Direct Feed and Gravity Feed Rainwater Systems Explained

A whole-house rainwater system collects water from your roof, filters it and stores it in a tank, usually buried in the garden. The question this page answers is what happens next: how the water actually reaches your toilet cisterns and washing machine. There are two arrangements. In a direct feed system, a submersible pump in the underground tank pushes water straight to each connected appliance whenever it is needed¹. In a gravity feed system, the pump moves water up to a small header tank in the loft, and gravity then feeds the toilets and washing machine from there².

The choice matters for three practical reasons. A direct feed system is the only option if your home has no loft space, for example a flat roof or a converted loft¹. A gravity feed system keeps working through pump failure or a power cut, because the header tank holds a reserve that drains down without any electricity¹. And a gravity arrangement uses far less energy over a year, because the pump fills the header tank in batches rather than starting for every single flush³.

Both systems deliver the same thing at the tap end: non-potable water for toilets, washing machines and the garden, which the makers estimate can cut your incoming mains water use by around 50 per cent when those uses are added together². Neither is for drinking. This page explains how each works, what each needs, and how to tell which one suits your house.

The two systems side by side

The difference between the two is entirely about where the water goes after the underground tank. In a direct feed arrangement the pump is the whole delivery system: every toilet flush and every wash cycle is a fresh demand on it. In a gravity arrangement the pump works ahead of demand, filling a loft tank, and the appliances draw from that tank by gravity, exactly the way a traditional cold water storage cistern feeds a gravity-fed shower.

The plumbing trade uses the same distinction for private water supplies generally. Dwr Cymru Welsh Water describes a direct system as one where the plumbing is connected straight to the supply, and an indirect system as one where the water is stored or pumped into a tank which then feeds part or all of the plumbing, with the tank sometimes also refilled from the mains⁵. A rainwater system with a header tank is an indirect system in that sense, and the mains top-up that most of them include is exactly the kind of arrangement the water regulations govern, covered on our page on rainwater regulations.

How a direct feed system works

In a direct feed system, water is fed directly from the submersible pump in the underground tank to each appliance². The pump is pressure sensitive, so it reacts automatically when water is needed¹. The sequence runs like this:

1. You flush a toilet, or the washing machine calls for water.
2. The pressure in the rainwater pipework drops.
3. The submersible pump registers the drop and starts.
4. It pumps until the cistern or machine is full.
5. It switches itself off automatically².

That on-demand behaviour is the system's main appeal: there is no loft tank to find space for, no extra plumbing between the pump and the appliances, and the pressure at the appliance is the pump's pressure. It is the arrangement the maker recommends when a property has no loft space, for example a flat roof or a loft that has already been converted¹.

The pump itself sits submerged in the tank. One maker describes its HydroForce pump as a submersible, pressure sensitive, electronically controlled rainwater tank pump, centrifugal and roto-dynamic with no reciprocating parts, manufactured in the UK and designed for rainwater harvesting systems⁶. The absence of reciprocating parts is the maker's own point about wear: fewer moving components in contact with the water. You can read more about pump types on our page on rainwater pumps.

The weakness of a direct feed arrangement is that the pump is a single point of failure. Every appliance depends on it, and it depends on electricity. To cover dry spells, direct feed systems are supplied with a mains backup device: the maker describes it as delivering a small amount of mains water to keep domestic appliances functioning when the underground tank has insufficient water due to lack of rainfall². One version, sold as Backup in a Box, is commonly used with direct feed systems where a submersible pump supplies water directly from the tank to appliances such as toilets or washing machines⁷. When the tank level becomes too low, the control unit opens a solenoid valve and allows approximately 200 litres of mains water to enter the tank¹. That is a measured top-up, not an open refill, and it is the point at which the water regulations on cross-connection and backflow prevention bite, which is why the whole subject has its own page on mains top-up rules.

Direct feed systems are sold as complete kits in a range of tank sizes, from a 3000 litre⁴ through 5000⁸ to 7500 litres⁹, each an underground domestic kit with mains water backup. There are also above-ground direct feed packages, such as a 7500 litre and a 9250 litre complete direct feed system intended to sit above ground¹⁰, which suit sites where excavation is impractical.

