Holding Tank Plumbing Layouts
General Installation Guidelines
When inlet or outlet hoses are routed overboard below the waterline and terminate inside the vessel below the waterline, they must be fitted with anti-siphon loops above the waterline to prevent siphoning.
For all installations:
- Use marine-grade sanitation hose throughout
- Secure all hose connections with double stainless steel hose clips
- Wherever possible, fit hose connections into the top of the tank
- Outlet fittings should include dip pipes extending to the base of the tank
- Breather vent plumbing should use 38mm (1½”) hose
- Deck rinse-out plumbing is typically 19mm (¾”), while all other plumbing should remain 38mm (1½”)
Waste Tank Level Monitoring
The layouts shown use the Tek-Tanks Tankminder level indicator, as it meets the minimum requirements for waste tank level indication.
Alternative options include:
- S3H Series
- Gobius C monitoring system
While invasive sensors require periodic maintenance, the Gobius C is our preferred solution. Its non-invasive design reads tank levels externally, eliminating leak risks and reducing maintenance requirements.
Toilet Flushing Systems
Most marine toilets use raw seawater for flushing via an inlet seacock (not shown). However, flushing from the onboard freshwater supply is often preferable where practical, as it helps avoid:
- Dirty or oily flushing water
- Unpleasant odours
- Salt build-up and scaling inside pipework
Toilets designed to draw their own flushing water must have a dedicated connection to the freshwater tank. When using freshwater systems, ensure a siphon break is installed.
Above & Below Waterline Installations
The diagrams below illustrate a range of common holding tank plumbing layouts for installations positioned either above or below the waterline.
These examples are intended as practical guidelines only. Every vessel is different, and there are many possible variations in fittings, pumps, pipe routing and tank arrangements. The layouts should provide a useful starting point for planning your installation.
If you require advice on a specific setup, our team is always happy to help discuss suitable options.
Gravity Discharge Systems
Gravity discharge systems operate most effectively with 2″ outlet hose. However, many existing seacocks are fitted with 1½” connections, and replacing them is often undesirable.
In these cases, we recommend:
- Installing the system initially with 1½” hose
- Testing the discharge performance
- Adding a pump later if required
If your boat is already out of the water and you want the best possible gravity discharge performance, upgrading to a 2″ seacock is strongly recommended.
Pump Installations
The above-waterline layouts show manual discharge pumps, although electric pumps can also be used.
Please note:
- No pump will completely prevent waste discharge while the tank is in use
- The seacock should remain closed while waste is stored in the tank
- Open the seacock only during discharge
We also recommend fitting a 1½” ball valve to prevent waste remaining inside the pump body, which can distort internal valves over time.
Hose Routing & Anti-Siphon Loops
All layouts show 1½” sanitation hose throughout, including vent lines.
Anti-siphon loops should ideally be positioned at least 10″ above the waterline. On sailing vessels, this measurement should be considered when the boat is heeled.
If either the toilet or the holding tank is positioned at or below the waterline, the installation should always be treated as a below-waterline system.
Hose Connections & Clipping
Any connection below the waterline must be double clipped.
Although not essential above the waterline, double clipping is still recommended for added security. For the most reliable seal, position the hose clamp bands opposite each other.
Tight Bend Installations
Sanitation hose generally has a minimum bend radius of approximately 200mm before collapsing.
Where tighter bends are required, swept 1½” elbows can be used to maintain flow and avoid hose restriction.
Deck Pump-Out Positioning
Deck pump-out fittings should ideally be positioned above the discharge pipework. This allows easier access for rodding through from the deck fitting if required.
This detail is not shown in the diagrams, as including it would reduce clarity in the renderings.
Layout Animations
Animations for each plumbing layout can be viewed by clicking the corresponding images.
