p*ss poor flow rate to newly installed shower

Ok, I thought I was on a roll with this one, situation is this:
Gravity system, CW tanks in loft with 28mm feed to large HW tank in basement (approx 7-8 metres of head between bottom of CW tank and top of HW tank).
I have installed a new thermostatic shower
http://www.victoriaplumb.com/tec10.jpg (also in the basement which has a large (8" wide) shower rose on it. The head of which is approx 1 metre above the top of the HW tank. I have taken 2 x 22mm feeds from both the HW tank (22mm tee off main outlet), and a tee off the 28mm CW feed to the tank. These run approx 3m under the floor up to the shower valve from which they rise approx 1m (in 15mm) to the shower rose (only 1 or 2 elbows max). Having connected it all up, the flow from the shower is absolute crap. The manufacturer states that the shower works with pressures from 0.5bar to 2 bar, what I get is basically a dribble out of all the holes. When I take the shower head off, I get a ok flow from the pipe, I haven`t measured it yet to see litres per minute or anything.
I don`t understand why I get such a poor flow rate when I have a decent head between tanks, my thoughts are as follows:
1.) The feeds to the shower are in 22mm but in the last 1/2 metre reduce to 15mm in order to connect to the shower valve which has 15mm compression inputs. 2.) If you look at the inlet to the shower rose when its unscrewed from the supporting pipe, you can see a white 'valve' which I can only assume is restricting the flow further. 3.) I have tee'd off the 22mm outlet from the HW tank rather than use a Surrey or Essex flange. 4.) The rose is larger than normal and the manufacturer isn`t telling the whole truth by saying it 'works' within those pressure parameters 5.) When I turn the tap on to the bath which is in the room above the shower, the flow rate is excellent (using the main 22mm feed from the HW tank).
What`s going wrong, dare I say it - do I need a pump? I though pumps were only designed for setups where the head available is next to nothing.
John
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7-8 metres is giving you about 0.6 bar isn't it? (30ft is one atmosphere is 1 bar - approximately) so your head is at the low end of the manufacturer's requirement and all the wiggles will make it worse.
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Chris Green ( snipped-for-privacy@x-1.net)

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This may be a flow regulator, which many showers are fitted with these days. Some have them where the hose meets the mixer. Remove this and see what happens.

Should not be a problem with that head and height. No air will be sucked in.

You should get a decent shower from that head. Just make sure there are no restriction at the top and bottom of the hose first. They may have put flow regulators in the hot and cold tails to the mixer too. You have approx 0.7 which should give a decent shower. The height from the shower head to the tanks water line is what you measure.
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certainly should - I get a decent shower with around 6' head (although I did need to get a large bore "for gravity systems" shower hose, the old one was for electric and was quite narrow giving a very poor flow rate)
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(high) minimum required by the mixer. Sounds like the valve or head is restrictive. Suggest you measure the flow with & without the head. If it's fine w/o the head then that sounds like the problem area. Just see how long it takes to fill a 2gal (10l) bucket & calculate from there.

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John wrote:

No, you ned a decent mains pressure system.
Rip it out and start again. :-)

In your case, that would seem to be the case. :-(

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