Connecting Multiple Water Tanks Together: Linking, Levels and Pumps

Adding storage to a home water backup system rarely means replacing a tank with a bigger one. More often, the practical answer is to link a second or third tank to what you already have. Done correctly, the tanks behave as a single reservoir: levels stay equal, one pump serves the whole bank, and you can keep expanding if your roof area or budget allows in stages.

This article walks through how that connection works, what can go wrong, and what to ask an installer before any pipework is cut.

Why link tanks instead of buying one big one?

A single large tank sounds simpler, but it comes with real constraints. Large-format tanks — anything above roughly 5,000 litres in a round configuration — are heavy, need a reinforced base, and are difficult to manoeuvre through a gate or into a confined yard. They can also be harder to source in certain regions.

Linked tanks let you add capacity in manageable steps: two 2,500-litre tanks give you 5,000 litres of storage, three give you 7,500, and so on. You can also mix slimline and round tank profiles where space is irregular — a narrow side passage alongside a broader tank pad, for example. The trade-off is more fittings, more potential leak points, and pipework that must be sized correctly from the start.

The equalisation principle: why the connection point matters

The core engineering principle behind linking tanks is communicating vessels: two containers connected below the waterline will always seek the same water level, provided the connecting pipe is not obstructed and is the same diameter throughout.

For this to work reliably:

• The connection must be at the base of each tank, not mid-wall. Some installers tap tanks halfway up, which limits equalisation to the upper portion of storage and wastes the lower half of each tank during drawdown.
• The connecting pipe diameter should match or exceed the inlet/outlet diameters already fitted to the tanks. Undersizing creates a bottleneck that slows equalisation and can cause one tank to drain while the other stays full during high-demand moments.
• All tanks in the bank should sit at the same height. Even a 50 mm difference in base elevation introduces a permanent level offset between tanks. On uneven ground, a compacted and levelled concrete pad for each tank is not optional — it is part of the system.

A useful detail to confirm with your installer: ask them to demonstrate equalisation by filling one tank while the other is empty, then watching the connecting pipe bring both tanks to the same level before the pump is commissioned.

Pipe sizing, valving and isolating tanks

A linked bank needs at least one isolation valve per tank on the connecting pipe. Without it, a cracked tank or a fitting that needs replacing forces you to drain the entire system. Gate valves or full-bore ball valves work; avoid anything with a restricted bore that would slow equalisation.

For a two-tank bank, a typical layout looks like this:

Point in the system	Recommended fitting
Tank base outlet (each tank)	BSP-threaded tank connector with rubber backing washer
Connecting pipe	HDPE or uPVC pressure pipe, same diameter as connectors
Isolation valve (each tank)	Full-bore ball valve
Pump inlet	Single draw-off point on one tank or on the connecting pipe
Float valve / level sensor	On one tank only — the system reads as one body of water

If you are adding a third tank to an existing two-tank bank, the new tank should be connected into the same base-level manifold, not daisy-chained off the second tank. Daisy-chaining creates unequal flow paths and the furthest tank may lag in level during rapid drawdown.

For more on the practical plumbing side of fitting tanks to a household supply, plumbing a water tank into your system covers inlet positions, overflow routing and pressure considerations in detail.

Choosing and sizing the pump for a linked bank

A linked bank still feeds one pump, which simplifies the electrical and control side. What changes is that the pump now has a larger reservoir to draw from, so it cycles less frequently and experiences less wear — provided the pump is sized correctly for the household’s peak demand, not just the total storage volume.

Key questions for your installer:

• What is the peak flow rate the household needs (litres per minute), accounting for simultaneous fixtures?
• Is the pump a pressure pump with a pressure vessel, or a variable-speed pump that ramps up on demand?
• Where will the pump draw from — a single outlet on one tank, or a tee on the connecting manifold?

Drawing from the connecting manifold rather than from one tank alone can slightly improve flow evenness across a large bank, though on a two- or three-tank system the practical difference is small.

If the system is gravity-fed from an elevated position rather than pump-fed, the same equalisation principle applies, but ensure the combined weight of full tanks on any elevated structure — a stand, a carport, or a roof — has been properly assessed. A full 2,500-litre tank weighs 2,500 kg. Three of them is 7,500 kg. Structural assessment is not optional.

Linking tanks correctly depends on pipework being sized and levelled properly from the start. Compare 3 free quotes — vetted installers, no obligation.

Tank size and configuration: thinking before you buy

Before purchasing additional tanks, it is worth revisiting what storage volume your household actually needs. A rough starting point is daily consumption multiplied by the number of days of independence you want. South Africans use on average about 237 litres per person per day, which is a useful upper bound for planning — though actual household consumption often varies considerably.

For many homes, the decision comes down to whether a single larger tank or a linked pair makes better sense for the site. The article small tank vs big tank works through that trade-off in more detail, including cost-per-litre differences and access constraints.

One practical note: if you are planning to link tanks you already own to tanks you are buying new, confirm the outlet thread standards match before ordering. Tank fittings in South Africa are not universally standardised, and adapting between different thread sizes adds cost and potential weak points.

Common mistakes installers should avoid

• Connecting tanks at different heights without levelling the base: results in permanent unequal levels and wasted capacity in the lower tank.
• Using undersized connecting pipe: creates a flow bottleneck that causes one tank to drain before the other equalises.
• No isolation valves: makes routine maintenance or tank replacement require draining the whole bank.
• Float valve on the wrong tank: if the float valve is on the tank furthest from the inlet, it may shut off supply before the closer tank is full, leaving capacity unused.
• Ignoring the structural load: particularly relevant for elevated installations and for older properties where concrete pads may not be rated for the combined wet weight.

A correctly installed linked bank is part of a broader backup water system that may include filtration, pressure management and a plan for recharging tanks during municipal supply windows or from a borehole.

If you are planning a linked-tank setup and want to check your design against what local installers actually build, get matched with up to three vetted specialists — no commitment required.

What to confirm with your installer before work starts

A short checklist worth having in hand when you speak to installers:

1. Will the tanks be levelled on a compacted base, and is that base included in the quote?
2. What pipe diameter will be used for the connecting manifold, and why?
3. Is there an isolation valve on each tank?
4. Where will the pump draw from, and what is its rated flow rate in litres per minute?
5. Where will the float valve or level sensor be positioned?
6. What is the overflow arrangement if tanks reach capacity during an unexpected mains surge?
7. Is any waterproofing or UV protection required for the connecting pipework in your specific installation site?

Getting clear answers to these questions before any digging starts is the best way to avoid a system that looks complete but performs poorly when you actually need it.