A fresh-water tank and a 12V pump give you running water at the sink — and a shower, if you fit one — without fetching a jug every time. Store drinking-quality water, lift it to the tap when you open it, and drain everything before a frost. Water is heavy, the wrong plastics can taint the supply, and a poorly matched pump will cycle or leak.
This guide covers portable containers and fixed tanks, internal versus underslung mounting, how to match a 12V pump and accumulator, and how to fill, vent and winterise the system. It pairs with our UK conversion walkthrough and beginner’s 12V electrics guide. Use drinking-water-rated materials, and follow the instructions for the exact parts you fit.
How a tank-and-pump system works
- A fresh-water store — removable container or fixed tank — with a filler, high-level breather, low-level outlet and, on a fixed tank, a drain.
- An inlet strainer so grit does not damage the pump.
- A 12V pump: submersible, in the container, or a diaphragm pressure pump that draws through a hose.
- Optionally, an accumulator on the pump outlet to store a little pressurised water and reduce on–off cycling.
- Pipework to the tap, shower and any water heater, on a fused 12V circuit with an isolation switch.
Grey water from sinks and showers belongs in a separate waste tank or container. The Campervan Shop is clear that a tank contaminated by fuel or waste must never be reused for drinking water.
Portable container, internal tank or underslung tank?
| Option | Best for | Pros | Watch-outs |
|---|---|---|---|
| Removable container (often 10–20 litres) with a submersible pump | Small vans, one sink, easy access to a tap | Easy to fill, clean and drain | Limited capacity; you carry the water |
| Fixed internal tank under a seat, bed or furniture | More capacity without crawling under the van | Easier to inspect; less exposed to dirt and frost | Uses living space; needs solid restraint when full |
| Underslung tank on the chassis | Preserving interior space | Frees the cabin; can allow a larger volume | Ground clearance and exhaust clashes; more frost risk |
The Campervan Shop’s fitting guide notes that an internal tank is easier to inspect and may be less exposed to cold weather, while an underslung tank frees interior space but brings mounting, clearance and frost issues. Never assume two vans with the same model name share the same underside — wheelbase, exhaust, spare wheel and AdBlue equipment all change what fits.
Size, weight and a real tank example
Water adds roughly 1 kg per litre, plus the empty tank, fittings and supports. A 75-litre tank therefore carries about 75 kg of water when full. That load has to sit on proper supports, not flimsy furniture panels, and it counts against remaining payload. You do not have to fill a large tank to the brim on every trip.
As a UK potable example, Ecosure’s 75-litre campervan tank uses WRAS-approved polyethylene (approvals 1202543 and 1201523). Published figures are 600 × 445 × 310 mm, 6.5 kg empty, an 8-inch screw-down lid and an outlet of up to ¾-inch BSP. Allow extra space for fittings and straps. The Foundation for Water Research cites a Caravan Club figure of around 23 litres as a general minimum daily supply in a caravan — a planning number, not a rule.
Choosing a 12V water pump
Whale and DIY Campervan draw the same line: a submersible pump sits in a container and suits a simple one-tap setup; a diaphragm pressure pump mounts outside a fixed tank and usually has a built-in pressure switch, so opening a tap starts the pump.
| Type | Example | Stated performance | Notes |
|---|---|---|---|
| Submersible | Whale freshwater range | 10.3, 13.2 or 15.8 L/min | Compact and quiet. Many models must not be run dry. |
| Onboard pressure pump | Whale Watermaster FP0814 | 8 L/min open flow, 2 bar (30 psi) cut-out, about 4 A, 5 A fuse | Vertical mount, 12 mm Quick Connect. Can run dry without damage, says Whale. Maximum suction lift 3 m. |
| Onboard pressure pump | Shurflo Trail King 7 | 7 L/min at 20 psi (1.4 bar) or 30 psi (2.0 bar) | Shurflo lists a 12 V three-chamber diaphragm pump. Maximum current is 4.5 A (20 psi) or 10 A (30 psi). |
Open-flow figures are measured with little outlet pressure. Real flow falls against pipe, filters, taps and height. Match the shut-off pressure to the lowest working-pressure limit of your taps, fittings and water heater — some leisure fittings are designed around modest pressures, and a much higher-pressure pump can cause problems elsewhere.
An accumulator on the pump outlet stores a little pressurised water so the pump does not slam on and off every time you crack a tap. The Foundation for Water Research describes this as the usual way to keep pressure in the system. It will not hide leaks. If you fit a Fiamma A20, Fiamma’s instructions say it is factory pre-charged to 0.9 bar for Fiamma pumps with a 1.5 bar pressure switch. For other pumps, set the pre-charge to the pump’s start (cut-in) pressure and never above 3 bar. Check it before installation and once a year.
