Low Pressure in the Shower: Causes by System Type
Gravity, combi or unvented, the fix is completely different depending on what you have.
Know your system first
A cold tank in the loft and a hot cylinder = gravity: your pressure is set by the height of that tank, and no shower head will change physics. A combi boiler = mains pressure: the boiler's flow rate is the ceiling. A sealed cylinder = unvented: mains pressure throughout, so a weak shower means a fault, not a design limit.
Cheap wins in order
Descale the shower head (overnight in vinegar). Check the isolation valve behind the shower is fully open, half-closed valves after past work are absurdly common. Clean the inlet filter on mixer showers. Only then think about pumps or boiler upgrades.
The right big fix per system
Gravity: a twin impeller pump or a move to unvented. Combi: check the boiler's flow rate against the shower's demand before blaming pipework. Unvented: a pressure-reducing valve set wrong or a failing expansion vessel, both engineer jobs, both quick.
Pressure and flow are not the same thing
Pressure is force, flow is volume, and a shower needs both. A gravity-fed system has plenty of flow but little pressure, which is why a mixer designed for mains supplies disappoints on it. Establishing which of the two you are short of decides whether the answer is a pump, a different valve, or a clean.
The cheap causes, checked first
A scaled shower head, soak it in descaler overnight. A partly closed isolation valve on the supply, often left that way after other work. A blocked filter in the shower valve inlet. In hard-water London these three account for most complaints, and all are free or nearly so to resolve.
When a pump is and is not the answer
A pump can transform a gravity-fed shower. It cannot legally be fitted to a mains-fed supply, and it will not help a scaled cartridge. If your cold runs strongly and the hot does not, you have a gravity hot supply and a pump is the right conversation. If both are weak, the problem is the incoming main.
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