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Shower pump sizing: positive or negative head, single or twin
How to tell whether a gravity-fed shower needs a positive head or a negative head pump, single or twin ended, and why combis and unvented cylinders are usually the wrong system for one.
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A shower pump boosts water that is already flowing from a cold water cistern and a hot water cylinder. That is a gravity-fed system, sometimes called open vented. Choosing a pump comes down to three questions: is the system really gravity-fed, is the shower in a positive or negative head position, and does the pump need to boost one supply or two?
Check the system first
Most domestic impeller shower pumps are made for gravity-fed systems with a cold cistern and a vented hot cylinder. If your hot water comes from a combi boiler or an unvented cylinder, the water is already at mains pressure and a standard shower pump is usually the wrong product. Do not fit one to either unless the pump maker's own page says that model is for that system. Boosting the mains is a different product and can be a regulated arrangement, so talk to a plumber and your water company. Our low water pressure guide explains how to tell the systems apart.
Positive head and negative head
"Head" is the height of the water in the cold cistern above the point where you draw water.
- Positive head: the cistern is above the shower, and water already flows from the shower head without a pump, even if it is weak. A typical example is a first-floor bathroom with the cistern in the loft above it.
- Negative head: the shower is level with or above the cistern, or the water does not flow on its own. Typical examples are a loft or attic shower, a shower in a flat-roofed extension at cistern height, or a bathroom where the cistern sits very low.
A standard (positive head) pump uses the flow of water to switch itself on. If there is little or no gravity flow at the shower, a positive head pump may never start. That is where a negative head or universal pump comes in. These are designed to start without relying on gravity flow.
Borderline cases are common. If the shower outlet is only slightly below the cistern, or the pipe runs are long and narrow, ask the pump maker or a plumber which type they recommend for the measured heights.
A quick self-check
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Single ended or twin ended
- Twin ended pumps have two impellers and boost the hot and cold supplies separately. They are the usual choice for a mixer shower fed from a cistern and cylinder.
- Single ended pumps have one impeller. Depending on the model, they boost one supply, or they boost already-mixed water after the shower valve.
Read what the maker says the single ended model is for before choosing it. The Bristan single ended pump at Wickes, for example, is described as boosting "hot, cold or pre-mixed" supplies.
Pressure figures
Pumps are sold by a bar figure, often 1.5, 2.0 or 3.0 bar. A higher figure is not automatically better. It has to suit the shower valve, the shower head, the size of the hot water cylinder and the capacity of the cold cistern. A strong pump on a small cylinder can empty the hot water quickly. The shower valve maker's instructions usually state what pressure range it is designed for.
Some retailer pages show two different figures, for example a "2.0 bar" title and a higher "maximum pressure" in the specification table. If that happens, ask the retailer or maker which figure is the pump's rated boost.
Where the pump goes and the electrics
Pump makers usually prefer the pump low down, near the hot water cylinder, with anti-vibration feet and flexible hoses. The Bristan pages at Wickes give the floor next to the cylinder as their "preferred pump location". Follow the specific maker's instructions for the pipework, including the hot water take-off from the cylinder.
A shower pump needs a fixed electrical supply. New electrical work, such as a fused spur, should be done by a qualified electrician. Some electrical work in homes must be notified under the Building Regulations; in England this is Part P. Electrical Safety First recommends using an electrician registered with one of the government-approved schemes. See electricalsafetyfirst.org.uk.
The pump and its fittings are also water fittings, so the Water Supply (Water Fittings) Regulations 1999 apply in England and Wales. Scotland and Northern Ireland have their own rules. A product listed as WRAS approved has been assessed as a product. That is not the same as a correct installation.
Shower pumps at Wickes and Amazon
We haven't tested any of these. They are examples of each type, with what the retailer's page says.
We haven't tested this. Figures are as the maker states them (checked 13 November 2026).
Salamander CT75Xtra positive twin, 2 bar. Wickes describes it as "suitable for positive head gravity fed systems", twin ended, for a thermostatic mixer shower. The page lists WRAS among its certifications and a 3 year warranty.
Bristan impeller twin ended, 2.0 bar. Wickes says to install it between the cold water tank or cistern and the hot water cylinder, boosting the hot and cold supplies. Its specification table lists a 3 year guarantee. It is not described as a negative head pump.
Bristan impeller single ended, 2.0 bar. Described as a single impeller for boosting hot, cold or pre-mixed supplies, including pumping blended water from the shower valve to the head. 3 year guarantee stated.
Bristan twin ended negative head, 2.0 bar. Wickes lists it as "suitable for domestic positive and negative head applications" and says it does not require a gravity flow of water to start. The title says 2.0 bar and the specification table lists a 4 bar maximum pressure, so check which figure applies to your use. 3 years guarantee stated.
Salamander CTFORCE30TU universal twin, 3 bar. Described as a universal (negative head) brass pump, "suitable for positive and negative head gravity fed systems". WRAS is listed among its certifications, with a 5 year warranty.
Stuart Turner Showermate Standard S2.0 bar single. The Amazon UK listing says Showermate Standard pumps are for open vented (gravity fed) systems in positive head applications.
Can I fit a shower pump to a combi boiler?
Not a standard gravity shower pump. A combi supplies hot water at mains pressure, and these pumps are made for a cistern-fed system. If the flow from a combi is poor, read the low water pressure guide and ask a plumber. Any work on the boiler itself is for a Gas Safe registered engineer.
Do I need a negative head pump for a loft shower?
Usually, if the shower is at or above the level of the cold cistern. Check with the pump maker using your measured heights.
Who should fit a shower pump?
A competent plumber for the pipework and a qualified electrician for the supply. Many pump makers also make their guarantee conditional on installation to their instructions.