I'm no expert, and am certainly no plumber, but I have my head around the difference between pressure and flow etc because fo the job I'm in.
You say that your shower STARTS OFF fine, but slows to a trickle. this means that you have some STORED PRESSURE in your system, but that the FLOW (to continuously replenish that pressure) is not sufficient.
Either that, or your incoming pressure is sufficient to fill the pipes above your shower (hence you get a short "gravity fed" shower, but the pressure is unable to give you more than a trickle at the eight of your shower head.
Neither of the above have anything to do with your shower.
First step is to establish whether your problem is incoming pressure or incoming flow. These are two different things.
Do you have plenty flow from your kitchen tap? If so, then your problem is not flow into your property, but could still be mains pressure, or flow within your property.
Take the shower head off and put your finger over the end of the hose. Can you hold the water back easily? If so, then the problem is likely to be a pressure problem. If the pressure slowly builds and builds until you can't hold it any more, then you are looking at a flow problem within your property. [i'm guessing by the fact that the shower "starts off" fine, that the shower hose isn't twisted or kinked]
Pressure problem: first step is to call the water board. They are legally obliged to give you 1-bar at the entrance to your house. This will give you a 10m rise or "head" of water. Is your shower head 10m (30ft) above the pavement level? They can come out and measure the pressure at your local access. Mine was 1.25Bar.
I was all set to install a "booster" system and breakout tank to incerase the pressure to my combi boiler, but then, without warning, the incoming pressure rocketed to 3 bar. Water board must have sorted something or pulled a drowned rabbit out of the mains or something like that! So I'm sorted now.
If this is your problem, and the water board can do nothing about it, then you can think about indirectly boosting the mains input. You can't pump directly from the mains to boost the pressure (because this will suck water away from your long suffering neighbours), but if you use the incoming supply to fill a tank (downstairs), then you can pump water from the tank to the combi. Not ideal, but could be the best option.
To resolve a flow problem, you need to understand where the restriction is. try other taps in the house to isolate where you might be getting a resistance to the flow. Are you getting fast flow from your kitchen tap? If so, the it's unlikely to be a flow problem in your property, and more likely a "mains" flow problem. If that's the case, then investigate something called an "accumulator". This will give you a pressurised "cylinder" of cold water inside your house, giving your combi boiler a ready supply until the pressure drops back (from lack of incoming flow to replenish the supply). At a push, you could also use the breakout tank and pump to get the same result. Until you pump all the water out of the tank that is. These solutions will give you a better shower (but not quite so long)
If you get plenty flow in the kitchen, and you've ruled out pressure, then you must have a restriction in your house somwhere. To get to the bottom of this, you need to check the pipes in your house. Do you have a long run of narrow pipe? several metres of 15mm pipe will not do your flow much good, which is why plumbers use 22mm wherever they can, and shorter lengths of 15mm to go to taps etc. Bath taps get 22mm almost right up to the outlet. Similarly, might you have an inline shutoff valve for your shower which is only partially open? (you should probebly have checked for that earlier actually.
I'm afraid I can't give you much more info than that just now. These are the thoughts of a bored engineer, rather than a time served plumber, but these are all the things I'd check for starters.
Failing all that, a local swimming pool is likely as good a place as any to get a decent shower!
Guy
