Power Shower Air Lock

[Tad]

I have a Wickes thermostatic power shower which was fitted to replace an existing gravity fed manual mixer shower a few years ago. Ever since it was fitted we've had problems with air locks, particulalry when someone else turns a hot water tap on anywhere else in the house.

The problem is usually avoided by very carefully turning on the shower at low pump speed, then gradually winding up the speed as the water heats up - but is very much a fine tuning exercise and appears to be getting worse.

The product manual states "High level hot feed pipe routed upward to the same level as the cold tank e.g. into loft space, should be avoided as air locks may result." This may be a bit of a clue as the feed T's off the main HW pipe close to the HW tank, about 12" below the top of the tank, then up into loft along the ceiling (just below the level of the CW tank) then drops down to the shower unit.

Apart from re-routing the HW feed under the floor boards (which is not an easy option), is there any other way of rectifying this problem? Would a Surrey Flange be the answer?

I assume the problem is that the pump is attempting to draw water quicker than the gravity feed is pushing, so drawing air down the vent. Is there such a thing as a one-way valve that could be fitted to the vent to prevent this? I'm not totally convinced of this argument as the shower (same model) in the ensuite doesn't have the same problem, although that backs on to the tank so only has a pipe length of less than a metre.

Could anything else be causing this problem?

 

[Agile]

"""The problem is usually avoided by very carefully turning on the shower at low pump speed, then gradually winding up the speed"""

You have no control over the speed of the pump so I dont understand what you are trying to say.

My best guess is that you have other hot takeoffs which are not fed through the pump!

Its always better if all hot is fed through the pump. That is not ideal though because twin impeller pumps are designed in the expectation that both ends will be delivering a flow.

You have more or less admitted that your configuration is not ideal.

Without seeing your installation its difficult for me to know what the worst aspect is. Certainly replacing the cold feed to the cylinder with 28 mm will be a good first step. Supplying everything through the pump might also help and fitting a suitable shower take off point lower down might also help.

Tony Glazier

 

[Gasguru]

Your not using speedfit of other such c**p are you?

 

[Tad]

Agile

Thanks for your speedy reply. I do not have a separate pump, the Wickes Thermostatic Power Shower is an all-in-one shower with an integral mains voltage pump. There is a separate flow control that goes from a trickle to ripping your skin off. To get the shower to work without getting an air lock, we have to turn on the shower at a very slow flow rate with the thermostat on cold and gradually increase the temperature - then gradually increase the flow up to full speed - sometimes reducing temperature / speed when the pump starts to make a strange noise. This is a very fine art.

The cold feed does not seem to be a problem - it's the hot water.

Gasguru
No, it's all 15mm copper pipe with capillary soldered joints. No leaks detected in the 5 years it's been running.

 

[Gasguru]

Only asking about speedfit and the like because its very restrictive on gravity systems.

 

[Agile]

OK now I better understand what you have.

I was referring to the cold FEED from your cistern INTO the cylinder. What pipe size is that in?

It sounds as if the problem may be that there is insufficient head between the cistern level and the outlet from the cylinder. What is this height in metres?

How close is the feed to the shower from the outlet at the top of the cylinder? What are the pipe sizes involved.

Tony

 

[Tad]

The cold feed is 22mm and there is approx 1.5m from the top of the tank to the bottom of the cistern. The outlet of the cylinder is in 22mm pipe with a 15mm T to the shower feed about 12" below that. The pipe work travels up into the loft space, across the hallway (approx 1.5m) then drops down to the shower unit which is probably half a metre higher than the top of HW tank.

If the problem was just a restriction of flow due to pipe size, or head of water, surely it would affect the other shower unit which operates fine. That's why I suspect the journey into the loft space as being the culprit - is there any other way of counteracting this (e.g. Surrey Flange or non-return valve) without re-routing the pipework under the floor and through a supporting wall - probably the reason why the initial plumber routed it through the loft.

 

[Tad]

Hi guys, it's me again.

On the assumption that it's the high level hot feed pipe into the loft that's the problem, where does the air come from that causes the airlock? Is it just bubbles collecting at the highest point or is it actually drawing air down the vent pipe?

If the latter, would fitting a double check valve to the vent pipe solve this problem - effectively creating a closed system but still allowing air to escape up the vent pipe?

Plan B is to do away with the Thermostatic power shower and install a separate shower pump with a thermostatic mixer. Would this arrangement still have problems with the high level feed - I assume not as the pump will be pushing the water uphill instead of sucking.

Plan C will be to do away with the power bit and just put in an electric shower off just a cold feed.

What do you think?

 

[Gasguru]

Don't install a non-return valve on the cylinder safety vent. The vent pipe allows the cylinder to boil without exploding in the event of a fault. The cylinder may also collapse if it is drained down.

Have you considered the following:

Have you fitted manual air vents on the loft shower pipework?

Have you checked the stored hot water is no more than 60 degrees. Hot water can boil around the pump impeller (due to the reduction in pressure) and will cause cavitation.

Try upgrading the feed pipework to 22mm.

Fit a non-return valve to the feed pipe just after the tee. Perhaps water is drawn back down the feed when taps are used. The non-return valve may also prevent air entering the feed as water runs past the tee.

Use a no pipe stop Essex flange ie take the hot water from the side of the cylinder. Fit "U" bit of pipe directly off the flange - this will further prevent air entering the feed.

 

[Swanmaint01]

Just read your note with interest. The correct method to draw off water from a cylinder, regarding your shower, is to use a Surrey Flange. The shower needs iit's own supply and should not be competing with other draw off points, ie taps.

Failing that, if you T down 450mm from the hot water outlet in the top of the cylinder, that should do the trick!

Good luck

 

[ScottishGasMan]

sorry to jump in on this thread but was curios about this

Have you checked the stored hot water is no more than 60 degrees. Hot water can boil around the pump impeller (due to the reduction in pressure) and will cause cavitation.

Whats cavitation? i Know ive heard of it before but dont know what it is now. and can the water around a CH pump do the same thing if its too hot?


ta
SGM

 

[Swanmaint01]

Cavitation happens in liquid when bubbles form and implode in pump systems or around the impellor. Pumps put liquid under pressure, but if the pressure of the substance drops or its temperature increases, it begins to vaporize, just like boiling water.

Yet in such a small, sensitive system, the bubbles can't escape so they implode, causing physical damage to parts of the pump or propeller.

cheers