How to choose a quiet water pump

Read the decibel number carefully, or ignore it

A single decibel figure is nearly always the running noise, measured at a stated distance, in conditions that suit the manufacturer. It is worth something, but only if you know what it is describing.

Ask at what distance. A figure at three metres and a figure at one metre are not comparable, and doubling the distance drops the level noticeably.

Ask what the pump was doing. Noise at full speed and noise at part load are different numbers, and a variable speed pump spends most of its life well below full.

For scale, a quiet suburban bedroom at night sits around 30 dB, normal conversation is around 60. The difference between a pump at 50 and a pump at 65 is not fifteen per cent, it is the difference between background and intrusive, because the scale is logarithmic.

Sound power and sound pressure are different numbers

This one catches out manufacturers, not just buyers, and it is worth two minutes because it explains figures that look wildly inconsistent between brands.

Sound pressure is what a microphone hears at a given spot. It depends on how far away you stand and what the room does to the sound, which is why a sound pressure figure is meaningless without a distance attached. Almost every dB number you see in pump marketing is sound pressure at one metre.

Sound power is the total acoustic energy the machine emits, independent of where you stand or what room it is in. It is the more rigorous measurement, it is what ISO 3744 produces, and it is the one used in standards and regulation.

The catch: for a machine the size of a house pump, the sound power figure runs roughly 15 dB higher than the sound pressure at one metre. Same pump, same test, two numbers fifteen apart. So a pump advertised at 47 dB and a pump declared at 62 dB may be identically loud, and you cannot tell which is quieter without knowing which quantity each brand quoted.

There is a second thing to ask: whether the figure is the measured value or the guaranteed one. A proper test adds an uncertainty margin, usually 3 dB, and the guaranteed figure is measurement plus margin. It is the number standards expect you to declare, and it is deliberately the cautious one. A brand quoting its raw measured value will look 3 dB better than a brand quoting its guaranteed value on an identical pump.

If you take one thing from this guide, take this: never compare two decibel figures without checking they are the same quantity. Ask whether it is sound power or sound pressure, if it is pressure at what distance, and whether the margin is included.

What a sound power number actually sounds like

Sound power figures feel abstract until you realise you already own a shelf of appliances that declare one. EU energy labels have carried a sound power value in dB(A) for years, on fridges, dishwashers, washing machines and vacuum cleaners. Same quantity, same weighting, directly comparable to a pump. Go and look at the label on your dishwasher, the number on it is the same kind of number as ours.

Typical declared sound power, roughly:

• Fridge freezer: about 32 to 42 dB(A)
• Dishwasher: about 39 to 49 dB(A)
• Washing machine on a wash cycle: about 47 to 62 dB(A). On a spin cycle: 70 to 82
• A Grewa Boost at rated duty: 67 for the Boost 4, 69 for the Boost 5, 75 for the Boost 8
• Vacuum cleaner: about 60 to 80 dB(A), capped at 80 by EU rules since 2017
• Petrol lawnmower: about 94 to 98 dB(A)

Why a 50 dB pump claim should stop you

Now hold that scale up against a pump advertised at 47 or 50 dB.

If that were sound power, the pump would be quieter than most washing machines on a wash cycle and about level with a good dishwasher, while moving several thousand litres an hour through a motor turning at high speed. The appliances down at that end of the scale are quiet because very little inside them moves quickly. A pressure pump is not one of those machines. At rated flow it is doing real work, and real work makes noise.

So the figure is almost certainly sound pressure at one metre. That corresponds to a sound power figure somewhere in the low sixties for a pump this size, which is to say: much the same as ours. The pumps are not fifteen decibels apart. The numbers are.

This is not us calling anyone a liar. Sound pressure at one metre is a legitimate, useful measurement and the industry has quoted it for decades. The problem is that it is usually quoted as a bare number, with no quantity named and no distance given, sitting on a comparison page next to figures produced a completely different way. Two honest numbers, no way to compare them, and the buyer carries the confusion.

The practical test: if a pump is advertised below about 55 dB with no quantity named, assume it is sound pressure and ask at what distance and at what flow. If the seller cannot tell you, the number is decoration.

The noise most people actually complain about

It is rarely the running noise. It is the starting and stopping.

A traditional pump has two states, off and full speed. Every time pressure falls to its cut-in point the motor slams to full, and every time it reaches cut-out it stops. Through a single shower that can happen repeatedly. Each transition is a thump you notice, in a way that steady running is not, and it carries through pipework and structure far better than motor hum does.

A variable speed pump changes speed to hold pressure instead of switching between two states. It starts gently, holds, and winds down. Fewer events, and softer ones. That is usually the single biggest difference between a pump you notice and one you do not, and it does not show up anywhere in a decibel rating.

While you are asking, ask what the pump does at rest. Some hold pressure silently. Others tick, or hum from the controller, and in a quiet house at night that is what you will hear.

What the motor has to do with it

A permanent magnet motor generally runs quieter than a standard induction motor at the same output, partly because it is more efficient so less energy becomes heat and vibration, and partly because it can be driven at a controlled speed rather than being switched on at full.

Cooling matters too. Some pumps use a fan, which adds its own noise and gives dust somewhere to collect. Others rely on the water and the casing to shed heat, which is quieter and has one less moving part.

Installation, which is free and often decisive

A pump bolted rigidly to a timber deck or a hollow wall turns that surface into a speaker. The same pump on a concrete pad, or on rubber feet or an anti vibration mat, can sound like a different machine.

Rigid metal pipe straight into the pump conducts vibration into the plumbing and around the house. A short flexible connector on either side breaks that path and it is a cheap part.

Then think about where. Against a bedroom wall is the obvious mistake. In a small hard sided enclosure is the less obvious one: enclosing a pump without any acoustic lining can make it louder by giving the sound a box to resonate in. And make sure the pump is properly primed, because a pump moving air with the water is far noisier than one that is not, and that particular noise means something is wrong rather than merely loud.

Common questions

What is a good decibel rating for a house pump?

Around 50 dB at a stated distance is quiet for a domestic pressure pump. Anything quoted well below that is worth questioning about the measuring conditions, and anything above about 65 will be noticeable indoors.

Will a pump cover fix the noise?

Sometimes, and sometimes it makes it worse. A purpose made acoustic cover with lining and ventilation helps. A plain plastic box traps heat and can amplify the sound. Never restrict airflow to a pump to make it quieter.

Why is my pump suddenly louder than it was?

A change in noise is usually a symptom rather than wear. Air in the lines, a failing bearing, a pressure vessel that has lost its air charge and is making the pump cycle rapidly, or something partly blocking the inlet. Worth investigating rather than living with.

Are submersible pumps quieter?

A pump sitting in water in a tank or bore is much quieter at the source, because the water absorbs the sound. The trade off is access: when something goes wrong you are lifting it out rather than walking up to it.