Sider]

softus wrote

PhiltheFrog wrote:
not external air getting in but being created from within the impeller.

Er, what?

Where did he write this or did you just make it up ?.

 

[Sider]

OOPs Sorry softus. I thought this was the first post you were referring to.
I should read them better.

 

[Sider]

Philthefrog wrote

To explain if I can: when a blade passes through water rapidly air bubbles are formed at the tip. This used to be a problem on ships propellers - the bubbles could erode the blades away - or cause problems to those caught up in them.

Where does the air come from. Surely a vacuum will be formed ?
Are you saying the water is air entrained or something and the impellers draw this air entrainment out.

 

[Sider]

Philthefrog wrote

and 15mm feeds to the pump from the CWT and HWC.

Someone may have mentioned it but these pipes sound much too small.
Do the manufacturer recommend 15mm.
I doubt it.

Any pump I have ever seen has a much larger pipe on the inlet compared to the outlet.

Have you calculated the NPSH required for this pump ?
Sounds like your NPSH is equal to or greater than the available NPSH.
"Plumbing Engineering Design Guide" describe cavation (amongst other things) as a "partial vacuum gradually being filled with vapour as the liquid at the interface boils at the reduced pressure in the cavity".

 

[simond]

Phil the frog

I'm not sure what i said that prompted the propellor cavitation story, but it was interesting, so thanks anyway.

My point was that the Warix flange is inserted via a dip tube in the HW cylinder, and many shower manufacturers like to see an airvent in the pipe that rises over the cyl and down to the pump. otherwise, it is v difficult to remove all trapped air on commissioning.

If the water was too hot and the microscopic air bubbles were being produced at the impeller tips, this does not explain why the pump runs OK after a long air blocked run.

 

[Softus]

The reference to external air not getting and being created by the impeller is a cavitation issue.
.
.
.
Hopefully that's right.

I think you've soundly misunderstood the phenomenon of cavitation. You can't create air inside a lump of water, any more than you can convert lead into gold.

 

[PhiltheFrog]

To all:

Thanks for your thoughts and suggestions - this particular topic seems to have some interest.

The Easiboost pump comes ready fitted with 15mm flexible connectors. These have new 15mm copperwork fitted to both the inlet and outlet sides, the cold water being feed directly from the CWT via a 15mm pipe and more or less in direct line from the CWT. The hot side has a 22mm take off from the Warix flange which then has a 15mm reducer. This pipe run is about 1.0m in length between the pump and HWT. All bends are swept. All this is in accordance with their recommendations: ' The minimum inlet size is 15mm for all pipework to and from the pump'.

The 22mm hot feed to the rest of the house and to the vent from the top of the Warix has a swept bend too. There is a short horizontal run some 300mm above the HWT and 450mm in length as per WRC recommendations before the T to the vertical vent/domestic HW pipework - so can I assume that any air bubbles inside the tank off the hot water coil inside will go into the vent?

To answer Simond's last point the system at the time being is bled every time before use - that's why it's 'working'. The HW inlet to the pump is or appears to be air-free but the HW outlet side is empty after use - all soft and flabby.

BTW for us not in the know what is: NPSH?

As soon as Management has finished her session in the bathroom the cylinder thermostat will be changed. I'll keep you posted.

 

[Nige F]

Micro air leak into system somewhere (on neg. pressure side )probably due to compression fittings not having a smear of compound round the olive-------and rising damp in the cupboard