A shower pump boosts a weak shower on a gravity-fed system: one with a cold water tank in the loft and a hot water cylinder. It is not for combi boilers or mains-pressure (unvented) systems. To choose one, work out whether your system is positive head (water flows from the shower head by gravity) or negative head (it does not, usually because the shower is close to the height of the tank). Then pick a twin impeller pump if you are boosting both hot and cold to a mixer shower, or a single impeller pump if you are boosting one supply, and a pressure rating that suits your tank and cylinder.
Check your system is suitable for a pump
- Suitable: a gravity-fed system with a cold water storage tank, usually in the loft, feeding a vented hot water cylinder. This is typical with a regular (conventional) boiler.
- Not suitable: combi boilers, unvented cylinders and anything fed directly from the mains. Salamander's installation guide, for example, says never to fit its shower pumps to the cold water mains or to a shared supply, and to contact the manufacturer for guidance before boosting supplies from a combi or water heater.
Regulation 5 of the Water Supply (Water Fittings) Regulations 1999 also requires notice to the water company before installing a pump or booster drawing more than 12 litres per minute that is connected directly or indirectly to a supply pipe. That is one reason shower pumps are designed to draw from stored water, not the mains.
Positive head or negative head?
"Head" is the height difference between the water level in your cold tank and the shower outlet. The higher the tank above the shower, the more gravity pressure you have.
Positive head
The shower head sits well below the bottom of the cold water tank, so water flows out of it by gravity when you turn the shower on, even if the flow is weak. A positive head pump uses a flow switch that starts the pump when it detects that gravity flow.
Negative head
The shower outlet is at or above the level of the tank outlet, so little or no water flows by gravity. This is common in loft conversions or where the shower is on the top floor close to the tank. A negative head pump uses a pressure switch that starts the pump when you open the shower valve.
How to check
Pump manufacturers publish a simple test using a measuring jug and a timer, so follow the one in the instructions for the pump you are considering. Salamander's, for example, is to open the shower valve fully to hot and then to cold and measure the natural, unpumped flow: it must be at least 1 litre in 30 seconds from each side (2 litres in 30 seconds mixed) for a positive head pump. Anything less than 2 litres a minute is treated as negative head. Its diagrams also show at least 600mm between the base of the cold water tank and the outlet for positive head. Some pumps are universal and work in either situation.
Single or twin impeller?
| Twin impeller | Single impeller | |
|---|---|---|
| Boosts | Hot and cold supplies separately | One supply |
| Typical use | Feeding both inputs of a mixer or thermostatic shower valve | Boosting a blended supply after a mixer, or a single hot or cold supply |
| Position | Before the shower valve | Usually after the shower valve, or on one supply |
Twin impeller pumps are the common choice for a standard mixer shower on a gravity system. Your shower valve's instructions will say how it should be fed.
Choosing the pressure rating
Shower pumps are rated in bar, typically from about 1.5 to 3.0 bar for domestic models. A higher rating gives more flow, but it also empties your hot water cylinder and cold tank faster. Before choosing a powerful pump, check:
- the capacity of your hot water cylinder and cold water tank, against the pump manufacturer's minimum recommendations (Salamander, for example, asks for 227 litres of cold storage per bathroom);
- that your shower valve and head are suitable for the pressure;
- how many outlets the pump will serve.
Installation points
- Hot water feed: a dedicated hot feed from the cylinder, usually through an approved top-entry or Essex-type flange, so the pump does not draw air from the vent pipe.
- Cold water feed: a dedicated feed from the cold water tank, taken from the opposite side to the mains inlet, never a branch off the mains.
- Isolation: full-bore isolators so the pump can be serviced; many pumps have them built into the flexible hoses. Manufacturers warn against restrictive service valves or non-return valves on the pump's supply pipework. See our guide to isolation valves.
- Flexible hoses and anti-vibration feet to reduce noise, and access for maintenance.
- Electrics: pumps are usually supplied from a fused connection unit. Electrical work in a bathroom, or adding a new circuit, can be notifiable under Part P of the Building Regulations in England, so use a registered electrician.
Fittings for the hot and cold feeds are in pipe systems and fittings, and our guide to push-fit vs compression fittings explains which suits which pipe.
Where to find shower pumps
Browse shower pumps and booster pumps, or the full pumps range. For the shower itself, see bathroom showers and shower fittings and spares. Always check each product's description for its head type, impeller type and pressure rating.
Frequently asked questions
Can I fit a shower pump to a combi boiler?
Not a standard shower pump. A combi takes its water directly from the mains, and shower pumps must not be fitted to the mains; manufacturers ask you to contact them before boosting a combi supply. If a combi shower is weak, the cause is usually low mains flow or a restricted shower head or valve.
What is the difference between a positive and negative head pump?
A positive head pump starts when it senses water flowing by gravity. A negative head pump starts when it senses a drop in pressure, so it works where there is little or no gravity flow.
Is a 3 bar pump better than a 1.5 bar pump?
Only if your cylinder, cold tank and shower can support it. A higher-rated pump uses stored water faster, so it can run the hot water out sooner.
Why does my shower pump keep running or cycling?
Common causes are a dripping tap or valve downstream, air in the pipework, or the wrong pump type for the head. Check for leaks first, then follow the manufacturer's troubleshooting guide.