No water from shower until i give the sink tap a quick blast.

[Quinla01]

Hello,

We have a pretty standard gravity fed system. Two cold water and one expansion tank in the loft and vented cylinder in the airing board airing cupboard on 1st floor. Pressure is terrible on both ground and first floor so we have a 4 bar pump that blasts both hot and cold water around the house and generally does a really good. The problem we have is with the shower in our en suite which was also working well until recently after the controls seized up and it was replaced with a Grohe system ( I was told they are good quality).

The components are:

• Grohe Rapido T Universal Concealed Thermostatic Mixer - 35500000
• Grohe Grohtherm Cube Thermostat 2-Way Diverter Bath Shower Trim - 19958000

Since having it fitted, when I turn on the shower, water doesn't flow immediately. Not a trickle and i don't hear the pump activate. If i do nothing at all, after about 10 seconds the pump will usually kick in and water will start to flow. Alternatively, if i give the basin tap in the same bathroom a quick blast the pump immediately kicks in and the shower works. So it's a bit annoying, but not end of world as I now instinctively turn on the basin tap, turn on the shower, turn off the basin tap. That said, i'd still like to get to the bottom of why I have to do this but without spending a lot of money to overcome a minor annoyance. The plumber who fitted it has gone AWOL so getting him back is not an option.

From what little i know about plumbing and heating systems and what research I've done online it sounds like it could be an airlock in the pipework coming into the shower. My logic is the airlock may be preventing flow required to activate the pump. It could also explain why even when the shower is on, despite the pressure being OK, it's only about 70% of the pressure i get from another shower in the main bathroom but this may just be because they are different showers. The previous shower was definitely a little more powerful. That said, if it were an airlock, would the air not be blasted out the shower once the pump kicks in and water starts flowing? Could the air gradually be coming in overnight (i shower in the morning). If so, where would it come from? We have no leaks. I've seen online videos of people showing how to remove an airlock from a tap with a hose pipe by connecting mains cold and hot to blast out the air but not sure how you'd accomplish something similar with a mixer shower most of which is concealed behind tiling and both hot and cold are on the pump.

Any advice / suggestions hugely appreciated.
Thanks

 

[ShaunCorbs]

I would say the pump is the wrong type for the new shower (need a negative head ) as it sounds like the shower valve is too restrictive for the gravity set up and your not getting enough flow for the pump to sense it to come on

 

[SimonG]

Def wrong pump. Negative heads the way to go.

 

[Quinla01]

Hummm, that's a shame. Here's the pump i have:

"stuart-turner-monsoon-twin-impeller-positive-shower-pump-46418-101360-18613"

I'm not about to go replace the pump when I have a really quick work around. Maybe at a later date though if i replace other showers & taps in the house so really good to know.

Out of interest, what is it on the shower that should have flagged alarm bells with the plumber that it wouldn't work with my pump? Is there some markings or something in the specifications of the shower that should have been obvious? To be fair, i didn't take any advise from the plumber when purchasing the shower.

Thanks again,
[automerge]1589877667[/automerge]

Def wrong pump. Negative heads the way to go.

Wrong pump or wrong shower? Our pump is on the first floor ( next to cylinder) as are all the showers. The tanks are in the loft so in that respect, from what i've read a positive pump is the correct type. Or maybe not?

 

[ShaunCorbs]

Min pressures normally high pressure ones start at 0.5 bar and low ones go below 0.2 bar

 

[DripsnDropsBognor]

I think most manufacturers state that you should be able to get 0.2 litres per minute under gravity pressure before the flow switch kicks in and activates pump. Could you install an air switch from shower tonsuring cupboard easily enough?

 

[SimonG]

Positive head works on flow, but need the head for it to register. Negative head works on a pressure differential.

 

[Quinla01]

If I understand you correctly, then i'm pretty sure we have the correct type of pump. After all, it's working really well on everything else in the house and used to work fine on the shower that was replaced. So that brings us back to the shower. Someone mentioned my shower may need a negative head pump if the flow isn't sufficient. I'm a bit lost at this point but can tell you that when the pump does kick in, the pressure and flow is immense so I don't think it's a physical restriction on anything end to end.

As to the question about an air switch, i'm not even sure what that is and what it would achieve.

Thanks again everyone.

 

[DripsnDropsBognor]

Shower you have is supposed to operate at minimum 1bar of pressure. For shower pump to activate the shower unit needs to produce 0.2 litres per minute under gravity to kick the pump over that's why you are having to kick it with the basin tap. Some pumps you can add a remote air switch to manually kick the pump on as you are doing with the basin tap usually somewhere you can reach without standing under the head
I'm not sure if your pump is compatible with this but I have seen it done

 

[Quinla01]

Shower you have is supposed to operate at minimum 1bar of pressure. For shower pump to activate the shower unit needs to produce 0.2 litres per minute under gravity to kick the pump over that's why you are having to kick it with the basin tap. Some pumps you can add a remote air switch to manually kick the pump on as you are doing with the basin tap usually somewhere you can reach without standing under the head
I'm not sure if your pump is compatible with this but I have seen it done

Got it, that makes sense now. I can pretty much reach the tap from the shower so it's pretty much the same as an air switch i guess. One other data point i noticed yesterday. On the shower system, we have a large shower head high level and then a small shower head on a flexible hose at a lower mid boy height level (i never use it). The smaller head on the hose does not have the issue (pump kicks in immediately) so another way to activate the large high level shower head from within the shower is to quickly turn the hose head on (turn left) then immediately turn large shower head on (turn right). Job done. First world problem hey. Still a bit annoying

thanks again for explanation.

 

[Stigster]

As you have now learned, the pump needs a minimum amount of water flowing through under gravity to move the flow switch and start the pump.

To get this minimum flow to start the pump, the important factor is not the location of the pump in relation to the cylinder or header tank, but the available head between the shower outlet and the head of water in the tank above. So, your highest shower head is too close to the header tank and will not produce enough flow to start the pump.

The lower shower head (and the tap you've been starting the pump with) is low enough below the header tank to get the required flow through the pump to start it. That is why the tap and the lower down shower outlets can both start the pump but the main shower head cannot.