--- title: "Why Some RVers Put a Bend in Their Sewer Hose. Does It Actually Help?" url: "https://camperfaqs.com/why-some-rvers-put-a-bend-in-their-sewer-hose" author: "Tory Jon" published: "2026-10-13T14:43:25-05:00" modified: "2026-10-13T14:43:28-05:00" --- # Why Some RVers Put a Bend in Their Sewer Hose. Does It Actually Help? If you leave your gray tank valve open on full hookups, a properly formed, water-filled bend can block campground sewer gases from traveling up the hose. With both tank valves closed and sealing properly, it offers little extra benefit. RVers commonly call that bend a homemade P-trap. The idea follows a familiar plumbing principle, but a dip in the hose needs more than the right appearance to work. And the smell it’s intended to stop can reach places beyond the inside of your RV. ## What a Sewer Hose P-Trap Looks Like If you’ve never seen one set up, this quick video from Unique Camping + Marine shows the whole thing in about a minute and a half. Their technician, Chris, builds a dip into the hose, pours water into it before connecting, and keeps the hose off the ground the entire time. [Watch this video on YouTube](https://www.youtube.com/watch?v=8XZy3AWnNQc) Looks simple enough. But why would anyone bother, especially when every drain inside the RV already has its own trap? The answer is on your roof. ## Your RV Can Turn Into a Chimney for the Campground Sewer Here’s something most RVers never think about. Open your gray valve at a full-hookup site, and you’ve connected the campground sewer straight to your gray tank. With no water barrier in the hose, sewer gas has an open path up the hose and into the tank. It doesn’t stop there. Your gray tank has a vent pipe that runs out through the roof. So the gas goes up the hose, through the tank, and out the top of your RV. From there it drifts down around your campsite and your neighbor’s. Darren Koepp, who runs the mobile repair service My RV Works in Washington state, [lays out this e](https://myrvworks.com/whats-that-smell-rv-sewer-hose-solutions/)[x](https://myrvworks.com/whats-that-smell-rv-sewer-hose-solutions/)[act path](https://myrvworks.com/whats-that-smell-rv-sewer-hose-solutions/). He goes further than most. He calls the popular advice to keep black closed and gray open flawed, because it can turn your rig into a vent for the park’s sewer. His preferred fix is simple: keep both valves closed until the tanks are about three-quarters full, then dump. If you’d rather leave the gray open, he says to put a water trap in the hose. But he’s honest about the trade-offs: - Sewer flies sometimes still get through. - The hose can wear out faster where water sits in the dip. - Valve seals can gum up when they never move, so he suggests opening and closing your valves every couple of days to wipe the seals. This is why an outside trap can matter even when every sink and shower drain inside your RV works perfectly. That smell never comes through a drain. It goes out the roof. ## A Bend Only Works If It Holds a Water Seal A dip in the hose isn’t automatically a trap. For it to block sewer gas, the water sitting in the low spot has to fill the hose from bottom to top. That’s the whole trick. A lazy sag doesn’t do that. You get a puddle along the bottom of the hose and a wide-open air gap above it. Gas slides right over the water, and now you have standing wastewater in your hose with no benefit at all. The test is simple: **the hump on the sewer side has to rise higher than the hose is wide.** With a standard 3-inch hose, the bottom of the hose at the top of that hump needs to sit more than 3 inches above the bottom of the dip. Any less, and there’s room for air to get over the water. It also has to stay that way. A bend that flattens out over a few days, or shifts every time water rushes through, can lose its seal without you noticing. And don’t overdo it. A sharp kink can choke off drainage. ## An Empty Bend Won’t Block Sewer Gas Opening an empty gray tank won't fill a newly arranged trap. The hose may have the intended shape, but the barrier is still missing. [Unique Camping & Marine’s hose-trap guide](https://uniquecampingmarine.com/blogs/news/how-and-why-to-add-a-p-trap-to-your-rv-sewer-hose?utm_source=chatgpt.com) recommends filling the trap before use. Owners can prefill the hose section before attaching it to the RV, or connect everything, open the gray valve, and run sink water to fill it. The same guide recommends straightening the hose before dumping a conventional black tank. If you’re