The clear plastic tube under a sink is a trap primer. It delivers a small amount of water to prime the drain pipe each time the tap is turned on to prevent sewer gases from rising up and entering the home.
What if you don’t have this clear plastic tube under your laundry room sink? If you have a high efficiency furnace in your basement, it likely has a trap primer that functions the same way. Look for a plastic hose draining into the floor drain next to your furnace.
Below we’ll discuss the concept of a trap primer and also why the plumbing underneath each sink has a u-shaped design and how this design actually provides safety to your home in several ways.
What function does a trap primer serve?
If you have a drain in the basement of your home, it undoubtedly leads into the public sewer system. This sewage system contains gases from the runoff that it collects and that last thing you want is for these fumes to enter your home through the floor drain.
So a trap primer is used throughout your home’s plumbing to prevent gases from coming up through the plumbing into your home. This involves draining small amounts of water into the drain to keep it wet and full which prevents gases from rising into your basement.
While you can’t see the drainage system under your home, you can take a peek underneath your laundry taps and other faucets and see what the drainage design looks like. Under your sink, you’ll see black PVC (plastic) plumbing with a u-shaped bend. This design is utilized to trap a small amount of water in the lowest part (the u-shape) to keep it full to prevent sewage fumes from entering your house.
So the “trap” is the bend in the plumbing and “priming” in this case refers to adding water to it, similar to how you’d prime a pump. By keeping the u-bend full of water, fumes will be stopped immediately and prevented from traveling further.
It actually provides a few other benefits as we’ll see below.
What is a P trap in plumbing?
Have you noticed the aforementioned curvy drain pipe in your home plumbing, located under sinks in the bathroom and kitchen? When water goes down the drain, it typically passes through the black plastic (PVC) plumbing piping beneath your sink before making its way down into the sewer.
It’s actually more of a u-shape than p-shape but the plumbing is done this way for a specific reason, several actually.
A p-trap:
- Prevents sewer gases from entering your home. The u-shape enables a small amount of water to sit in the pipes and block gases from rising up into the home itself.
- May prevent valuables from being lost. If you drop something like a ring or other small valuable item in your drain, the p-trap may trap the item – hence the name trap – and enable you to get it back by opening the pipe and retrieving it rather than having it washed away.
- Collects hair and other debris. Instead of clogging your drains, this stuff can often be removed from the p-trap itself, possibly saving you a costly visit from a plumber.
- Prevents odors from rising up. More so in commercial toilets that may not have water in it, a trap primer can help to prevent sewage smells from coming up through the plumbing.