Pipework, strainer and tank fittings
Keep the suction side of an external pump short, unkinked and no narrower than the maker allows. Whale’s Watermaster instructions specify a strainer on the inlet and 12 mm Quick Connect ports. Shurflo’s typical notes call for a minimum ½-inch (13 mm) inside-diameter reinforced intake hose and a strainer. Mount the pump on its anti-vibration feet, on a solid board — Whale asks for a surface at least 19 mm thick — and leave access to clean the strainer.
Use food-grade drinking-water hose and fittings. The Foundation for Water Research advises WRAS-approved materials even when there is no direct mains connection, because non-approved plastics can support bacterial growth and cause taste or odour.
Fit tank connections before you strap the tank in, then leak-test them. The Campervan Shop’s typical layout is a high breather above the fill point (without it the tank can pull a vacuum); a low outlet, usually ½-inch, running to the pump; a lockable body-side filler with about 40 mm pipe, or a hose-coupling fill point; and a low-point drain (they describe an optional 19 mm tap). Extra taps branch off after the pump. An unpressurised tank must not be left closed against mains pressure through a blocked vent. Secure the tank for a full load, and do not work under a van supported only by a jack.
Wiring and installation order
Treat the pump as its own motor circuit. Use the fuse the manufacturer specifies (5 A automotive for the Whale FP0814; follow Shurflo’s sheet for the Trail King you buy) and size the cable so voltage drop does not starve the motor. Mark that fused spur on your plan using our wiring diagram guide. Fit an isolation switch you can reach quickly, and switch the pump off when the tank is empty or the van will stand unused.
- Choose portable, internal or underslung from space, payload, frost risk and how you will fill the tank.
- Measure the bay or underside, including fittings and access, against the tank drawing for your exact van.
- Fit the breather, filler, outlet and drain; leak-test before final mounting.
- Secure the empty tank, then run a short suction hose to the strainer and pump.
- Fit the accumulator if you are using one, then drinking-water pipe to each outlet.
- Wire the dedicated fused circuit and isolation switch.
- Fill slowly, watch the breather, bleed air at the taps, and check every joint before closing furniture or underbody covers.
If you later add a water heater, tee it in after the pump to the heater maker’s pressure limits and give it its own drain.
Filling from a site tap, frost and hygiene
The Water Supply (Water Fittings) Regulations 1999 apply to fittings used where water is supplied by a water undertaker. The Foundation for Water Research says any caravan that takes water directly from a water main should use WRAS-approved materials and an effective backflow preventer, so water from the van cannot return into the supply. Keep your own clean, food-grade filling hose. Whale’s Watermaster mains hook-up (EM9433) includes a factory-set pressure regulator at a nominal 1.5 bar (maximum 1.8 bar), which matters because a closed onboard tank must not see raw mains pressure.
Water expands when it freezes, so drain the tank, pump, filters, taps, pipes and water heater before freezing weather. Insulation on an underslung tank only slows heat loss: The Campervan Shop notes that wrapping does not generate heat, and a wrapped tank can still freeze in a long cold spell. If you have a Truma boiler, Truma’s instructions say disconnect power to the pump, open the hot-water taps and drain valve, and make sure all the water leaves (10 litres). Truma does not accept warranty claims for frost damage. For stale water, the Foundation for Water Research’s advice is “if in doubt, flush it out”.
FAQ
Is a submersible pump enough, or do I need a pressure pump?
A submersible pump in a removable container is usually enough for one sink. Once you have a fixed tank and several outlets — especially a shower — a self-priming diaphragm pressure pump is the more flexible choice.
Why does the pump pulse or keep clicking on and off?
Rapid cycling often means there is no accumulator, a small leak, or an oversized pump. Check joints first, then the accumulator pre-charge against the pump’s cut-in pressure.
Can the pump run if the tank is empty?
Only if the manufacturer says so. Whale states that the Watermaster onboard pump can run dry without damage; many submersible pumps must not be run dry. Switch the pump off when the tank is empty either way.
How big should the fresh-water tank be?
It depends how you camp. A 10–20 litre container can serve drinks, cooking and washing up if you refill often. Showers or multi-day off-grid trips push people toward 60–100 litres or more. Every litre is about a kilogram.
Do I have to drain the system in winter?
Yes if there is any risk of frost. Drain the tank, pipes, pump, filters and water heater. An underslung tank should not be left full in freezing weather, even if it is insulated.
Choose the simplest store that matches how you travel, then give the pump clean water, the right pressure, a short suction hose and a way to drain every last drop.