P-Traps, S-Traps and Flush Inlets: Understanding Toilet Connections
Separate the toilet waste outlet from the flushing inlet, understand P-trap and S-trap terminology, and read connection positions from the actual pan drawing.
Basterfield EngineeringUpdated
A Toilet Has Two Different Water Paths
The flushing connection brings water into a WC pan. The waste outlet carries the discharge away. They belong to different parts of the installation, yet short descriptions such as back entry or P-trap are sometimes treated as though they describe the whole toilet.
Keep the two paths separate when reading a product schedule or drawing. Ask where flushing water enters the pan, then ask where the waste outlet leaves it. Only after those questions are answered should the surrounding pipe route be considered. This prevents a correct flushing choice from concealing an incompatible waste connection.
What P-Trap and S-Trap Describe
A trap retains water as part of the separation between the fixture and the drainage system. The familiar P and S names describe outlet arrangements. In common sanitaryware terminology, a P-trap has a horizontal outlet and an S-trap has a downward outlet. The exact dimensions and position still come from the fixture drawing.
Twyford’s Classic sanitaryware range, for example, distinguishes P-trap and S-trap pans. Polypipe’s drainage guidance also illustrates the horizontal-versus-vertical outlet terminology. These references explain the language; they are not evidence that a particular connector or alteration will suit every stainless steel pan.
What Top Entry and Back Entry Describe
Top entry and back entry describe the flush inlet on the pan. They do not identify the waste outlet. A top-entry pan receives its flushing connection from above; a back-entry arrangement connects at the rear. The chosen cistern or flush valve and flush pipe must suit that inlet.
The Walcro 106 range demonstrates the distinction in its named variants: 106T is the top-entry arrangement and 106B is the back-entry arrangement. A valve inlet connection size is a further specification again. It describes the supply connection to the valve, rather than the pan’s waste outlet or the overall length of the assembly.
Read the Four Combinations Separately
The B17659 range makes the two decisions explicit. B17659(PB) combines a P-trap with back entry; B17659(PT) combines a P-trap with top entry. B17659(SB) combines an S-trap with back entry, while B17659(ST) combines an S-trap with top entry.
Changing the second description therefore does not necessarily change the trap type. Likewise, changing the trap type does not automatically alter the flushing inlet. Keep the complete code and both plain-language descriptions together in the equipment schedule. The product photograph may illustrate the range; it should not override the selected model’s stated connections.
Position Matters as Well as Direction
Knowing the direction is only the start. A horizontal outlet still has a centre height and a relationship to the wall and fixture. A downward outlet still has a position relative to the finished wall and floor. Those positions need to agree with the actual connection arrangement, not simply resemble it in a photograph.
Use the finished surfaces as references when reviewing dimensions. A new floor finish or wall lining can alter the apparent position of an existing connection. Mark what has been measured on site and what has been taken from the proposed product drawing, so the installer can resolve the relationship before fixing the pan in place.
Mounting Is Another Question
Wall mounting, floor mounting and shrouding describe other aspects of the fixture. None should be used as a shortcut for identifying the trap or flush inlet. The mounting arrangement also needs its own coordination with the supporting structure and the services behind or below the pan.
The B17659 examples are wall-mounted pans with a supplied wall bracket, but their named trap and inlet combinations differ. That is why a schedule that says only wall-hung stainless steel toilet is incomplete as a connection description. An installer needs the selected drawing and configuration, as well as the proposed position in the room.
Use Drawings to Resolve the Match
For a replacement, record the existing pan identity if it is known, the waste outlet position, the flush inlet arrangement and any limitations on access. Compare those observations with the new fixture drawing. Do not assume that an adaptor can resolve every difference, or that moving one connection leaves every other part of the installation unaffected.
Once the connection language is clear, cistern-versus-valve selection becomes a separate discussion about compatible equipment and the available services. Keeping inlet, outlet and mounting information distinct helps everyone describe the same installation accurately, from the initial drawing through to maintenance records.