Accumulator Tank: Do You Need One for Better Water Flow?

What an accumulator tank actually does

An accumulator is a pressure vessel connected to your incoming cold main. Inside is a cushion of air, usually behind a rubber membrane or bladder. When nobody is using water, the mains slowly fills the vessel and squeezes that air. When you open several taps at once and the mains alone cannot keep up, the compressed air pushes the stored water back out to join the supply.

The result is more litres a minute at the outlets for as long as the stored water lasts, and then the flow falls back to what the mains alone can deliver. The vessel refills when demand drops. That is why accumulators are sized by how much water you use at your busiest moment and for how long, not by the size of the house.

Two words cause most of the confusion here. Pressure is how hard the water pushes when nothing is running, measured in bar. Flow is how much comes out when a tap is open, measured in litres per minute. An accumulator helps flow. It cannot make static pressure higher than your mains provides.

Test your mains before you buy anything

You can do a rough version of the test yourself. With every other tap and appliance off, open the kitchen cold tap fully and time how long it takes to fill a bucket of known size. Divide the litres by the seconds and multiply by sixty to get litres per minute. Then open a second cold tap, ideally the bath, and see how much the kitchen flow falls.

• Good pressure and good flow that holds up with two taps open: you do not need an accumulator.
• Good pressure but flow that collapses when a second tap opens: this is where an accumulator earns its place.
• Low pressure even with one tap open: an accumulator on its own will not help, and the supply itself needs looking at.
• Flow that dropped suddenly after years of being fine: suspect a fault, not the mains, and do not buy a tank to cover it.

An engineer will measure static pressure with a gauge and dynamic flow at a set pressure. Unvented cylinder and combi makers each publish the minimum supply they need, so these two readings decide what can be fitted.

When an accumulator makes an unvented cylinder possible

An unvented cylinder gives mains pressure hot water at every outlet, but it can only pass on what the mains supplies. In older London houses the supply pipe from the street is often narrow, long or shared, so a static reading looks fine while the flow sags as soon as a shower and a basin run together.

Adding an accumulator on the cold supply to the cylinder and the cold outlets can carry the house through the morning rush on that kind of supply. It is a common way to make an unvented cylinder installation workable where the flow test is borderline. The design has to include the cold side as well as the hot, otherwise the showers go cold as the hot flow outruns the cold feeding the mixer.

If you are still deciding between systems, including whether a combi or system boiler suits the house, our unvented vs vented cylinder guide explains what each one needs from the mains, and how to choose a hot water cylinder goes through the rest of the decision.

When an accumulator will not help

In each of these the answer is repair, not storage. It is worth ruling them out first, because they are cheaper to fix than an accumulator is to fit.

• A failed or partly closed stopcock, or a pressure reducing valve that has stuck, which throttles the supply inside your own house. Our guide to the pressure reducing valve explains how to spot that one.
• Low static pressure from the street at every time of day. More storage at the same low pressure still delivers weak water.
• A single weak tap or shower, which is almost always a local problem such as a blocked cartridge, a scaled shower head or a clogged strainer.
• Weak hot water on a vented system fed from a cold water tank in the loft. That is gravity, not the mains, and our guide to low hot water pressure covers the real fixes.

Accumulator, booster pump or a new supply pipe?

These three are often confused, and they solve different problems. An accumulator stores mains water and gives it back at peak times, with no pump and no noise. A booster pump increases pressure, and a pumped system needs care: water regulations limit how much a pump may draw directly from the mains without the water company's consent, which is why pumped set ups normally draw from a break tank or a pumped accumulator rather than straight off the main. A new, larger supply pipe from the boundary fixes the cause rather than working round it, but it means digging and usually involves the water company too.

Where the street pressure is good and the only weakness is an old, narrow supply pipe, replacing the pipe is the cleaner long term fix. Where replacing it is impractical, an accumulator is often the next best thing. Where pressure itself is too low, a pumped solution may be the only way to get mains style performance.

Space, siting and upkeep

Accumulators are not small. The size needed for a family house can rival a hot water cylinder, and they are heavy when full, so the floor has to take the weight. They usually go in a basement, under the stairs, in a garage or in a utility room on the cold main, close to where the supply enters.

Keep an accumulator somewhere cool and frost free. It holds cold drinking water, and stored cold water should stay cold, so a warm airing cupboard or a spot beside the boiler is the wrong place for it.

The air charge needs checking from time to time, and the membrane can fail after years in service. A failing accumulator shows itself as flow that surges and drops, taps that spit air, or banging pipes as the pressure swings. If your pipes have started knocking since an accumulator went in, our guide to water hammer explains what else can cause it.

Is an accumulator worth it?

It is worth it when three things are true: the static pressure is decent, the flow drops away when more than one outlet runs, and you want a mains fed system such as an unvented cylinder, or a mixer shower with a combi boiler. In that case it can be the difference between a system that works and one that disappoints every morning.

It is not worth it when the flow test is already good, when pressure is low at every hour of the day, or when a fault inside the house is the real cause. Any installer who recommends one without measuring both pressure and flow first is guessing.

Book a flow test or a survey

We measure the incoming mains before recommending any hot water system, and we will tell you if the supply is fine as it is, if a fault needs fixing, or if storage or a new supply pipe is the honest answer. Every unvented cylinder job is carried out by a G3 qualified engineer, and we cover the whole of London.

Call 020 4652 5111 to book. Our hourly rate and call-out charge are on the pricing page, agreed before we attend.