The Hymer Grand Canyon is one of the best-built campervans on the market. But even on a Mercedes Sprinter platform with German engineering, the water system has its weak points — and when something goes wrong at a campsite in the middle of nowhere, you need answers fast.

Whether you're dealing with water leaking from the bathroom sink, a pump that refuses to start, a mysteriously empty fresh water tank, or the dreaded frozen grey water tank in winter, this guide covers every common water-related problem Grand Canyon owners encounter. Each section includes the likely cause, a step-by-step quick fix, and what to do if the quick fix doesn't work.

Quick Diagnosis

1. Bathroom Sink Leaking Water

This is probably the single most common water complaint among Hymer Grand Canyon owners. You wash your hands, and a few minutes later you notice water pooling on the floor under the bathroom sink — or worse, dripping into the area below.

Why it happens

The Grand Canyon's bathroom uses compact plumbing with push-fit pipe connections. Over time, the vibration from driving on rough roads loosens these joints. The drain fitting where the waste pipe connects to the sink basin is a known weak point — it's a small plastic component that can crack, especially around the plug hole connection. The swivel mixer tap is another common culprit: its internal cartridge or the hose connections at its base can develop leaks.

On older Grand Canyon models, Hymer used 10mm rubber hose with jubilee clips for the water supply lines. These are actually more reliable than the semi-rigid push-fit systems used by some other manufacturers, but the hose clamps can still loosen over time.

Quick fix on the road

1. Locate the leak source. Place dry paper towels around every connection under the sink. Run water, then check which towel gets wet first. This tells you whether the leak is on the supply side (pressurized) or the drain side (gravity).
2. If the drain pipe has separated: Push the pipe firmly back into its connector. If it won't hold, wrap the joint tightly with self-amalgamating silicone tape. It will hold temporarily until you can replace the fitting.
3. If the tap is dripping from the base: Check that the supply hoses are fully pushed home. Tighten any jubilee clips with a flat screwdriver. If the leak comes from the swivel joint of the mixer tap itself, the cartridge inside may need replacing.
4. If the sink basin itself is cracked: A hairline crack around the waste outlet is a known Hymer issue. Clean and dry the area thoroughly, then apply two-part epoxy or marine-grade silicone sealant. This holds for weeks until you can source a replacement.

2. No Water Coming from the Taps

You turn the tap and nothing happens. Or you get a weak dribble that immediately dies. This is the most frustrating water issue because it can have multiple causes, and you need to work through them systematically.

The troubleshooting sequence

According to Hymer's own HelpCenter, check these in order:

1. Is the fresh water tank actually empty? On the Grand Canyon S, verify the level through the HYMER Connect touchscreen or app. If you've been parked in cold weather, the frost monitor may have auto-drained your tank (see Section 4).
2. Is the drain valve closed? If left open — even partially — the tank will empty itself. Check both the fresh water drain and the Truma boiler drain valve (the red knob).
3. Is the 12V supply turned on? The water pump needs 12V power. If not connected to shore power, make sure the habitation battery system is active.
4. Is the water pump switch on? There's a dedicated pump switch on the control panel. It's easy to accidentally turn off.
5. Check the pump fuse. On the Grand Canyon S, all electrical components are grouped in the rear boot. Find the fuse labeled for the water pump (typically 7.5A or 10A). Replace if blown.
6. Test the microswitch on the tap. Hymer taps have microswitches that activate the pump when opened. If you can't hear the pump at all, try a different tap — if the pump runs from the kitchen but not the bathroom, you've found your bad microswitch.

3. Water Pump Problems

Pump runs continuously (won't stop buzzing)

If your pump starts running the moment you turn on the 12V system and never stops, a microswitch inside one of the taps is stuck in the "on" position or has short-circuited. This is well-known across Hymer models.

To diagnose: find the wires leading from each tap (they trail from the base of the fitting). Disconnect them one at a time. When the pump stops, you've found the faulty tap. The microswitch needs replacing — on some taps it can be changed independently; on sealed taps, the entire assembly needs swapping.

Some Grand Canyon owners have gotten tired of repeated microswitch failures and upgraded to a Shurflo pressure-switched pump. This eliminates microswitch dependency entirely — the pump pressurizes the whole system and activates automatically when pressure drops. More reliable long-term, though it requires some wiring changes.

