Do I need a shower pump?

Whether weak flow is a pump problem at all, the system types that can and cannot be pumped, and what to check before spending anything.

Difficulty: beginner
Time: 20 min
Read: 3 min
Safety: caution

Short answer

A pump only helps a gravity-fed system with a cold tank in the loft and a hot cylinder. If you have a combi or an unvented cylinder, a pump is not an option and the answer lies elsewhere. Measure the flow rate first — many weak showers are a scaled head, not a system fault.

Pumps are sold as the cure for every weak shower, and for one particular system they genuinely are. For the other two they are neither permitted nor useful, so the first question is always which system you have.

Safety

A pump may only be fitted to a gravity-fed system with stored hot and cold water. It must never be fitted to a combi boiler, an unvented cylinder or a direct mains supply — doing so can damage the appliance and risks drawing water back into the public main, which is a breach of the Water Supply (Water Fittings) Regulations. Pump wiring is fixed electrical work and needs an electrician.

What you'll need

A bucket of known volume
A watch or phone timer

Step by step

Identify the system.Cold tank in the loft plus a cylinder in a cupboard means gravity — pumpable. A boiler with no tanks anywhere means combi — not pumpable. A cylinder with no loft tank means unvented — not pumpable.
Measure the flow rate before deciding anything.Fill a 10-litre bucket from the bath tap and time it. Divide 600 by the seconds taken to get litres per minute. Below 8 is weak; above 12 is good.
Rule out the cheap causes.Descale the head, check the hose, clean the inlet filter, and confirm the stopcock and isolating valves are fully open. These account for most weak showers.
Check the head of water on a gravity system.Measure vertically from the bottom of the cold tank to the shower head. Under about a metre, a mixer will always be feeble and a pump is the correct answer.
Confirm both supplies can be pumped.A twin-impeller pump boosts hot and cold together. Both must come from storage — a shower with a mains cold and a gravity hot needs a single-impeller pump and careful design.
Check the cold tank can keep up.A pump can draw 20 litres a minute or more. A small or poorly fed loft tank empties and the pump runs dry, which destroys it quickly.
Take the hot feed from a Surrey or Essex flange.A pump drawing from a plain cylinder outlet pulls air off the top of the cylinder and becomes noisy and unreliable. This is the detail installers most often skip.
Plan for noise and access.Pumps are audible and need servicing. An airing cupboard is fine; under a bedroom floor with no access is not.
Consider the alternatives if you cannot pump.On a combi, a higher-flow boiler or a flow-optimised head. On an unvented or mains system, an accumulator. On any system, fixing the actual restriction first.

Tips

Do the bucket test before and after any change. It turns an argument about perception into a number.
A pump makes a weak system fast; it cannot make it hotter or make the cylinder bigger. Expect to run out of hot water sooner.
If the cold tank was sized for taps and a bath, it will need upgrading before it can feed a pumped shower.

Common mistakes

Fitting a pump to a combi or unvented system — Those supplies are already at mains pressure. Pumping them is prohibited, can damage the appliance, and creates a backflow risk to the public supply.
Buying a pump before descaling the head — A scaled head can halve the flow. Fitting a pump behind it simply forces more water at the same blockage and disguises a free fix.

Alternatives

Fix the restriction: Descale the head, replace a kinked hose, clean inlet filters and fully open every valve. Free, and it resolves a large share of weak showers.
Cold water accumulator: A pressurised vessel that stores mains water and releases it at a higher flow when demanded. Suits mains-pressure systems where a pump is not allowed.
Whole-house pump: Boosts every gravity-fed outlet rather than the shower alone. Costs more, but it also fixes slow baths and weak basin taps.

Questions people ask

How do I know if my system is gravity fed?

Look in the loft. A cold water storage tank up there, feeding a hot cylinder in a cupboard, is a gravity system. No tanks at all means a combi or an unvented cylinder, neither of which may be pumped.

Will a pump increase my water bill?

It increases how much water you use, because a stronger shower uses more per minute. It also increases hot water demand, which usually shows up on the fuel bill before the water one.