staying, rebuild and refill the trap after you dump and flush. ## Doesn’t the RV Already Have Traps Inside? RV sink and shower drains use odor barriers, including conventional water-filled traps that can dry out or devices such as [Wavin’s HepvO waterless valve](https://wavin.us/hepvo?utm_source=chatgpt.com), which uses a membrane that opens for drainage and closes afterward. Those devices protect the living space at the drains; an outside hose trap is intended to stop campground gases before they enter the gray tank. Persistent odors inside still call for checking the RV’s plumbing. An outside bend can't fix a leaking connection or a faulty drain barrier. ## There’s Even a Product for It Long before anyone sold a gadget for this, RVers built the dip with whatever they had on hand. They’d shove a coffee can or a rock under the hose, then pull it out before dumping the black tank. The weak spot is that hoses move. A hose propped over a coffee can can slide off, flatten out, or shift a little every time water rushes through. Once the shape goes, so does the seal. That’s the problem the [EZ RV Solutions Sewer P-Trap](https://www.amazon.com/EZ-RV-Solutions-Sewer-P-Trap/dp/B0H45XH1D7?&linkCode=ll2&tag=camperfaqs-20&linkId=1f41b588074e5121b56ab24da6f88bf5&language=en_US&gaOptInStatus=true&ref_=as_li_ss_tl) is built to solve. It snaps onto a standard 3-inch sewer hose and locks it into a trap shape, so the dip stays the same depth day after day. It’s aimed at RVers who leave the gray valve open on full hookups, and it’s marketed as a barrier against both sewer gas and sewer flies. I’d take the fly claim with a grain of salt. A good water seal should keep most bugs out, but the repair tech mentioned earlier has seen flies get past a trap anyway. Think of it as a big improvement over an open hose, not a guarantee. ## Straighten the Hose Before Dumping the Black Tank The critics have a point here. A trap is supposed to hold liquid, but when you dump the black tank, you want everything out of that hose. A dip is the last thing you want during a dump. Winnebago says the same thing. Its [2017 Spirit operator’s manual](https://winnebago.com/Files/Files/Winnebago/Resources/OperatorManuals/Operator2017/17Spirit.pdf) warns that bends and sags in the hose can trap waste, and tells owners to keep the hose free of low spots when dumping. If you leave your gray valve open during your stay, the routine looks like this: 1. **Close the gray valve a day or so before you dump.** That lets clean-ish rinse water build up. Keep an eye on the level so the tank doesn’t fill up on you. 2. **Straighten the hose.** Take out the bend so the hose runs downhill to the sewer. 3. **Dump the black tank, then close that valve.** 4. **Open the gray valve.** The gray water rinses out the hose behind the black. When you pack up, lift the hose and let any leftover water run into the sewer connection before you disconnect. Skip that step and the dip you built on purpose will hand you a hose full of wastewater. As always, your RV’s manual has the final word on dumping and flushing. ## Should You Leave the Gray Valve Open? Advice differs. Unique Camping & Marine favors leaving the gray valve open on sewer hookups, paired with a trap and regular cleaning. [Watch this video on YouTube](https://www.youtube.com/watch?v=PBQ-vZfthLI) [Jayco’s 2024 towable owner’s manual](https://www.jayco.com/uploads/rvs/manuals/619-Jayco-Family-Towable-Manual---2024---WEB.pdf?utm_source=chatgpt.com) advises against leaving drain valves open continuously while camping, particularly in cold weather. Follow the instructions for your RV and the campground’s sewer connection requirements. Some campgrounds require the entire hose to stay off the ground, including any trap section. Keep a conventional black tank valve closed between dumps. The tank needs liquid to carry toilet waste out; a hose bend cannot replace that water. ## So, Is the Homemade Trap a Good Idea? It depends on how you camp. **If you leave your gray valve open on full hookups:** yes. A solid, water-filled dip blocks the path from the campground sewer to your tank and to your roof vent. Just make sure it actually holds a seal, and fill it when you set up. **If you keep both valves closed until you dump:** it’s not doing much. The closed valves are already doing the job. **On dump day:** take the bend out. A dip that’s helpful all week becomes a problem the minute the black tank opens. A lazy sag that never quite seals gives you the downside of a trap without the benefit.