How a gravity feed system works

A gravity feed system adds one stage: the pump moves water from the underground tank, through a control unit, into a header tank in the loft, and the toilets and washing machine are then fed by gravity from that tank³. The maker's own product, the Rain Director, works exactly this way, pumping from the underground tank through a control unit into what it calls a Smart Header Tank within the loft space³.

The header tank changes how the pump behaves. Instead of starting for every flush, the pump runs to fill the header tank, then rests while the loft reserve covers demand. The maker states the Rain Director uses 6 to 8 times less energy than a traditional system by completely filling the Smart Header Tank and then letting gravity supply the appliances³. For a household watching its electricity use, that is the practical difference between the two arrangements.

The reserve in the loft is also what carries the house through trouble. The maker's guidance is blunt about the benefit: if the underground tank runs low, the control unit switches to mains water and feeds the header tank directly, keeping toilets and the washing machine working during low rainfall, pump failure, or a power cut¹. So a gravity system fails soft in three separate ways:

• Dry weather: the control unit delivers mains water directly to the header tank³.
• Pump failure: the water already in the header tank still drains to the toilets by gravity¹.
• Power cut: the same applies, no electricity is needed for the gravity leg¹.

One installation detail from the maker is worth noting before the plumber arrives: always plumb the outside tap or taps directly from the pump so you get full pump pressure, and do not feed them from the header tank³. A loft tank delivers gravity pressure only, which is fine for filling a cistern but weak at the end of a hose. Gravity pressure depends on height: Yorkshire Water explains that the lower your home is in relation to the storage tank, the higher the pressure, and the closer it is, the lower¹². A header tank a metre or two above a cistern gives a modest but steady flow, which is why power showers, which Cambridge Water notes use almost twice as much water as gravity-fed showers, are a different animal altogether¹³.

Gravity feed kits are sold in the same range of underground tank sizes as direct feed ones: 1500¹⁴, 5000¹⁵, 7500¹⁶ and 15,000 litres¹⁷, each built around the Rain Director control unit and header tank.

Why many systems need a header tank

The header tank is not an afterthought; for many homes it is the heart of the design. Enduramaxx, a tank maker, puts it plainly: water recycling systems often require a header tank to provide a steady supply of water¹⁸. The Rain Director is described by its maker as a gravity fed rainwater management control unit and header tank, and it is specified to nearly all customers when a small header tank can be situated in a loft space³.

Three reasons recur across the makers' material:

• Steady supply: the loft tank buffers demand, so appliances see a constant gravity-fed flow rather than pump starts and stops¹⁸.
• Energy: filling a tank in batches and letting gravity do the delivery uses 6 to 8 times less energy than a traditional arrangement, the maker states³.
• Resilience: the tank's contents keep flushing toilets through a power cut or pump failure, and the control unit can fall back on mains water¹.

The header tank itself is a modest piece of kit. Makers supply small horizontal tanks for exactly this duty, chosen for their low weight so a loft floor can carry them¹⁹. A loft reserve is a meaningful buffer for a household, and it is worth checking what your loft floor can safely carry before choosing a size, a point covered with tank sizing generally on our tank sizing page.

The principle is the same one used on private water supplies at village scale. In a Drinking Water Inspectorate case study of a contaminated private supply, the arrangement described is that water is pumped to a covered holding reservoir before being gravity fed to the properties²⁰. Your header tank is that arrangement in miniature: pump once, store high, deliver by gravity.

What a rainwater system is not for

The permitted uses are the non-potable ones. A domestic rainwater harvesting system can be used for laundry, watering gardens, toilet flushing and cleaning¹⁸, and the direct feed system's maker sums up the classification: rainwater is classed as non-potable, allowing water to be used for toilets and washing machines, plus garden use². Enduramaxx describes the same scope for large-scale storage: rainwater stored in its tanks can be used for non-potable applications such as toilet flushing, garden watering and washing clothes using a washing machine²¹. If you want the detail on what is and is not safe, our page on what harvested rainwater can be used for and our page on drinking rainwater cover it.