Pump runs but delivers weak flow

1. Cracked manifold. The plastic manifold connecting to the pump output is a known failure point. If cracked, the pump recirculates water back into the tank. Lift the inspection cap and watch for water leaking back while the pump runs.
2. Air in the system. After winterization or storage, run every tap (hot and cold) until sputtering stops. If air keeps returning after a few hours, you likely have a failing non-return valve.
3. Clogged tap aerator. Unscrew the nozzle at the end of the tap and clean out sediment. A 30-second fix that can dramatically improve flow.
4. Pump dying. Submersible pumps last a few years of regular use. If it sounds labored, time for replacement. Always photograph wiring before disconnecting.

4. Losing Water Due to Temperature — The Frost Monitor

You wake up on a cold morning, go to brush your teeth, and discover your fresh water tank is completely empty. No leak visible anywhere. This baffles many new Grand Canyon owners, but it's an intentional safety feature.

What's happening

Hymer vehicles are equipped with a frost monitor that automatically opens a drain valve when the outside temperature drops below approximately 4°C (39°F). This empties the fresh water tank to protect it and the connected plumbing from frost damage. It's a protective function — not a malfunction.

The Grand Canyon's fresh water tank is inside the vehicle (which helps with insulation), but the waste water tank is external. This is why Hymer designed the van as a three-season setup and explicitly cautions against use in very cold winters without additional preparation.

What to do about it

For mild cold (0°C to 5°C): Keep the Truma diesel heater running overnight. Interior warmth keeps components above the frost trigger temperature. On the Grand Canyon S, pre-set this through the HYMER Connect app.

For serious cold (below -5°C): The frost monitor is doing its job. Fill smaller containers and keep them inside the heated cabin overnight. Some owners bypass the frost monitor for controlled winter camping, but this is at your own risk — a burst tank from frozen water is far more expensive than refilling the next morning.

5. Winterizing Your Grand Canyon's Water System

If you're storing your Grand Canyon for winter or parking it in freezing conditions for more than a few days, proper winterization is essential. Failing to drain the system can lead to burst pipes, cracked pump, damaged faucets, or a ruptured tank — repairs easily exceeding €1,000.

1. Drain the fresh water tank completely. Use the drain valve — look for the triangular valve wheel on top of the tank. Turn approximately two full revolutions for full drain. The first partial turn drops to 20% (the "running dump" for travel).
2. Drain the Truma boiler. The safety drain valve (red knob) is typically accessible in the wardrobe area near the boiler. Make sure it's fully open.
3. Open all taps. Turn on every tap (kitchen and bathroom, hot and cold) and flush the toilet to drain residual water. Leave them open.
4. Use compressed air (recommended). Connect a compressor with pressure regulator (max 30 PSI) to the city water inlet. Blow out remaining water from low points and bends.
5. Add RV-specific antifreeze. Pour 2–3 liters of non-toxic propylene glycol RV antifreeze into the tank. Run the pump and open each tap until pink liquid flows. Never use automotive antifreeze.
6. Protect the P-traps. Pour antifreeze down each drain (bathroom sink, kitchen sink, shower) to prevent trap seals from freezing.

6. Frozen Pipes and Grey Water Tank

Even with the frost monitor active, the externally mounted grey water tank is vulnerable to freezing. A frozen grey waste tank means your sinks and shower won't drain — water just backs up.

Prevention

Keep the interior heated. Since the Grand Canyon's fresh water lines run through the heated interior, they're generally protected above 5°C. The vulnerable points are the external grey water tank and waste outlet pipes. Hymer offers optional 12V heated pads; aftermarket tank heaters (€80–150) are a solid retrofit option.

If pipes are already frozen

1. Heat the interior gradually. Run the Truma heater. Open cabinet doors under sinks to circulate warm air to pipes.
2. Use a hair dryer on accessible pipes. Never a heat gun or open flame. Work from the tap end backward toward the tank.
3. Pour warm (not boiling) water over the grey tank. Thermal shock from boiling water can crack the tank.
4. Inspect for damage once thawed. Frozen water expands and cracks pipes, fittings, and valves. Check every connection before resuming use.

7. Truma Boiler Not Producing Hot Water

The Grand Canyon uses a Truma Combi system (typically 6kW) for both space heating and hot water. When hot water stops working, it can be gas, electrical, water, or a combination.

