Pumps suit gravity systems with adequate flow and insufficient head. The pump choice matters; the feeds matter more. And you cannot pump directly off the mains – that is not permitted.
First, does your system suit one?
Boosting a mains supply with a domestic pump is not permitted under the Water Supply (Water Fittings) Regulations, because it can draw pressure down for your neighbours. Pumps on gravity systems draw from stored water, which is why those are allowed. Anybody offering to pump your mains is proposing something they should not.
| Your system | Will a pump help? |
|---|---|
| Cold tank in the loft, hot cylinder in a cupboard | Yes. This is exactly what pumps are for |
| Combi boiler | No. Mains pressure already, and it cannot be pumped |
| Unvented cylinder | No. Mains pressure already |
| Electric shower | No. Flow is limited by the heating element |
| Gravity hot, mains cold | Single-impeller pump on the hot only, with care over balancing at the mixer |
| Poor mains flow everywhere | No – that is a supply problem. See low water pressure |
The distinction that decides it: a gravity system usually has adequate flow and insufficient pressure, which is what a pump adds. A restricted mains supply has adequate pressure and insufficient flow, and a pump cannot create water that is not arriving.
The feeds matter more than the pump
Most failed pumps were not faulty. They were starved, or they were drawing air, and both are installation problems rather than product problems.
- A dedicated hot draw-off from the cylinder. A flange, or an Essex or Surrey fitting, taking water from a point where it will not draw air. Teeing into the existing hot pipe near the vent is the classic mistake, because that is precisely where air collects.
- A dedicated cold feed from the tank, not shared with other draw-offs that will rob it.
- Adequate tank capacity. A pump moves a lot of water. A small cold tank will empty during a long shower and the pump will run dry, which destroys it.
- Adequate tank refill rate, so it keeps up.
- Full-bore isolation valves on both feeds, so the pump can be serviced.
- Flexible connections on the pump, to stop vibration transmitting into the pipework.
- No restrictions – undersized pipe, sharp bends or old gate valves on the feeds all cost you what the pump is adding.
Air ingress is the commonest cause of a pump that cuts in and out, hunts, or makes a rattling noise. It is nearly always a feed taken from the wrong place.
Positive head, negative head, and why it matters
Getting this wrong means a pump that will not start.
- Positive head – the water level in the cold tank is above the pump, by a stated minimum, and the pump is started by the flow when you open the shower. Simpler and cheaper.
- Negative head – the pump is at or above the tank water level, so there is no gravity flow to start it. A negative-head pump has a sensor that detects the pressure drop when you open the outlet. Also called universal head.
- Single or twin impeller – twin pumps both hot and cold, which is what a mixer shower needs. Single pumps one side, for a system with gravity hot and mains cold.
- Rating – bigger is not better. An oversized pump can outrun the tank’s ability to supply it, which is how pumps run dry.
A positive-head pump in a negative-head position simply will not start, and it is a surprisingly common misdiagnosis – the pump gets returned as faulty when the problem was the specification.
Noise, and where it goes
Pumps are not silent, and where they are sited determines whether that matters.
| Position | Noise | Notes |
|---|---|---|
| Airing cupboard next to a bedroom | Audible, and at six in the morning | The most common position and the most common complaint |
| Loft | Further from living space | Needs frost protection and proper boarding, and access for servicing |
| Under a bath | Very poor access | Avoid – it will need servicing eventually |
| Downstairs cupboard or utility | Good, where the feeds allow | Frequently the best compromise |
Mitigations: anti-vibration mounts, flexible connections on both feeds and the outlets, a solid level base rather than floorboards, and not fixing it rigidly to a stud wall that will act as a soundboard. All of that is standard on a good installation and frequently absent on a poor one.
The alternative worth considering
Before fitting a pump, it is worth asking whether converting to an unvented cylinder would be a better answer.
- A pump solves the shower. It is cheaper, quicker, and it leaves the loft tank in place.
- An unvented conversion solves the whole house – mains pressure at every outlet, no loft tank, no pump. It costs considerably more, it is notifiable work, and it depends entirely on whether the incoming main can feed it.
The measurement decides: static pressure and dynamic flow at the stopcock. If the main is good, conversion is worth pricing. If it is marginal, a pump is the sensible answer and a conversion would disappoint – see converting from vented to unvented and shower types and water pressure.
Weak shower on a gravity system? A pump usually transforms it – and the feeds matter more than the pump.
Questions we get asked about this
Will a pump fix my weak shower?
If you have a gravity system with a loft tank, yes, and it will transform it. On a combi or unvented cylinder it will not help, and pumping the mains is not permitted.
Why does my pump cut in and out?
Usually air being drawn into a feed, or the tank running low. Both are feed problems rather than pump faults, and both are why the pump will eventually fail if left.
My new pump will not start.
Frequently a positive-head pump in a negative-head position – there is no gravity flow to start it. The specification is wrong rather than the pump.
Are pumps noisy?
They make a noise, and where it is sited determines whether that matters. Anti-vibration mounts, flexible connections and a solid level base make a real difference.
Should I get the biggest pump available?
No. An oversized pump can outrun the tank’s ability to supply it, which makes it run dry and destroys it. It should be matched to the system.
Is an unvented conversion better than a pump?
It solves more – mains pressure everywhere rather than at one shower – and it costs considerably more and depends on the incoming main. The flow measurement at the stopcock decides which makes sense.
Regulations and standards referenced on this page
- The Water Supply (Water Fittings) Regulations 1999 (SI 1999/1148) — UK Statute Law Database
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