Two further limits shape whether a system makes sense at all. First, pipework: water companies recommend, and sometimes require, black and green HDPE pipe for rainwater, so that the non-potable supply can never be confused with the mains¹⁷. Our page on pipe labelling sets out what that means in practice. Second, cost and disruption: rainwater harvesting systems are typically more suited to new-build properties, where the cost of plumbing them in is negligible, and retrofitting into an existing property can be expensive¹⁸. If you are laying pipes anyway, a system is cheap to add; if it means lifting floors, it is a bigger decision.

A note on the mains side of the house, because it is often asked alongside: if you ever face a boil water notice or similar instruction on your mains supply, follow the water company's advice, and remember that normal mains use flushes the system through, as South West Water explains for households without a storage tank, where water is fed directly from the mains²². Your rainwater system does not change what you should do about your drinking water.

Underground tanks: collecting, filtering and storing

Whichever feed arrangement you choose, the front half of the system is the same: collect, filter, store. The direct feed system's maker describes it as designed for homeowners who want to collect, filter and store rainwater from their roof for domestic use and garden irrigation¹. Water arrives through the downpipes, passes through a filter, and settles in a tank that is usually buried, because buried tanks stay dark and cool and free up the garden.

Filtering happens before storage, not after. Inline filters are made for exactly this duty: one maker's compact rainwater harvesting filter is for inline filtration of rainwater in tanks both above and below ground²³. Keeping leaves and grit out of the tank protects the pump and keeps the water reaching your appliances cleaner. The filter types and how to maintain them are covered on our rainwater filters page and our maintenance page.

The tank itself comes in two broad families. Enduramaxx makes rainwater harvesting tanks, also called vertical storage tanks or water storage tanks, which provide the water storage element of a rainwater harvesting system, available as above-ground or underground tanks²⁴. Above ground, they can be vertical or horizontal, standard or slimline, low profile, or designed for restricted spaces²⁵. Below ground, the maker is clear about one requirement: underground rainwater harvesting tanks require a pump to work as part of a rainwater harvesting system²⁵, which is why every system on this page is built around one.

The F-Line flat tanks are the makers' answer to the biggest objection to underground systems, the dig. The tanks are flat and the installation depth is 60% less than other rainwater tanks, and on the larger sizes the maker states the excavation can be up to 70% less, meaning little earth excavation, easy handling and less cost¹⁵. How deep to dig in practice is covered on our underground tank depth page, and the tank designs themselves on our underground tanks and above-ground tanks pages.

Scale runs from small domestic set-ups to far larger stores, but the principle is the same: collect rainwater, filter it, and hold it underground until the household needs it. For an ordinary home, a domestic rainwater harvesting system covers laundry, watering the garden, toilet flushing and cleaning²¹. That is the yardstick to size against: enough stored water to keep those jobs running through a dry spell, without paying for capacity the roof catchment cannot refill.

One practical point on delivery, because it surprises people: on garden systems, the delivery is kerbside and offloading is the responsibility of the customer, though the maker notes shallow dig tanks can be lifted off the vehicle by hand with enough people²⁶. Plan for bodies and trolleys on delivery day.

Which arrangement suits your house

The decision comes down to three questions, in this order:

1. Do you have a loft you can put a small tank in? If not, a flat roof or a converted loft, the maker's guidance is that a direct feed system is the only suitable option¹.
2. How much do you value keeping toilets working through a power cut or pump failure? If that matters to you, and for a prepping-minded household it usually does, the gravity system's header tank reserve is the deciding feature¹.
3. How much does the pump's energy use matter? The gravity arrangement's batch filling uses 6 to 8 times less energy than a traditional system, the maker states³.

For most houses with an ordinary loft, the makers' own guidance points to gravity feed: the Rain Director is specified to nearly all customers when a small header tank can be situated in a loft space³. Direct feed earns its place where there is no loft, or where the household prefers the simpler pipework and accepts that a pump failure stops the rainwater supply until it is fixed.

If your interest is emergency water rather than everyday savings, read the two together: a gravity system's header tank is a small household reserve by design, and our pages on rainwater in a power cut and on water in an emergency pick up that thread. For the garden-only end of the scale, a water butt or a garden system may be all you need, and our water butt or underground tank comparison sets out the trade-off.