No hot water on gas

1. Check the gas supply. Open a stovetop burner to verify flow. The boiler requires more gas pressure than the hob.
2. Check for error codes. E516H and E517H indicate gas supply or ignition problems. Reset the Truma by pressing the brown button until the orange LED illuminates. Repeat up to 3 times.
3. Bleed the gas lines. Light a stovetop burner for a minute to establish flow, then retry the Truma.
4. Check the safety window switch. Some installations have a safety switch connected to a window. If open, the boiler won't fire.

No hot water on electric (230V)

1. Verify mains power is reaching the van (check other 230V appliances).
2. Check the Truma's 230V fuse on the Combi's power electronics board.
3. Reset the 230V overheat switch on the Truma unit.
4. Test the heating elements with a multimeter (healthy = ~58 ohms). Replacement elements cost ~€130 aftermarket.

Hot water works briefly, then dies

Almost always an airlock. Classic symptom: hot water flows for a minute, sputters, stops. Water drains back from the boiler because of a failed or missing non-return valve on the cold water feed. The permanent fix: install a non-return valve (check valve) on the cold supply pipe to the Truma. Under €10. Solves it permanently.

8. Grey Water Won't Drain

Clogged tank: The most common cause. Open the cleaning cover on the tank, drain, then rinse thoroughly. Hair, food particles, and soap residue build up over time.

Jammed electric valve: You'll hear clicking but no flow. There's a small manual override knob on the valve face (roughly shirt-button sized). Turn it manually. Often in a hard-to-reach spot — long-nose pliers help.

Broken valve handle: The plastic handle can snap in cold weather. Replacement is straightforward — a knurled collar unscrews, the waste pipe pulls out.

Van not level: Waste water can "back up" if the van is tilted. Level it and try again.

9. Water Level Display Showing Wrong Values

Your HYMER Connect shows 75% when the tank is full, or empty when it isn't. The measuring probes get coated with mineral deposits, soap residue, and biofilm. The fix: drain the tank, remove the inspection cap, and clean the probe with a soft brush and mild cleaning solution. Avoid abrasives. If cleaning doesn't work, the probe needs replacing (dealer-level repair).

10. Fresh Water Tank Cap Breaking

The tank cap has thin plastic retaining lugs that break easily — a surprisingly common issue. Once snapped, it won't seal properly, leading to potential contamination and spillage. Replacement caps with more robust lugs are available from Hymer parts suppliers (typically €15–25, two minutes to swap). Carry a spare.

11. Water Tastes or Smells Bad

After the van sits unused for a few weeks, bacterial growth in stagnant water causes unpleasant taste or smell — and can make you ill.

1. Drain the entire system. Fresh tank, boiler, all lines.
2. Fill with sanitizing solution (Puriclean, Milton, or equivalent). Follow dilution instructions.
3. Run through every tap (hot and cold) and shower until you detect the solution.
4. Let sit 12–24 hours.
5. Drain and flush with fresh water through every outlet until clean.

Do this at the start of every season and after any period of more than two weeks unused. Running the Truma hot water to maximum temperature periodically helps kill bacteria in the boiler tank.

12. Water Running Into the Van from the Awning

Grand Canyon owners with a pop-top and Thule roll-out awning often find rain water running down the gap between the awning mounting and the van body. The awning is mounted high to accommodate the pop-top, creating a larger gap than on fixed-roof vans.

The most effective DIY fix: foam pipe insulation cut in half and pushed into the gap. Cheap, removable, allows the awning to roll out and the roof to pop. For a more permanent solution, self-adhesive rubber drain channels above the sliding door redirect water flow. Some owners use magnetic tape with waterproof fabric strips that attach when deployed and remove for driving.

13. Essential Repair Toolkit

Frequently Asked Questions

The Grand Canyon's water system is well-engineered for a campervan, but like any compact plumbing setup subjected to road vibration, temperature swings, and irregular use, it has known weak points. The good news: nearly every issue has a straightforward fix that doesn't require professional help.

The key to keeping your water system trouble-free is regular inspection (especially under sinks and around the Truma boiler), proper seasonal winterization, and carrying a basic repair kit. Most fixes take 15–30 minutes with basic tools.

Safe travels and dry floors.