(see previous posts - the saga continues!)

So following advice from The Scruff and others, I now have a shiny new TMV, Immersion element and various new pipework to match. All in aid of making the pumped upstairs shower work reliably. I have also cleared a vast amount of scale and crud from the loft tank as this was fouling the inlet to the pump and stopping it.

Fundamentally the main remaining issue is the simple head of water - natural flow with the shower at the normal height is only about 1L/min, which is only *just* enough to trigger the flow-switch in the shower pump.

To fix this, I'd like to raise the loft tank, I can get about another meter fairly easily, which should (I think) make an appreciable difference. BUT... I'm worried there won't he enough head pressure in the mains to fill the tank at the higher level. Flow into the tank when empty is only about 2 - 2.5 L/min - will raising the tank another meter in the loft compromise this?

Also (see previous posts if you want) ALL the cold taps and other cold water in the house (except this one shower) feed from the rising main; we already get odd noises after flushing if the loft tank is also drawing water...

The house is about 100 years old, so quite tall - loft is about 6-7 M above ground level and all the supply pipework looks to be 15mm

Can I safely raise the tank?

Could/Should I be pumping the rising main in the loft (or elsewhere) to improve flow/head?

Thanks as always.

You can safely raise the tank which will increase the pressure only slightly but will not change the flowrate to the shower (unpumped).
You cannot pump mains water so I would expect the existing flowrate of the mains into the tank will not change.

Surprised that raising the tank by 1M will not change the flow into the shower pump?

I'd have thought that flow would be governed by pressure (i.e. Head) and diameter of pipe. Current total head of water is only about 1M, so doubling this would make a difference, I'd have thought?

Good news that the rising main won't need to be adjusted; though I am still slightly concerned as I foresee that flow into this will diminish as demand at other cold taps increases.

I suppose the 'right' solution for that would be to make some/all of the other cold taps use the header tank instead of the main... Definitely out of scope on this project!

It will make a slight difference. Raising the pressure from 0.1 to 0.2 bar

What's the flow like at the kitchen tap? 2.5l is very low I'd be tempted to change the inlet valve to the cistern!

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The mains water flowrate will be shared by the number of taps that are turned on so yes the flowrate into the tank can fluctuate.
The supply flowrate to the pump is determined by the size of the supply pipes. For example if they are in 15mm at the moment, by increasing the size to 22mm, you will get a greater flowrate (pressure will be the same no matter what the size of pipe). Also the number of restrictions like bends can hamper the flowrate.

Hi and Happy N.Y to the Experts and readers.
Can't for the life of me remember seeing a set up with TRV on the hot inlet side of a twin pump!
In my small brain the pump is trying to "suck" from the TRV and this must play havoc with the internals!
My own system works great no TRV before the pump only after on 15mm to individual services and this arrangement works fine, also a TRV for 15mm might not cope with a bath/shower supply.
My advice, for what it is worth, would be to try without TRV on inlet to pump before you embark on tank lift.

Edit: forgot to mention my pump is on the floor level of the cylinder and not like yours on a shelf half way up the cylinder with subsequent head pressure loss!

Thanks all.

TMV is on the hot inlet side to help regulate temp from the solar HW in summer (this can easily run 80C ++ and will damage the pump etc); The pump has flow-switches that only switch it on when the natural flow exceeds a specified value; Salamander spec this as 2L/min natural flow, but experimentally 1L/min seems *just* sufficient, and this should prevent the 'sucking' problem. The exercise here is to boost that natural flow even slightly so that we get reliable operation of the pump instead of, as at present, it giving up the ghost about 1/2 the time when the head in the tank drops.

Position of the pump in the cupboard is (should be) irrelevant, since the required natural flow is governed by the head pressure to the shower head (I get about 2L/min by lowering the shower head to the level of the bath taps...).

Pressure at the kitchen tap is good enough that I can't measure it by holding a jug under (it just splashes straight out, in contrast to the situation in the loft, similarly at the Bath tap as well) so I don't think we have a problem there.

Changing the inlet to the tank is definitely in the plan, it is massively coated in scale outside and I dread to think what the internals are like... Ideally I'd change the tank too, as demand will only increase with the kids growing up, but the loft hatch will only accommodate a 25 Gal one; so adding a second in series would be the way forward there (and out of scope for the time being).

Upping the head pressure by even a small amount looks worthwhile - all I need is to get to the point where it will reliably trigger the shower pump. There are a few bends in the cold supply to the pump, but these are all essential due to the installation; pipework is in 22mm as far as the junction where the cold feed splits to the TMV/Pump Cold-Side and 15mm thereafter (all within manufacturer's spec for the pump).

At some point; an extension and/or revision of the whole bathroom is on the cards, so I can change a number of things then - with a bigger capacity in the loft, I could get the downstairs shower (cold-feed Electric) and bathroom fed from the attic as well, which should even out some of the pressure variation in the supply to the loft, fit new HW tank with proper shower flange etc etc...